GRIDLEY CITY COUNCIL AGENDA: Regular Meeting of 10-5-26 Page 1 of 3 “Our purpose isto continuously enhance our community’s vitality and overall quality of life. We are committed to providing high quality, cost-effective municipal services and forming productive partnerships with our residents and regional organizations. We collectively develop, share, and are guided by a clear vision, values, and meaningful objectives.” The Public is encouraged to attend and participate in person. Comments from the public on agenda items will be accepted until 4 pm on October 5, 2026, via email to [email protected] or via the payment/document drop box at Gridley City Hall and will be conveyed to the Council for consideration.
You may view using the following link, ID, and passcode:
https://us06web.zoom.us/j/86445204626?pwd=CM1RLornP1csfSmCMOS7egaWsSnPxr.1 Passcode:494184
Webinar ID: 864 4520 4626
CALL TO ORDER - Mayor Farr
ROLL CALL
PLEDGE OF ALLEGIANCE – Councilmember Sanchez
PROCLAMATION – None
INTRODUCTION OF NEW OR PROMOTED EMPLOYEES - None
COMMUNITY PARTICIPATION FORUM - Members of the public may address the City Council on matters not listed on the agenda. The City Council may not discuss nor take action on any community participation item brought forward by a member of the community. Comments are requested to be limited to three (3) minutes.
CONSENT AGENDA
1. City Council Minutes
City Council review and approval of City Council minutes Recommended Action(s):
a. Approve regular City Council meeting minutes dated September 21st, 2026
ITEMS FOR CONSIDERATION
2. Inter-Agency Services and Collaboration Ad Hoc Committee Gridley City Council – Regular Meeting Agenda
Monday, October 5th, 2026; 6:00 pm
Gridley City Hall, 685 Kentucky Street, Gridley, CA 95948 GRIDLEY CITY COUNCIL AGENDA: Regular Meeting of 10-5-26 Page 2 of 3 City Council to discuss and consider establishing the Inter-Agency Services and Collaboration Ad Hoc Committee
Recommended Action(s):
a. Establish the Ad-Hoc Committee and authorize Mayor to appoint two Councilmembers of the City Council to serve on the committee
3. Butte Subbasin Groundwater Sustainability Plan Periodic Evaluation Report for Water Years 2021-2025
The City Council to review and approve the Butte Subbasin Groundwater Sustainability Plan Periodic Evaluation Report for Water Year 2021-2025 and supplemental appropriation for 2026-2027 cost share
Recommended Action(s):
a. Adopt Resolution No. 2026-R-038: A Resolution of the City of Gridley approving and authorizing the filing (with Department of Water Resources of the state of California) of the attached periodic evaluation report for water years 2021 through 2025 pursuant to water code section 10733.8
b. Adopt Resolution No. 2026-R-039: A Resolution of the City of Gridley approving a supplemental appropriation and authorizing payment of the fiscal year 2026-27 cost-share contribution to the County of Butte Groundwater Sustainability Agency
COUNCIL COMMITTEE REPORTS - Brief reports on conferences, seminars, and meetings attended by the Mayor and City Council members, if any.
CITY ADMINISTRATOR REPORTS - Brief updates and reports on conferences, seminars, and meetings attended by the City Administrator, if any.
DEPARTMENT UPDATE REPORTS – Brief updates and reports on City services as it pertains to each department, if any.
POTENTIAL FUTURE CITY COUNCIL ITEMS - (Appearing on the Agenda within 30 days):
Ord. No. 852-2026: 2nd Reading by Title Only and Adoption 10/19/2026 Ord. No. 851-2026: 2nd Reading by Title Only and Adoption 10/19/2026 Electric Distribution System Study 10/19/2026
Records Retention RFP 10/19/2026
Code of Conduct Handbook 10/19/2026
CLOSED SESSION – None
GRIDLEY CITY COUNCIL AGENDA: Regular Meeting of 10-5-26 Page 3 of 3 ADJOURNMENT – adjourning to a regular meeting on October 19th, 2026.
NOTE 1: POSTING OF AGENDA- This agenda was posted on the public bulletin board at City Hall at or before 6:00 p.m., October 2nd, 2026. This agenda along with all attachments is available for public viewing online at www.gridley.ca.us and at the Administration Counter in City Hall, 685 Kentucky Street, Gridley, CA.
NOTE 2: REGARDING UNSCHEDULED MATTERS – In accordance with state law, it shall be the policy of this Council that no action shall be taken on any item presented during the public forum or on unscheduled matters unless the Council, by majority vote, determines that an emergency situation exists, or, unless the Council by a two-thirds vote finds that the need to take action arose subsequent to the posting of this agenda.
GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 1 of 6 “Our purpose isto continuously enhance our community’s vitality and overall quality of life. We are committed to providing high quality, cost-effective municipal services and forming productive partnerships with our residents and regional organizations. We collectively develop, share, and are guided by a clear vision, values, and meaningful objectives.”
CALL TO ORDER
Mayor Farr called the meeting to order at 6:00 pm.
ROLL CALL
Present: Roberts, Calderon, Farr, Johnson, Sanchez
Absent: None
Arriving after roll call: None
Staff Present: Elisa Arteaga, City Administrator
Todd Farr, Police Chief
Landon Little, Deputy City Attorney
Martin Pineda, Finance Director
Ryan Carlson, Electric Director
Jerry Cox, Public Works Director
Christopher Smith, Principal Planner
Carmen Santana, Deputy City Clerk
Pamela Roberts, Code Enforcement
PLEDGE OF ALLEGIANCE
Vice Mayor Johnson led the Pledge of Allegiance.
PROCLAMATION – None
INTRODUCTION OF NEW OR PROMOTED EMPLOYEES - None
COMMUNITY PARTICIPATION FORUM
Joantha Guthrie from Waste Management spoke to report the upcoming Free Dump Day scheduled for October 10th.
Mike Trainer, 755 Berry Patch Ct., spoke to inform the City of an upcoming Cal Cities training opportunity for the City Attorney.
CONSENT AGENDA
Gridley City Council – Regular Meeting Minutes
Monday, September 21, 2026; 6:00 pm
Gridley City Hall, 685 Kentucky Street, Gridley, CA 95948 GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 2 of 6
1. City Council Minutes
City Council review and approval of City Council minutes Recommended Action(s):
a. Approve regular City Council meeting minutes dated August 3rd, September 8th, 2026
b. Approve special City Council meeting minutes dated August 20th, August 24th, August 28th, 2026
ROLL CALL
Motion: Johnson
Second: Roberts
Action: approve consent agenda as presented
Ayes: Calderon, Farr, Roberts, Johnson, Sanchez
Noes: None
Absent: None
Abstain: None
Motion passed, 5-0
ITEMS FOR CONSIDERATION
2. Emergency Operation Plan Update
City Council to review and adopt the Emergency Operation Plan update Recommended Action(s):
a. Adopt Resolution No. 2026-R-036: a Resolution of the City Council of the City of Gridley adopting the updated Emergency Operations Plan for Gridley Fire Chief Dyer and Battalion Chief Zarate presented a PowerPoint outlining the significant changes to the Emergency Operations Plan (EOP) and facilitated a discussion with the City Council regarding the updates.
ROLL CALL
Motion: Roberts
Second: Johnson
Action: approve Resolution No. 2026-R-036: a Resolution of the City Council of the City of Gridley adopting the updated Emergency Operations Plan for Gridley Ayes: Calderon, Farr, Roberts, Johnson, Sanchez
Noes: None
Absent: None
GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 3 of 6 Abstain: None
Motion passed, 5-0
3. Discussion and Direction Regarding Procurement of On-Call City Engineering and Electric Utility Engineering Services
The City Council provide direction regarding the procurement structure for on-call City engineering and electric-utility engineering services and authorize staff to prepare and issue the corresponding Requests for Qualifications (RFQs).
Recommended Action(s):
a. Staff recommends that the City Council provide direction regarding the procurement structure for on-call City engineering and electric-utility engineering services and authorize staff to prepare and issue the corresponding Requests for Qualifications (RFQs).
City Administrator Elisa Arteaga introduced the discussion regarding the procurement structure for on-call civil engineering and electric utility engineering services for the City.
ROLL CALL
Motion: Johnson
Second: Roberts
Action: direct staff to prepare and issue two separate requests for qualifications for on-call city engineering and civil infrastructure services and on-call electric utility engineering services Ayes: Calderon, Farr, Roberts, Johnson, Sanchez
Noes: None
Absent: None
Abstain: None
Motion passed, 5-0
4. Tentative Parcel Map 2-26 (Cunningham); APN 009-061-001; 999 Indiana Street / 1445 Pecan Street
City Council to consider approval Resolution 2026-R-037: a Resolution of the City Council of the City of Gridley approving tentative parcel map no. 2-26 (APN 009-061-001) for the division of one parcel into two (2) parcels located at 999 Indiana St. and 1445 Pecan St.
Recommended Action(s):
a. Approve Resolution 2026-R-037: a Resolution of the City Council of the City of Gridley approving tentative parcel map no. 2-26 (APN 009-061-001) for the division of one parcel into two (2) parcels located at 999 Indiana St. and 1445 Pecan St
GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 4 of 6 Principal Planner Christopher Smith presented Tentative Parcel Map 2-26 (Cunningham) to the City Council for consideration.
ROLL CALL
Motion: Johnson
Second: Calderon
Action: approve Resolution 2026-R-037: a Resolution of the City Council of the City of Gridley approving tentative parcel map no. 2-26 (APN 009-061-001) for the division of one parcel into two (2) parcels located at 999 Indiana St. and 1445
Ayes: Calderon, Farr, Roberts, Johnson, Sanchez
Noes: None
Absent: None
Abstain: None
Motion passed, 5-0
5. Introduction and first reading by title only of Ordinance No. 851-2026 Amending Title 17 (Zoning) of the Gridley Municipal Code to Implement 2022-2030 Housing Element Programs and Comply with State Housing Law
City Council to receive introduction of Ordinance No. 851-2026 by reading of title only, and set the second reading and adoption of the ordinance for a tentative date of October 5th, 2026
Recommended Action(s):
a. Approve Open the public hearing and receive public testimony; b. Introduce Ordinance No. 851-2026 (Attachment 1) by reading of title only, and c. Set the second reading and adoption of Ordinance for a tentative date of October 5th. 2026.
Principal Planner Christopher Smith introduced the first reading, by title only, of Ordinance No. 851-2026.
Nora Hogan, 740 Berry Patch Court, expressed concerns regarding the proposed changes in Ordinance No. 851-2026.
Lee Underwood, 725 Berry Patch Court, also expressed concerns regarding the proposed changes in Ordinance No. 851-2026.
ROLL CALL
Motion: Roberts
Second: Sanchez
GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 5 of 6 Action: Introduce by reading of title only of Ordinance No. 851-2026: Amending Title 17 (Zoning) of the Gridley Municipal Code to Implement 2022-2030 Housing Element Programs and Comply with State Housing Law
Ayes: Calderon, Farr, Roberts, Sanchez
Noes: None
Absent: Johnson
Abstain: None
Motion passed, 4-1
6. Introduction and first reading of title only of Ordinance No. 852-2026 Removal of Additional Three-Foot Story Setback Requirement (Gridley Municipal Code Section
17.78.020(A)(2)(a))
City Council to receive introduction of Ordinance No. 852-2026 by reading of title only, and set the second reading and adoption of the ordinance for a tentative date of October 5th, 2026
a. Approve Open the public hearing and receive public testimony; b. Introduce Ordinance No. 852-2026 (Attachment 1) by reading of title only, and c. Set the second reading and adoption of Ordinance for a tentative date of October 5th. 2026.
Principal Planner Christopher Smith introduced the first reading by title only of Ordinance 852- 2026.
ROLL CALL
Motion: Roberts
Second: Sanchez
Action: Introduce by reading of title only of Ordinance No. 852-2026: Removal of Additional ThreeFoot Story Setback Requirement (Gridley Municipal Code Section 17.78.020(A)(2)(a)) Ayes: Calderon, Farr, Roberts, Sanchez, Johnson
Noes: None
Absent: None
Abstain: None
Motion passed, 5-0
COUNCIL COMMITTEE REPORTS
Councilmember Roberts commended the Police Department for its assistance to a Gridley resident.
Vice Mayor Johnson reported that West Nile virus activity remains elevated in the County.
CITY ADMINISTRATOR REPORTS –
GRIDLEY CITY COUNCIL MINUTES: Regular Meeting of 9-21-26 Page 6 of 6 City Administrator Arteaga provided a report on her recent meetings, including meetings with City staff and discussions with the City of Biggs regarding the electrical contract. She also reported that a proposed ad hoc committee will be brought forward for consideration at the next regular City Council meeting.
DEPARTMENT UPDATE REPORTS – None
POTENTIAL FUTURE CITY COUNCIL ITEMS - (Appearing on the Agenda within 30 days):
Noise Ordinance Amendment 10/5/2026
AMI Metering 10/5/2026
Electric Distribution System Study 10/5/2026
Records Retention RFP 10/5/2026
Code of Conduct Handbook 10/19/2026
CLOSED SESSION – None
ADJOURNMENT – adjourning to a regular meeting on October 5th, 2026.
City Council Agenda Item #2
Staff Report
Date: October 5th, 2026
To: Mayor and City Council
From: Elisa Arteaga, City Administrator
Subject: Establishment of an Inter-Agency Services and Collaboration Ad Hoc Committee Recommendation
Staff recommends that the City Council discuss and consider establishing the Inter-Agency Services and Collaboration Ad Hoc Committee and authorize the Mayor to appoint two Councilmembers of the City Council to serve on the Committee.
If approved, the Committee would be tasked with meeting with representatives of other public agencies, including neighboring cities, the County, and other governmental entities, for the limited purpose of reviewing identified existing contractual service arrangements, service levels, associated costs, and cost-sharing methodologies, and evaluating opportunities for shared services, cooperative projects, community events, and other mutually beneficial activities.
Background
The City of Gridley and the City of Biggs have an existing relationship involving the electrical maintenance services. As costs, operational needs, service expectations, and community priorities change over time, periodic review of these arrangements provides both cities with an opportunity to evaluate contracts and whether the existing structure continues to meet the needs of their respective communities.
Staff recommends establishing a two-member City Council ad hoc committee to facilitate focused discussions with representatives of the City of Biggs and develop recommendations for consideration by the full City Council. In addition to reviewing the existing electrical service agreement, there may be opportunities to collaborate on community events, projects, equipment and resource sharing, professional services, grant opportunities, and other activities that provide mutual benefit.
Discussion
The proposed Inter-Agency Services and Collaboration Ad Hoc Committee would provide a forum for representatives of the two cities to review all existing contractual relationship and identify opportunities to improve coordination, establish appropriate cost-sharing methodologies, and explore other areas of cooperation.
The Committee's assigned scope would include:
• Reviewing the contractual arrangements for services between the cities; • Reviewing current service levels, responsibilities, and associated costs; • Evaluating whether existing cost-sharing or reimbursement provisions appropriately reflect the services and resources being provided;
• Discussing potential modifications to existing service arrangements; x Regular
Special
Closed
Emergency
• Identifying opportunities for shared services, equipment, personnel, professional services, or other municipal resources;
• Discussing cost sharing for community events, programs, and projects that provide benefits to both communities;
• Identifying potential joint projects, grant opportunities, or other collaborative efforts; and • Developing findings and recommendations for presentation to the full City Council.
The Committee would serve solely in an advisory capacity. It would not have authority to amend or enter into contracts, commit or expend City funds, establish City policy, or otherwise bind the City.
Any proposed amendments to an existing agreement, new agreements, financial commitments, appropriations, or policy decisions resulting from the Committee's discussions would be presented to the full City Council for consideration and approval at a properly noticed public meeting.
Committee Composition and Term
The Inter-Agency Services and Collaboration Ad Hoc Committee would consist of two members of the City Council appointed by the Mayor. Appropriate City staff may participate in Committee meetings as necessary to provide administrative, financial, operational, contractual, or technical information. The Committee would be established for the limited purpose of completing the assigned review and inter-agency discussions described in this report.
Following completion of its review, the Committee would present its findings and recommendations to the City Council. The Committee would conclude its assigned work upon presentation of its final recommendations to the City Council unless the Council provides additional direction or establishes a new defined assignment.
Brown Act Consideration
The proposed Committee is intended to function as a limited-purpose ad hoc advisory committee consisting solely of less than a quorum of the City Council and assigned to address the specific matters identified by the Council.
The Committee's scope and duration are therefore intended to be limited to reviewing the existing service arrangements, evaluating identified cost-sharing and collaboration opportunities, and returning recommendations to the City Council.
Staff will coordinate with the City Attorney regarding the Committee's structure and meeting procedures to ensure compliance with applicable Brown Act requirements. If the Committee's responsibilities subsequently develop into continuing subject-matter jurisdiction, the City should reevaluate whether the Committee should instead operate as a standing committee subject to applicable open-meeting requirements.
Fiscal Impact
There is no significant direct fiscal impact associated with establishing the Committee other than staff time associated with preparing for and participating in meetings.
The Committee's review may identify potential changes to existing service costs, reimbursement methodologies, shared expenses, or opportunities for cost savings. Any recommendation having a fiscal impact would be presented to the City Council with the appropriate financial analysis before implementation.
Compliance with City Council Strategic Plan or Budget Goal The proposed action supports the City's goals of fiscal responsibility, intergovernmental collaboration, efficient delivery of municipal services, and responsible management of City resources.
Reviewing existing service arrangements and exploring opportunities for shared costs and resources can assist the City in ensuring that services are provided efficiently and that costs are appropriately allocated among participating agencies.
City Council Agenda Item #3
Staff Report
Date: October 5, 2026
To: Mayor and City Council
From: Elisa Arteaga, City Administrator
Subject: Approval of the Butte Subbasin Groundwater Sustainability Plan Periodic Evaluation Report for Water Year 2021-2025 and Supplemental Appropriation for FY 26-27 Cost Share
RECOMMENDATION
Staff recommends City Council:
a. Adopt Resolution No. 2026-R-038: A Resolution of the City of Gridley approving and authorizing the filing (with Department of Water Resources of the state of California) of the attached periodic evaluation report for water years 2021 through 2025 pursuant to water code section 10733.8
b. Adopt Resolution No. 2026-R-039: A Resolution of the City of Gridley approving a supplemental appropriation and authorizing payment of the fiscal year 2026-27 cost-share contribution to the County of Butte Groundwater Sustainability Agency
BACKGROUND
The City of Gridley is one of the participating agencies within the Butte Subbasin working collaboratively to comply with the requirements of California's Sustainable Groundwater Management Act (SGMA).
In 2019, the City and the other participating agencies entered into a Cooperation Agreement for the development and implementation of the Butte Subbasin Groundwater Sustainability Plan (GSP). The GSP was subsequently adopted by the participating agencies and submitted to the California Department of Water Resources (DWR) in 2022. DWR approved the Butte Subbasin GSP in 2023.
As part of the ongoing implementation and evaluation requirements of SGMA, Water Code section 10733.8 provides for DWR's periodic review of groundwater sustainability plans and their implementation. The participating agencies have prepared the attached Periodic Evaluation Report for Water Years 2021 through 2025, dated September 24, 2026.
DISCUSSION
The Periodic Evaluation Report provides a comprehensive review of implementation of the Butte Subbasin GSP during the five-year evaluation cycle. Among other matters, the report addresses:
• Groundwater conditions and information reported through the annual reporting process; • New information developed or collected during the evaluation period; x Regular
Special
Closed
Emergency
• Changes to monitoring locations and monitoring networks; • The status of DWR Recommended Corrective Actions;
• Progress on projects and management actions identified in the GSP; • Updates to the well inventory and land subsidence monitoring network; and • Adjustments to sustainable management criteria, including groundwater-level Minimum Thresholds and Measurable Objectives.
•
Based upon the Periodic Evaluation, the participating agencies have determined that groundwater conditions within the Butte Subbasin remain sustainable and that no significant undesirable conditions were observed or reported during the evaluation cycle.
The report also documents the agencies' work to address DWR's Recommended Corrective Actions and explains technical adjustments made during implementation of the GSP.
In particular, groundwater-level and interconnected-surface-water Measurable Objectives and Minimum Threshold values were recalculated and restated beginning with the 2023 water year to account for groundwater elevation variances from elevations contained within the SGMA Portal. This adjustment was intended to allow groundwater readings to be interpreted consistently and is documented in a Technical Memorandum included as Appendix 4B of the Periodic Evaluation.
The resolution further recognizes that certain technical modifications made during the evaluation period—including changes to the well inventory, revisions to the land subsidence monitoring network, and adjustments to monitoring and reporting criteria—do not constitute an amendment to the GSP.
Approval of the resolution will constitute the City's approval of the Periodic Evaluation Report.
Following approval by the participating members under the Cooperation Agreement and the applicable Groundwater Sustainability Agency governing body, the Periodic Evaluation will be submitted to DWR in accordance with Water Code section 10733.8.
The resolution also authorizes GSA staff, participating member agencies, and consultants to respond to DWR inquiries following submission of the Periodic Evaluation. An additional resolution is attached to the staff report for supplemental appropriation and authorizing the payment of the 2026-2027 CostShare Contribution.
FISCAL IMPACT
Approval of the Periodic Evaluation Report does not, by itself, authorize a new city project or expenditure. However, there are costs associated with participation in the Butte Subbasin GSP, groundwater monitoring, reporting, and implementation that will require a supplemental appropriation for FY 26-27 cost-share and allow for existing groundwater management arrangements.
The city will continue to budget annually for future cost-share contributions.
ENVIRONMENTAL REVIEW
Approval of the Periodic Evaluation Report is an administrative action associated with the City's ongoing participation in groundwater sustainability planning and reporting. The resolution does not independently approve or authorize construction of a project or physical modification of the environment. Any individual projects or management actions requiring environmental review will be evaluated separately, as applicable.
COMPLIANCE WITH CITY COUNCIL STRATEGIC PLAN OR BUDGET GOALS Approval of the Periodic Evaluation supports the City's goal of maintaining reliable and sustainable water resources, protecting critical public infrastructure, and complying with applicable state groundwater management requirements.
ATTACHMENTS
1. Resolution No. 2026-R-038: Approving and Authorizing Filing of the Periodic Evaluation Report for Water Years 2021–2025
2. Resolution No. 2026-R-039: Authorizing Payment of FY 26-27 Cost Share and Approving Supplemental Appropriation
3. Butte Subbasin Groundwater Sustainability Plan Periodic Evaluation Report, dated September 24, 2026
4. Periodic Evaluation Fact Sheet
RESOLUTION NO. 2026-R-038
1 A RESOLUTION OF THE CITY OF GRIDLEY APPROVING AND AUTHORIZING THE FILING (WITH DEPARTMENT OF WATER RESOURCES OF THE STATE OF CALIFORNIA) OF THE ATTACHED PERIODIC EVALUATION REPORT FOR WATER YEARS 2021 THROUGH 2025 PURSUANT TO WATER CODE SECTION 10733.8.
The Governing Board of the City of Gridley (Hereinafter referred to as “Agency”) does hereby make the following findings, determinations, and take the following actions:
1. Agency, along with other parties constituting Agencies joining in the Butte Subbasin Groundwater Sustainability Plan, entered into a Cooperation Agreement in 2019 to develop the Butte Subbasin Groundwater Sustainability Plan (hereinafter referred to as the “Plan”) which Plan was adopted by each of the Agencies and submitted to the Department of Water Resources in 2022 pursuant to Water Code section 10733.4.
2. The Butte Subbasin Groundwater Sustainability Plan was approved by the Department of Water Resources (DWR) in 2023; and
3. Water Code section 10733.8, at least every five years, provides for a review by the Department of Water Resources of the annual reports supplied regarding the implementation and conditions pursuant to the Plan; and
4. The attached Periodic Evaluation Report describes the contents of the annual reports, new information gathered, and the conditions observed and reported during the five-year period (evaluation cycle) regarding the Plan, changes in monitoring locations and networks, and the status of actions undertaken related to Recommended Corrective Actions reported by the DWR in regard to the Plan, the status of Projects, and Management Actions under and pursuant to the Plan; and
5. The Agency finds that the groundwater conditions within the Butte Subbasin are sustainable and that no significant undesirable conditions have been observed or reported during the evaluation cycle of the Periodic Evaluation Report. The Agency further finds that Recommended Corrective Actions by the DWR have been addressed and are being conducted as reported in the attached Periodic Evaluation Report. Agency finds that minor alterations in the well inventory, revisions to land subsidence monitoring network and sustainable management criteria definitions and groundwater level Minimum Thresholds and Measurable Objectives adjustments made as documented in the Annual Reports to DWR, each of which modification is not an RESOLUTION NO. 2026-R-038
2 Amendment of the Groundwater Sustainability Plan but instead an adjustment in the reporting and measuring criteria for the Plan; and
6. The groundwater level and interconnected surface water Measurable Objectives and Minimum Thresholds values in the Plan were recalculated and restated in the 2023 water year and subsequent annual reports to account for groundwater elevation variances from the elevations in the SGMA Portal which are used after the adjustment. The adjustment allows readers to interpret the readings consistently. This adjustment was approved by the Agencies and is thoroughly documented in a Technical Memorandum attached to the Periodic Evaluation as Appendix 4B.
Now therefore, the Agency resolves as follows:
1. Each of the above findings is true and correct.
2. The Periodic Evaluation Report attached hereto, dated September 24th, 2026, is hereby approved by this Agency and upon approval and adoption by the other Members in the Cooperation Agreement and approval by the Groundwater Sustainability Agency Board shall be submitted to the DWR in compliance with the requirements of Water Code section 10733.8. Following submission to the Department of Water Resources, the staff of the Groundwater Sustainability Agency, its members and its consultants are directed and authorized to promptly respond to any inquiries of the Department of Water Resources.
This Resolution was adopted by the City Council of the City of Gridley on this 5th day of October, 2026 and remains in full force and effect without amendment by the following vote:
AYES: ________________________________________
NOES: ________________________________________
ABSTAIN:_______________________________________
ABSENT: _______________________________________
By:
______________________________ ____________________________ Elisa Arteaga, City Clerk Michael Farr, Mayor
RESOLUTION NO. 2026-R-038
3 Agency Names:
Biggs West Gridley Water District
Butte County
Butte Water District
City of Biggs
City of Gridley
Colusa Groundwater Authority
Glenn County
Reclamation District 1004
Reclamation District 2106
Richvale Irrigation District
Western Canal Water District
RESOLUTION NO. 2026-R-039
A RESOLUTION OF THE CITY COUNCIL OF THE CITY OF GRIDLEY APPROVING A SUPPLEMENTAL APPROPRIATION AND AUTHORIZING PAYMENT OF THE FISCAL YEAR 2026-27 COST-SHARE
CONTRIBUTION TO THE COUNTY OF BUTTE GROUNDWATER SUSTAINABILITY AGENCY WHEREAS, the City of Gridley participates in regional groundwater sustainability planning and implementation activities within the Butte Subbasin in accordance with the Sustainable Groundwater Management Act (“SGMA”); and
WHEREAS, the City works cooperatively with other participating agencies within the Butte Subbasin to implement the adopted Groundwater Sustainability Plan (“GSP”) and to meet applicable monitoring, reporting, planning, and groundwater management requirements; and WHEREAS, participating agencies share certain costs associated with administration, technical services, monitoring, reporting, implementation, and other activities necessary to carry out groundwater sustainability responsibilities within the Butte Subbasin; and WHEREAS, the City of Gridley’s Fiscal Year 2026-27 cost-share contribution payable to the County of Butte Groundwater Sustainability Agency is $12,790.91; and WHEREAS, sufficient expenditure authority was not included in the City’s adopted Fiscal Year 2026-27 budget for this cost-share contribution, requiring a supplemental appropriation; and WHEREAS, the City Council finds that payment of the City’s cost-share contribution supports the City’s continued participation in regional groundwater sustainability efforts and implementation of the Butte Subbasin Groundwater Sustainability Plan.
NOW, THEREFORE, BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF GRIDLEY AS FOLLOWS:
Section 1. The foregoing recitals are true and correct and are incorporated herein by reference.
Section 2. The City Council hereby authorizes payment of the City of Gridley’s Fiscal Year 2026- 27 cost-share contribution to the County of Butte Groundwater Sustainability Agency in the amount of $12,790.91.
Section 3. The City Council hereby approves a supplemental appropriation in the amount of $12,790.91 to the following expenditure account:
Account No.: 630-4630-53600
Amount: $12,790.91
Purpose: FY 2026-27 Groundwater Sustainability Agency Cost-Share Contribution RESOLUTION NO. 2026-R-039
Section 4. The Finance Director is hereby authorized and directed to make the necessary budget adjustments and accounting entries to implement this Resolution and process payment of the authorized cost-share contribution.
Section 5. The City Administrator, Finance Director, and their designees are authorized to take such administrative actions as may be reasonably necessary to carry out the purposes of this Resolution.
Section 6. This Resolution shall become effective immediately upon its adoption.
PASSED AND ADOPTED by the City Council of the City of Gridley at a regular meeting held on October 5th, 2026, by the following vote:
AYES: ______________________________________________________________ NOES: ______________________________________________________________ ABSENT: ______________________________________________________________ ABSTAIN: ______________________________________________________________ ATTEST:
______________________________ ____________________________ Elisa Arteaga, City Clerk Michael Farr, Mayor
Time Horizon: 20 Years
(Through 2042)
Purpose: Sets the base
information, defines
sustainability metrics, and
establishes long-term goals
The Master Plan
(GSP)
Time Horizon: 1 Year
(Due April 1)
Purpose: A basic progress
tracking tool documenting
current conditions,
estimated pumping, and
year-over-year storage
changes, and progress on
implementation.
The Annual Check-In
(Annual Reports)
The Deep Dive
(Periodic Evaluation)
Time Horizon: 5 Years
(Due every 5 years)
Purpose: An implementation
evaluation tool. Assesses 5
years of implementation,
addresses state feedback,
adds new science and data,
and determines if the GSP
needs updating.
Is ourgroundwateringoodshape?
Status: On Track
The first Periodic Evaluation assessment concludes that the Subbasin continues to be managed sustainably.
Land Subsidence
Status: Not Occurring
Status: Stable
Change in Storage
(Cumulative change stabilized around
-50 TAF, showing rebound by 2025).
Status: Stable
Groundwater Levels
Every five years, State law – the Sustainable Groundwater Management Act (SGMA) – requires the Butte Subbasin's
11 local groundwater agencies to closely review
implementation of groundwater management in the
subbasin [Butte SGMA fact sheet].
This is the first such in-depth evaluation of the groundwater sustainability plan (GSP) check-in, covering water years 2021–2025. It is essentially a comprehensive report card that shows whether the plan continues to support
groundwater sustainability and what new information has improved subbasin understanding.
Whatisa“Periodic Evaluation,”andwhy
is itimportant?
Butte Subbasin Groundwater: A Five-Year Comprehensive Review Yes. Groundwater conditions have stayed stable
over the past five years, even enduring through
two dry years that required more pumping than
usual.
[Learn more: 2025 Annual Report brochure]
No red flags were raised, and the Subbasin's
GSP continues to provide a meaningful roadmap
without needing significant changes.
Much of that stability comes down to how the
Subbasin operates: when rainfall is abundant,
the Subbasin relies on surface water for
irrigation (from the Feather River, Sacramento
River, or Butte Creek); when supplies tighten,
groundwater fills the gap.
This conjunctive-use approach, paired with
cooperative management across all 11 agencies,
is central to why the Subbasin has weathered
recent dry years without lasting groundwater
issues.
BUTTE SUBBASIN | GROUNDWATER SUSTAINABILITY PLAN | JULY 2026 www.buttebasingroundwater.org/
First Periodic Evaluation | Water Years 2021-2025
The current Groundwater
Sustainability Plan (GSP)
remains the appropriate
guiding policy for the region.
Pumping volumes stayed
within sustainable yields.
No Plan Amendment
is required at this time.
Groundwater Sustainability Indicators
Seawater Intrusion
Not applicable (Butte Subbasin is
not located near the coast).
Status: Stable
Water Quality
(Some localized, naturally occurring
water quality challenges identified).
Status: Stable
Interconnected
Surface Water
The Focus Area What DWR Asked What We Did
Wherearewe now,andwhat’s next?
DWR approved the Butte Subbasin GSP in July 2023 and asked the GSAs to address three Recommended Corrective Actions (RCAs):
TheState recommendedthat 3 key items in theGSPbeaddressedby the First Periodic Evaluation.Howdidweaddress them,andwhat changed? The Draft Periodic Evaluation will be available in
August/September 2026. The Final Periodic
Evaluation is due to the State in January 2027.
The agencies will keep expanding the monitoring
network, apply new state guidance on interconnected
streams once it's released, and continue working with
neighboring subbasins on shared challenges.
Your Input Matters! Attend your GSA meetings and
public workshops to stay involved.
www.buttebasingroundwater.org/
Butte Subbasin Periodic Evaluation | Water Years 2021-2025 Fine-Tuning How We
Track Groundwater
Levels
Look closer at how declining
groundwater levels could affect
local supply wells.
Built a detailed well inventory,
studied threshold impacts, and
expanded assessment of
potential impacts of groundwater
uses and users and documented
the findings.
Upgraded our monitoring network
using advanced satellite data
and ground instruments.
Installing new shallow monitoring
wells, verified the health of
groundwater-dependent habitats,
and partnering with neighboring
subbasins [Interconnected Surface
Water factsheet].
Sharpen the approach to
tracking sinking land
(subsidence), even though it isn't
currently occurring here.
Clarify how groundwater
pumping affects nearby streams
and wetlands.
Getting Clearer on
Land Subsidence
Understanding
Connection between
Surface Water and
Groundwater
Where can I learn more or sharea
comment?
KeyMilestonesAhead
Aug/Sept 2026
Draft Periodic
Evaluation
released
January 2027
Final Report
submitted to the
State
Learn More: Visit
buttebasingroundwater.org
or scan the QR code to sign
up for your GSA’s interested
parties list.
Share a Comment by Sept 9: To
comment on the Periodic Evaluation:
email the Facilitation Team
([email protected]).
Comments due September 9, 2026
Read & Review: Take a deeper dive
into groundwater conditions by
reading the most recent Annual
Report.
1 Capitol Mall, Suite 340
Sacramento, CA 95814
elmontgomery.com
September 24, 2026
Butte Subbasin Periodic Evaluation
Prepared for:
BUTTE SUBBASIN GROUNDWATER SUSTAINABILITY AGENCIES
Left and right photos courtesy of Andy Scheer
Center photo courtesy of California Department of Water Resources
BUTTE SUBBASIN GSAS
September 24, 2026
Butte Subbasin Periodic Evaluation
Final Draft to GSAs
Prepared for:
Butte Subbasin GSAs
Prepared by:
Montgomery & Associates
1 Capitol Mall, Suite 340, Sacramento, California
With support from:
Davids Engineering, Inc.
ACKNOWLEDGEMENTS / LIST OF CONTRIBUTORS
This Periodic Evaluation was developed by:
Montgomery & Associates – Project Lead
Lisa Porta, P.E., Project Manager
Pete Dennehy, P.G., C.Hg., Technical Lead
Hannah Vaughn-Hulbert, Water Resources Scientist
Luis Mendez, GIS Specialist
Davids Engineering – Water Budgets and Projects and Management Actions Brandon Ertis, P.E.
With contributions from:
Consensus Building Institute
Tania Carlone – GSA Meetings Coordination and Facilitation Butte County Department of Water and Resource Conservation Technical Staff Under the Supervision of:
Draft Butte Subbasin Periodic Evaluation, September 2026 Page i Contents
ACRONYMS & ABBREVIATIONS.............................................................................................................VIII EXECUTIVE SUMMARY.........................................................................................................................ES-1 1 INTRODUCTION ................................................................................................................................. 1 2 NEW INFORMATION COLLECTED ................................................................................................... 3 3 STATUS OF RECOMMENDED CORRECTIVE ACTIONS ................................................................. 5
3.1 Recommended Corrective Action 1........................................................................................... 5
3.2 Recommended Corrective Action 2........................................................................................... 5
3.3 Recommended Corrective Action 3........................................................................................... 6 4 GROUNDWATER CONDITIONS RELATIVE TO SUSTAINABLE MANAGEMENT CRITERIA......... 8
4.1 Chronic Lowering of Groundwater Levels .............................................................................. 10
4.1.1 Chronic Lowering of Groundwater Levels SMC Defined in the 2022 GSP....................... 10
4.1.2 Current Chronic Lowering of Groundwater Levels Conditions Relative to SMC .............. 11
4.1.3 Evaluation of Chronic Lowering of Groundwater Levels SMC ......................................... 15
4.2 Depletion of Interconnected Surface Water............................................................................ 20
4.2.1 Depletion of Interconnected Surface Water SMC Defined in the 2022 GSP.................... 21
4.2.2 Current Depletion of Interconnected Surface Water Conditions Relative to SMC............ 21
4.2.3 Evaluation of Depletion of Interconnected Surface Water SMC....................................... 23
4.3 Reduction of Groundwater in Storage..................................................................................... 23
4.4 Degraded Groundwater Quality ............................................................................................... 24
4.4.1 Degraded Groundwater Quality SMC Defined in the 2022 GSP...................................... 24
4.4.2 Current Degraded Groundwater Quality Relative to SMC................................................ 24
4.4.3 Evaluation of Degraded Groundwater Quality SMC......................................................... 27
4.5 Land Subsidence....................................................................................................................... 27
4.5.1 Land Subsidence SMC Defined in the 2022 GSP............................................................ 27
4.5.2 Current Land Subsidence Conditions Relative to SMC ................................................... 28
4.5.3 Evaluation of Land Subsidence SMC .............................................................................. 35 5 STATUS OF PROJECTS AND MANAGEMENT ACTIONS.............................................................. 44
5.1 Background ............................................................................................................................... 44
5.1.1 Overview of Butte Subbasin GSP and PMA Approach .................................................... 44
5.1.2 Categories of Butte Subbasin PMAs (from GSP)............................................................. 45
5.1.3 Types of Butte Subbasin PMAs and District Projects (introduced in PE)......................... 46
5.2 Summary and Status of PMAs ................................................................................................. 46
5.2.1 Ongoing PMAs................................................................................................................. 54
5.2.2 Planned PMAs................................................................................................................. 54
5.2.3 As Needed PMAs ............................................................................................................ 57 Draft Butte Subbasin Periodic Evaluation, September 2026 Page ii
5.2.4 New PMAs....................................................................................................................... 57
5.3 Estimated Benefits of Active PMAs ......................................................................................... 57
5.4 Tracking, Administering, and Reporting of PMAs.................................................................. 60
5.4.1 Revised PMA List for DWR Module................................................................................. 60
5.4.2 Process of Reviewing and Adding PMAs......................................................................... 63 6 BASIN SETTING BASED ON NEW INFORMATION OR CHANGES IN WATER USE.................... 64
6.1 Hydrogeologic Conceptual Model ........................................................................................... 64
6.1.1 Butte Subbasin Analysis .................................................................................................. 65
6.1.2 DWR Sacramento Valley Regional Investigation............................................................. 66
6.2 Groundwater Conditions .......................................................................................................... 66
6.2.1 New Water Quality Information........................................................................................ 67
6.2.2 Groundwater Dependent Ecosystem Analysis................................................................. 75
6.3 Water Use Changes and Associated Water Budget............................................................... 80
6.3.1 Water Use for WYs 2021-2025........................................................................................ 81
6.3.2 Land Use or Cropping Pattern Changes.......................................................................... 83
6.3.3 Hydrology and Surface Water Supply Reliability.............................................................. 86
6.3.4 Updates to Current and Projected Water Budgets........................................................... 88
6.3.5 Updates to Sustainable Yield and Changes in Storage ................................................... 90
6.3.6 Subbasin Conditions and Assessment of Overdraft......................................................... 91
6.4 Model Updates........................................................................................................................... 92
6.4.1 Key Model Update Findings and Recommendations for Future Modeling Efforts............ 92 7 MONITORING NETWORKS.............................................................................................................. 95
7.1 Groundwater Level and Groundwater Storage....................................................................... 95
7.1.1 Status of GSP Data Gaps for Groundwater Level and Groundwater Storage.................. 97
7.1.2 Monitoring Network Assessment for Groundwater Level and Groundwater Storage..... 100
7.1.3 2027 Representative Monitoring Networks .................................................................... 103
7.1.4 Sustainable Management Criteria Updates ................................................................... 107
7.2 Interconnected Surface Water................................................................................................ 115
7.2.1 Status of GSP Data Gaps for Interconnected Surface Water ........................................ 117
7.2.2 Monitoring Network Assessment for Interconnected Surface Water.............................. 119
7.2.3 2027 Representative Monitoring Network for Interconnected Surface Water ................ 120
7.2.4 Sustainable Management Criteria Updates for Interconnected Surface Water.............. 122
7.3 Groundwater Quality............................................................................................................... 126
7.3.1 Status of GSP Data Gaps for Groundwater Quality ....................................................... 130
7.3.2 Monitoring Network Assessment for Groundwater Quality............................................. 130
7.3.3 Sustainable Management Criteria Update for Groundwater Quality .............................. 131
7.4 Land Subsidence..................................................................................................................... 131 Draft Butte Subbasin Periodic Evaluation, September 2026 Page iii
7.4.1 Status of GSP Data Gaps for Land Subsidence ............................................................ 134
7.4.2 Monitoring Network Assessment for Land Subsidence.................................................. 134
7.4.3 2027 Representative Monitoring Network for Land Subsidence .................................... 135
7.4.4 Sustainable Management Criteria Updates for Land Subsidence.................................. 137
7.5 SGMA Monitoring Network Module Reconciliation .............................................................. 137 8 GSA AUTHORITIES AND ENFORCEMENT ACTIONS.................................................................. 139
8.1 Funding .................................................................................................................................... 139
8.1.1 Butte Water District - Proposition 218 Outreach Approach............................................ 142
8.1.2 Reclamation District 2106 - Proposition 218 Outreach Approach.................................. 143
8.2 County Well Ordinances and Permitting Processes ............................................................ 143
8.2.1 Butte County.................................................................................................................. 144
8.2.2 Colusa County ............................................................................................................... 145
8.2.3 Glenn County................................................................................................................. 146
8.3 County Drought Resilience Plan Updates............................................................................. 148
8.3.1 Butte County.................................................................................................................. 149
8.3.2 Colusa County ............................................................................................................... 149
8.3.3 Glenn County................................................................................................................. 150
8.4 Domestic Well Mitigation Programs in Glenn and Colusa Counties................................... 150
8.5 General Plan Updates ............................................................................................................. 150
8.5.1 Butte County General Plan ............................................................................................ 150
8.5.2 Colusa County General Plan ......................................................................................... 154
8.5.3 Glenn County General Plan........................................................................................... 154
8.5.4 City of Biggs and City of Gridley General Plans............................................................. 155
8.5.5 Effects of Land Use Plans Outside Subbasin ................................................................ 155 9 OUTREACH, ENGAGEMENT, AND COORDINATION WITH OTHER AGENCIES....................... 156
9.1 Inter-agency Coordination Within the Subbasin .................................................................. 156
9.1.1 Governance Structure.................................................................................................... 158
9.1.2 Subbasin Cost Sharing for SGMA Implementation........................................................ 158
9.2 Public Outreach and Engagement......................................................................................... 159
9.2.1 Public Participation Venues ........................................................................................... 159
9.2.2 Communications, Public Notices, and Informational Materials ...................................... 162
9.3 Coordination with Other Agencies ........................................................................................ 162
9.3.1 DWR Technical Support Services.................................................................................. 162
9.3.2 Annual Groundwater Monitoring .................................................................................... 163 10 OTHER INFORMATION.................................................................................................................. 164
10.1 Consideration of Adjacent Basins ......................................................................................... 164
10.1.1 Inter-Basin Coordination in the North Sacramento River Corridor ................................. 165 Draft Butte Subbasin Periodic Evaluation, September 2026 Page iv
10.1.2 Inter-Basin Coordination in the Sutter Buttes Area ........................................................ 170
10.1.3 Additional Inter-Basin Coordination Efforts .................................................................... 172
10.2 Challenges Not Previously Discussed .................................................................................. 173
10.2.1 Assistance Needs for Plan Implementation ................................................................... 173
10.3 Legal Challenges..................................................................................................................... 174 11 SUMMARY OF KEY TAKE-AWAYS AND CONTINUED IMPLEMENTATION APPROACH ......... 175 12 REFERENCES ................................................................................................................................ 178 Tables
Table ES-1 Summary of Sustainable Management Citeria for Each Applicable Sustainability Indicator in the Butte Subbasin............................................................................................................... ES-4 Table 2-1. Summary of New Information Since the Last Periodic Evaluation (Initial GSP)............................ 3 Table 2-2. Missing Information in Initial GSP................................................................................................. 4 Table 4-1. Current Groundwater Conditions Relative to Sustainable Management Criteria as Defined in the GSP ........................................................................................................................................... 9 Table 4-2. Summary of Wells Completed Deeper Than 50 Feet At-risk to Be Dry Under MT Conditions.... 18 Table 4-3. Summary of Wells Completed Less Than 50 Feet Projected to Be Dry Under MT Conditions... 19 Table 4-4. Potential Risks to Critical Infrastructure in the Subbasin and Subsidence BMP......................... 38 Table 5-1. Summary and Status of Projects and Management Actions....................................................... 48 Table 5-2. Planned Projects and Management Actions............................................................................... 56 Table 5-3. Estimated Benefits of Completed or In Progress Projects and Management Actions................. 59 Table 6-1. Summary of Active Cleanup Program Sites in the Butte Subbasin............................................. 73 Table 6-2. Summary of Butte Subbasin NDVI‑Based Vegetation Health Categories for Likely GDE Areas (2020 –2025)............................................................................................................................ 76 Table 6-3. Summary of GDE Regional Range of 2020 to 2025 NDVI Annual Averages ............................. 80 Table 6-4. GSP and PE Current Surface Water and Groundwater Use for WYs 1971-2018 and 2021-2025................................................................................................................................ 88 Table 6-5. Estimated Groundwater Extractions, Storage Change, and Sustainable Yield for Four Water Budget Scenarios..................................................................................................................... 91 Table 7-1. Groundwater Level RMS Assessment Summary...................................................................... 101 Table 7-2. 2027 Groundwater Level Representative Monitoring Network Well Construction Information.. 106 Table 7-3. Summary of MO/MT Revisions for Groundwater Level Representative Monitoring Network.... 107 Table 7-4. Summary of 2027 Measurable Objectives and Minimum Thresholds for Groundwater Levels RMS....................................................................................................................................... 109 Table 7-5. Butte Subbasin Surface Water Gages...................................................................................... 115 Table 7-6. Butte Subbasin Interconnected Surface Water RMS Assessment Summary ........................... 119 Table 7-7. 2027 ISW Representative Monitoring Network Well Construction Information ......................... 120 Draft Butte Subbasin Periodic Evaluation, September 2026 Page v Table 7-8. Summary of MO/MT Revisions for ISW Representative Monitoring Network ........................... 122 Table 7-9. Summary of 2027 Measurable Objectives and Minimum Thresholds for ISW RMS ................. 123 Table 7-10. Summary of ISW and Groundwater Level SMC Comparison ................................................. 126 Table 7-11. Summary of Groundwater Quality Monitoring Network........................................................... 128 Table 7-12. Summary of Land Subsidence Extensometer Monitoring Network Well Construction Information ............................................................................................................................................... 135 Table 8-1. Butte Subbasin GSAs’ Sources of SGMA Implementation Funding ......................................... 140 Table 8-2. Public Meeting Schedule – Proposition 218 Process for Butte Water District........................... 142 Table 8-3. Public Meeting Schedule – Proposition 218 Process for RD 2106 ........................................... 143 Table 9-1. Butte Subbasin Public Engagement Meetings.......................................................................... 160 Table 11-1. Summary of Updated SMC..................................................................................................... 176 Figures
Figure 1-1. Groundwater Sustainability Agencies in the Butte Subbasin....................................................... 2 Figure 4-1. Primary Aquifer Spring Groundwater Elevations Relative to Measurable Objective for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) .................... 13 Figure 4-2. Very Deep Aquifer Spring Groundwater Elevations Relative to Measurable Objective for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) ..................... 13 Figure 4-3. Primary Aquifer Fall Groundwater Elevations Relative to Measurable Objectives for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) .................... 13 Figure 4-4. Very Deep Aquifer Fall Groundwater Elevations Relative to Measurable Objectives for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) ..................... 14 Figure 4-5. Primary Aquifer Fall Groundwater Elevations Relative to Minimum Thresholds for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) .................... 14 Figure 4-6. Very Deep Aquifer Fall Groundwater Elevations Relative to Minimum Thresholds for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) .................... 15 Figure 4-7. Location of Supply Wells that Could Lose Access to Groundwater if Water Levels Reach MT . 17 Figure 4-8. ISW Spring Groundwater Elevations Relative to MO................................................................. 22 Figure 4-9. ISW Fall Groundwater Elevations Relative to MO..................................................................... 22 Figure 4-10. ISW Fall Groundwater Elevations Relative to MT.................................................................... 23 Figure 4-11. 2025 EC Data for Butte Subbasin RMS Wells......................................................................... 26 Figure 4-12. Cumulative Land Subsidence, WY 2020-2025 (from WY 2025 Annual Report) ...................... 29 Figure 4-13. Annual Land Subsidence for WY 2025 and Conveyance Canal Infrastructure........................ 30 Figure 4-14. Land Subsidence and Groundwater Elevation at Extensometer 18N01E35L001M................. 32 Figure 4-15. Land Subsidence and Groundwater Elevation at Extensometer 19N01E35B002M ................ 33 Figure 4-16. Land Subsidence and Groundwater Elevation at Extensometer 20N01E18L001M................. 34 Figure 4-17. Subsidence BMP Management Scenarios (DWR, 2026) ........................................................ 36 Figure 4-18. Subsidence Monitoring Locations and PLSS Sections............................................................ 41 Draft Butte Subbasin Periodic Evaluation, September 2026 Page vi Figure 6-1. Maximum Nitrate Concentration (mg/L) from 2000 to 2025....................................................... 68 Figure 6-2. Maximum Arsenic Concentration (ug/L) from 2000 to 2025 ...................................................... 70 Figure 6-3. Maximum 1,2,3-TCP Concentration (ug/L) from 1989 to 2025 .................................................. 72 Figure 6-4. Active Contamination Sites in the Butte Subbasin..................................................................... 74 Figure 6-5. Butte Subbasin GDE Regions ................................................................................................... 77 Figure 6-6. Regional NDVI Averages and Health (2020-2025).................................................................... 78 Figure 6-7. Annual Surface Water Diversions and Groundwater Extractions for WYs 2021-2025............... 82 Figure 6-8. 2024 Land Use in the Butte Subbasin ....................................................................................... 85 Figure 6-9. Water Use Type in the Butte Subbasin (updated 2025) ............................................................ 87 Figure 6-10. Butte Subbasin Groundwater Budget, as Simulated by BBGM............................................... 89 Figure 6-11. Groundwater Pumping and Change in Storage from WY 2025 Annual Report ....................... 90 Figure 7-1. 2022 GSP Primary and Very Deep Aquifer Groundwater Level Representative Monitoring Site Network.................................................................................................................................... 96 Figure 7-2. GSP-identified Primary Aquifer Water Level Monitoring Data Gaps.......................................... 97 Figure 7-3. Primary Aquifer Groundwater Level Representative Monitoring Network Data Gap Filling Efforts ................................................................................................................................................. 99 Figure 7-4. Revised Primary Aquifer Groundwater Level Representative Monitoring Site Network with Identified Wells to be Removed.............................................................................................. 102 Figure 7-5. 2027 Primary Aquifer Groundwater Level Representative Monitoring Site Network................ 104 Figure 7-6. 2027 Very Deep Aquifer Groundwater Level Representative Monitoring Network .................. 105 Figure 7-7. 2027 Measurable Objectives for Groundwater Level Primary Aquifer RMS Wells................... 111 Figure 7-8. 2027 Minimum Thresholds for Groundwater Level Primary Aquifer RMS Wells...................... 112 Figure 7-9. 2027 Measurable Objectives for Groundwater Level Very Deep Aquifer RMS Wells .............. 113 Figure 7-10. 2027 Minimum Thresholds for Groundwater Level Very Deep Aquifer RMS Wells ............... 114 Figure 7-11. 2022 GSP Interconnected Surface Water RMS Network ...................................................... 116 Figure 7-12. Revised Interconnected Surface Water RMS Network with Remaining Data Gaps............... 118 Figure 7-13. 2027 Interconnected Surface Water RMS Network............................................................... 121 Figure 7-14. Measurable Objectives for Interconnected Surface Water RMS Wells.................................. 124 Figure 7-15. Minimum Thresholds for Interconnected Surface Water RMS Wells..................................... 125 Figure 7-16. Water Quality Representative Monitoring Network................................................................ 129 Figure 7-17. GSP Land Subsidence Monitoring Networks with Cumulative Subsidence Measured by InSAR ............................................................................................................................................... 133 Figure 7-18. Revised 2027 Land Subsidence RMS Monitoring Network ................................................... 136 Figure 9-1. Groundwater Sustainability Agencies in the Butte Subbasin................................................... 157 Figure 10-1. Northern Sacramento Valley Subbasins................................................................................ 166 Draft Butte Subbasin Periodic Evaluation, September 2026 Page vii
APPENDICES
Appendix 3A – Well Inventory of Supply Wells in the Butte Subbasin Appendix 4A – RMS Groundwater Level Hydrographs
Appendix 4B – MT/MO Revision Technical Memorandum
Appendix 4C – 2025 Groundwater Quality Trend Monitoring Program Results Appendix 6A – Hydrogeologic Conceptual Model Evaluation Appendix 6B – GDE Health Evaluation Methodology
Appendix 6C – Water Use Analysis Methodology
Appendix 7A – GSP Revised 2027 Broad and RMS Monitoring Networks Appendix 8A – Second Amendment to Cooperation Agreement Appendix 10A – Interbasin Coordination Factsheet
Appendix 10B – Interconnected Surface Water Factsheet
Appendix 10C – Interbasin Coordination Framework 5-Year Implementation Update Appendix 11A – Adopted Resolution by the Butte Subbasin GSAs (Placeholder) Draft Butte Subbasin Periodic Evaluation, September 2026 Page viii
ACRONYMS & ABBREVIATIONS
µg/L ................micrograms per Liter
μS/cm .............microsiemens per centimeter
1,2,3-TCP .......1,2,3 - Trichloropropane
AEM ...............aerial electromagnetic
AF ..................acre-feet
AFY................acre-feet per year
APNs ..............assessor's parcel numbers
BAB ...............Butte Advisory Board
BBGM ............Butte Basin Groundwater Model
bgs ..................below ground surface
BMP ...............Best Management Practice
BWD ..............Butte Water District
BWGWD........Biggs-West Gridley Water District
CBI .................Consensus Building Institute
CDEC .............California Data Exchange Center
CGA ...............Colusa Groundwater Authority
COC ...............constituent of concern
CODSS ...........canal operations decision support system DROP .............Drought Resilience and Outreach Program DWR ..............California Department of Water Resources EC ..................electrical conductivity
EIR .................Environmental Impact Report
FRRAWMP....Feather River Regional Agricultural Water Management Plan FSS .................Facilitation Support Services
GAMA ...........Groundwater Ambient Monitoring and Assessment GDE ...............groundwater dependent ecosystem
GEEEO ..........Groundwater Extraction Estimates from Earth Observations GPS ................Global Positioning System
GSAs ..............Groundwater Sustainability Agencies
GSP or Plan ....Groundwater Sustainability Plan
HCM ..............hydrogeologic conceptual model
IBC .................inter-basin coordination
ILRP ...............Irrigated Lands Regulatory Program
IM ...................interim milestones
InSAR ............Interferometric Synthetic Aperture Radar ISW ................Interconnected Surface Waters
LEA ................Local Enforcement Agency
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ix MAR ..............Managed Aquifer Recharge
MBR ...............multi-benefit recharge
mg/L ...............milligrams per Liter
MO .................Measurable Objectives
MT..................Minimum Thresholds
NCCAG..........Natural Communities Commonly Associated with Groundwater NDVI..............Normalized Difference Vegetation Index NRO ...............Northern Region Office
NSRC .............North Sacramento River Corridor
NSV................Northern Sacramento Valley
NSV IRWM ...Northern Sacramento Valley Integrated Regional Water Management PE ...................Periodic Evaluation
PLSS ..............Public Land Survey System
PMAs .............projects and management actions
RCAs ..............recommended corrective actions
RD ..................Reclamation District
RID .................Richvale Irrigation District
RMS ...............representative monitoring site
SEWD ............Sutter Extension Water District
SGMA ............Sustainable Groundwater Management Act SMC ...............Sustainable Management Criteria
Subbasin .........Butte Subbasin
SWRCB..........State Water Resources Control Board
TDS ................total dissolved solids
TM..................Technical Memorandum
TNC................The Nature Conservancy
TSS .................Technical Support Services
UR ..................undesirable results
USGS .............United States Geological Survey
WCWD ..........Western Canal Water District
WD .................Water District
WY .................Water Year
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-1
EXECUTIVE SUMMARY
The Butte Subbasin’s first Periodic Evaluation (PE) written assessment covers the evaluation cycle between Water Years 2021 and 2025, immediately following the Groundwater Sustainability Plan (GSP) adoption in December 2021. This evaluation includes the information collected over the past five years and summarizes data analysis presented in the last five Annual Reports submitted to California Department of Water Resources (DWR). New information collected includes locally measured groundwater level and groundwater quality data, aerial electromagnetic (AEM) and subsidence information provided by DWR, additional evaluation of constituents of concern (COC), a refined supply well inventory, and groundwater-dependent ecosystems (GDE) health assessment. Together, this data and analysis allowed the Groundwater Sustainability Agencies (GSAs) to clarify information presented in the GSP and address DWRprovided Recommended Corrective Actions (RCAs). The resulting assessment concluded that the GSP continues to be the appropriate guiding policy and implementation plan for sustainable groundwater management in the Butte Subbasin and does not necessitate an amendment at this time.
Three Recommended Corrective Actions are Addressed
In its July 27, 2023, review and approval of the Butte Subbasin GSP, DWR identified three RCAs to guide plan implementation (DWR, 2023a). The three RCAs were addressed in this first PE as follows:
1. RCA 1: Revise the sustainable management criteria (SMC) for the chronic lowering of groundwater levels; evaluate effects on beneficial users if water levels reach the minimum thresholds.
a. Data gap addressed through well inventory review
b. Updated SMC per Annual Reports documented in Periodic Evaluation (PE) and confirmed by the GSAs
c. Beneficial users analysis conducted
2. RCA 2: Revise the SMC for land subsidence
a. Addressed by revising Land Subsidence monitoring network with best available data
b. DWR Best Management Practice (BMP) reviewed to confirm and adapt SMC
3. RCA 3: Improve the understanding and management of streamflow depletion caused by groundwater pumping
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-2 a. Addressed through installation of new shallow monitoring wells b. Evaluated GDE health
c. Awaiting DWR guidance for review
d. Conducting inter-basin coordination (IBC) with neighboring subbasins Groundwater Conditions Remain Stable
Groundwater conditions were found to be stable and the Subbasin is on track to continue to meet its sustainability goal, defined in the GSP as:
The sustainability goal for the Butte Subbasin is to maintain, through a cooperative and partnered approach, locally managed sustainable groundwater resources to preserve and enhance the economic viability, social well‐being, and culture of all beneficial uses and users without experiencing undesirable results.
The Subbasin did not experience any undesirable results in this first evaluation cycle, despite the occurrence of two surface water curtailment years, which caused increased groundwater pumping compared to average extraction volumes. This Subbasin continues to demonstrate optimal conjunctive use management of its water resources through the cooperative management by the 11 GSAs.
Sustainable Management Criteria Have Been Confirmed
During this evaluation cycle, the GSAs identified calculation errors associated with the original development of the Minimum Thresholds (MTs) and Measurable Objectives (MOs) for the groundwater level and interconnected surface waters (ISW) representative monitoring networks in the GSP. The MO/MT values in the GSP were calculated incorrectly due to the use of different groundwater level data than are currently available on the SGMA Portal. The error was corrected and the MTs and MOs were recalculated and documented in the Water Year (WY) 2023 Annual Report (Butte Subbasin GSAs, 2024) and have been referenced ever since. The updated MTs and MOs provide a similar level of protection to beneficial users as the ones shown in the GSP; therefore, they are in line with the policy intent and protection level described in the GSP.
The GSAs have formally adopted these updated MTs and MOs as documented in the Resolution of October 2026, signed by all GSAs. A copy is provided in Appendix 11A To address RCA 2, the subsidence network was revised to include the newest DWR Interferometric Synthetic Aperture Radar (InSAR) data extent and three extensometers. This revision to the network, along with the RCA 2 and the newly released Subsidence Best Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-3 Management Practice (BMP) document by DWR, prompted the GSAs to review the SMC for subsidence. As a result, the MTs and MOs for subsidence are unchanged; however, the GSAs decided to modify the undesirable result definition as follows:
“An undesirable result occurs when 25% of any Public Land Survey System (PLSS) Township (6 x 6 sections) exceeds the MT for two consecutive years, within the same InSAR pixels.”
The MT does not change and is set at 0.5 foot over five years within the same InSAR pixels. An undesirable result occurs if a cumulative amount of subsidence of 0.5 foot is measured with InSAR data over a five-year period, two years in a row.
This definition is different from the GSP, where the undesirable result was related to 25% of Global Positioning System (GPS) monitoring locations falling below the MT (Butte Subbasin GSAs, 2021). Since the GPS stations are no longer operational, this change was necessary with the revised land subsidence monitoring network.
The occurrence of an undesirable result will be confirmed with review of groundwater level and extensometer data and presence of critical infrastructure that could be damaged. The GSAs are not responsible for managing changes in land surface elevation due to other factors (e.g., tectonics) besides lowering of groundwater levels.
The GSAs consider this change in management approach to be consistent with the intention of the GSP and does not necessitate a GSP Amendment. The GSAs have adopted this new undesirable result definition in the Resolution of October 2026, signed by all GSAs (Appendix 11A).
Based on these changes, the revised SMC are shown in the summary table below.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-4 Table ES-1: Summary of Sustainable Management Citeria for Each Applicable Sustainability Indicator in the Butte Subbasin Sustainability
Indicator Evaluation Status MT Definition Undesirable Result Identification MO Definition Chronic Lowering
of Groundwater
Levels and
Reduction of
Groundwater
Storage
No indication of undesirable results
There were no groundwater level
measurements below the MT between
WY 2021 and 2025.
For each RMS well, the shallower of
either:
i. the shallowest 7th percentile of
nearby domestic wells, or
ii. the range of measured
groundwater levels, or 20 feet
(whichever is greater), below the
observed historic low.
If the resulting value is shallower than
the observed historic low, the MT is
set 10 feet deeper than the observed
historic low.
When 25% of each aquifer’s
RMS wells fall below their MTs
for 24 consecutive months (11
of 41 wells in the Primary
Aquifer and 3 of 10 in the Very
Deep Aquifer).
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well.
Depletion of
Interconnected
Surface Water
No indication of undesirable results
There are no groundwater level
measurements below the MT between
WY 2021 and 2025.
10 feet below the measured historical
low groundwater elevation at each
RMS well.
When 25% of the RMS wells (3
of 12) fall below their MTs for
24 consecutive months.
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well.
Degraded Water
Quality
No indication of undesirable results
One well routinely exceeds the MT due
to naturally occurring salinity. Otherwise,
there are no MT exceedances between
WY 2021 and 2025.
The higher of either 900 μS/cm or the
measured historical high of EC for
each RMS well.
When 25% of RMS wells
(2 of 8) exceed their MT for 2
consecutive years.
Measured EC less than or equal
to 700 μS/cm at each RMS well.
Land Subsidence No indication of undesirable results
There were no subsidence monitoring
sites with a subsidence rate greater than
the MT between WY 2021 and 2025.
0.5 foot of subsidence over a 5-year
period within the same InSAR pixel.
When 25% of any PLSS
Township (6 x 6 sections)
exceeds the MT over 2
consecutive years, within the
same InSAR pixels.
Up to 0.25 foot of subsidence
over a 5-year period, within the
same InSAR pixels. (0.05 foot
over 1 year; 1 foot over 20
years).
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-5 Projects and Management Actions are on Track
The GSP identified a number of projects and management actions (PMAs) the GSAs and their partners could implement to continue to manage the Subbasin sustainably. The PMAs were categorized as ongoing, planned, or as needed at the time of GSP development. Most projects are being implemented by the Districts as part of their normal operations and are not necessarily needed to reach sustainability in the Butte Subbasin. Summarized below are the three categories of projects and their general status:
1. Ongoing PMAs – Status: completed
System Modernization for Biggs-West Gridley Water District (WD), Richvale Irrigation District (RID), Western Canal WD
2. Planned PMAs – Status: in progress
• Butte Water District – dual source irrigation system
• Subbasin-wide multi-benefit recharge (MBR) – now Rancho-Llano Seco recharge project
• Subbasin-wide grower education (now moved to outreach and engagement) • Subbasin-wide additional shallow monitoring wells (Technical Support Services [TSS] funding received, installation in progress)
3. As-Needed PMAs (18 total) – Status: not started or no longer needed These projects were deemed no longer needed by the GSAs to advance groundwater sustainability in the Subbasin and have been reclassified as “other District projects” that are being completed outside of SGMA compliance needs, but still promote good stewardship of conjunctive water resources management in the Subbasin.
One new PMA (a conjunctive use project) was identified in the time since GSP adoption, and it has been fully implemented and was completed in WY 2024. DWR included a total of 26 PMAs in their new online PMA Module to be updated annually by the GSAs. After further review of this list of projects, the GSAs decided to simplify the list to only include projects necessary for GSP implementation that have a clear goal and linkage to groundwater sustainability. A total of eight projects now remain in the DWR Module and will be tracked by the GSAs. An annual roundtable discussion among GSA managers will support identification of new PMAs, as needed, to maintain Subbasin sustainability.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-6 Additional Information Solidifies Understanding of Basin Setting The Butte Subbasin hydrogeologic conceptual model (HCM) was re-analyzed during the evaluation period by California State University, Chico, Earth and Environmental Sciences Department (CSU Chico) Professor Dr. Todd Greene, Geologist. Professor Greene reviewed the geologic layering in the subbasin based on DWR’s 2022 AEM geophysical surveys and supporting lithology and geophysical data. The analysis verifies the HCM presented in the GSP provides aquifer stratigraphy for the existing RMS monitoring wells and was envisioned as a guide for selecting the location and screen intervals for future monitoring well sites.
Additional surface water quality related to constituents of concern (COCs) Arsenic, Nitrate, and 1,2,3-Trichloropropane (1,2,3-TCP) was reviewed to supplement missing information in the GSP and support ongoing discussions on managing elevated arsenic levels in wells near the Sutter Buttes. Efforts to further characterize and manage groundwater quality degradation due to arsenic in this area are underway and are being addressed through the Sutter Buttes Water Quality Working Group in collaboration with DWR. This Working Group, composed of GSA representatives, was proposed in the Butte Subbasin GSP.
An assessment of vegetation health conditions using Normalized Difference Vegetation Index (NDVI) data within GSP-mapped “Likely a GDE” regions was conducted over the 2020–2025 period. Results of this evaluation indicate that vegetation across most GDE regions was generally healthy throughout the evaluation cycle, indicating no effects on GDEs due to groundwater pumping.
Conjunctive Use of Surface and Groundwater Remains the Key to Sustainability To meet water demands, the subbasin relies primarily on surface water when supplies are available and supplements demands with groundwater during periods of reduced surface water availability. Surface water is crucial for maintaining groundwater sustainability in the Butte Subbasin.
Subbasin Water Supplies Continue to be Reliable
During drought and low‑flow years, limited surface water supply leads to increased groundwater pumping and reduced recharge, causing declines in groundwater storage, while wet years bring greater surface water use and more stable or increasing storage. Although curtailments have occurred during a few critical years in the past (e.g., 2015, 2021, and 2022), surface water supply in the Subbasin has generally been reliable over the long term. As a result, this variability is not expected to significantly alter overall water budget assumptions due to limits on curtailments under the Settlement Agreement; curtailment on the Feather River cannot exceed 50% in a given year or 100% in a seven-year consecutive period based on the Settlement Agreement.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-7 One state-wide program that may threaten the surface water supply reliability in the Subbasin in the future is a series of agreements to be approved by the State Water Resources Control Board (SWRCB) as an implementation pathway for an updated Bay-Delta Plan. Currently, the Healthy Rivers and Landscapes approach is the preferred alternative developed by the Sacramento Valley water rights holders. This approach would have little effect on recharge and groundwater levels, and ensure continued groundwater sustainability. However, if the unimpaired flow approach put forth by the State Board gets implemented, then reservoir levels will be impacted causing potential shortages for fall and future irrigation seasons. These impacts will likely affect subbasin recharge and sustainability overall.
Updated Water Budgets Provide a Refined Sustainable Yield Because curtailments on the Sacramento and Feather Rivers limited surface water supplies from 2020 to 2022, increased pumping in these dry and critical water years caused average groundwater extraction (198,100 acre-feet [AF]) to exceed the GSP’s current estimate during the evaluation period. As wetter conditions returned in 2023–2025, surface water availability significantly increased, reducing reliance on groundwater. Consequently, groundwater extraction declined to an average of 168,200 AF in those years.
The three projected water budgets in the GSP representing no climate change, 2030 climate change, and 2070 climate changes conditions remain unchanged for this PE. These projections continue to represent the future projected conditions based on the best available information at the time of this PE.
In the GSP, four sustainable yield values were calculated based on estimated groundwater extractions and groundwater storage change under current conditions and three projected water budget scenarios. DWR commented in the GSP determination letter that a single sustainable yield value would be preferred to better describe the management approach for the Subbasin (DWR, 2023a). To better reflect actual sustainable conditions based on observations from the most recent 10-year period, this PE updated the sustainable yield to 182,300 AF using groundwater extractions and storage change estimates from WY 2015 to 2024 from the Butte Basin Groundwater Model (BBGM). This 10-year period includes three years of surface water curtailment (2015, 2021, and 2022) and seven years of full surface water supplies. The resulting value is within the range of projected conditions from the GSP water budgets.
Refined Monitoring Networks Provide Robust Data Acquisition During this evaluation cycle, some of the Subbasin’s monitoring networks have changed through data gap filling and to address DWR’s RCAs.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-8 • Groundwater level and storage network assessment resulted in two wells being removed from the Primary Aquifer Network due to questionable measurements with the presence of oil or missing perforation data
• DWR-funded TSS well installation will fill data gaps in two phases: currently, five new wells were sited and are awaiting installation; in a second phase, four new wells are also planned to be installed. These wells will address shallow groundwater level monitoring for GDEs and ISW data gaps.
• The groundwater quality monitoring network assessment identified potential problematic wells that need to be further assessed during the next implementation phase. • The subsidence monitoring network was revised to include the most recent InSAR coverage from WY 2025 provided by DWR, as well as three existing extensometer locations.
GSAs Continue to Collaboratively Manage the Subbasin, Engage Stakeholders, and Interact with GSAs in Neighboring Subbasins
The 11 GSAs in the Butte Subbasin continue to collectively manage groundwater and comply with SGMA under one approved GSP. The GSA collaborative approach is described in a Cooperation Agreement1 that was approved by all GSAs in the fall of 2019. The GSAs have continued to coordinate on various actions and enforcement authorities since the GSP was adopted and submitted. No governance changes have been implemented, and the 11 GSAs continue to operate and implement the GSP per the governance structures established during GSP development.
During this evaluation period, the GSAs have made progress on developing and implementing a funding approach for GSP implementation activities.
The Butte Subbasin is actively engaged in a range of IBC activities that reflect both the hydrologic interconnectedness of the Northern Sacramento Valley's groundwater subbasins and the shared regional commitment to sustainable groundwater management under SGMA. The GSAs have participated in IBC and the Sutter Buttes Working Group, which are key activities completed by the GSAs during this evaluation cycle and supported by the descriptions below.
At the regional scale, the Northern Sacramento Valley Integrated Regional Water Management (NSV IRWM) program provides a standing forum for information sharing across the region. The Counties of Butte, Colusa, Glenn, Tehama, Shasta, and Sutter have entered into a Memorandum of Understanding to promote coordination, collaboration, and communication on 1 Cooperation Agreement Among Groundwater Sustainability Agencies in the Butte Subbasin Draft Butte Subbasin Periodic Evaluation, September 2026 Page ES-9 shared water resources. Together, these six counties developed and are implementing the NSV IRWM Plan, a collaborative framework that engages multiple agencies, stakeholders, tribes, and the public to identify regional water-related needs and advance projects and programs that provide multiple benefits across the region, including GSP implementation.
At the corridor scale, the Butte Subbasin is leading and participating in two structured inter-basin coordination efforts. The North Sacramento River Corridor (NSRC) Inter-Basin Coordination effort brings together GSAs from seven subbasins—Red Bluff, Antelope, Los Molinos, Corning, Vina, Colusa, and Butte—to advance coordinated groundwater management through the Periodic Evaluation process and into the next five-year implementation period. This effort builds on a foundational inter-basin coordination framework developed between 2020 and 2022, a completed technical joint GSP evaluation, and a collaboratively developed Framework Implementation Work Plan that are expected to be reflected consistently across all participating subbasins' Periodic Evaluations. A separate Sutter Buttes Water Quality Working Group coordinates across the Butte, Colusa, Sutter, North Yuba, South Yuba, and Yolo Subbasins to address water quality concerns arising from the unique volcanic geology of the Sutter Buttes area, with a focus on engaging DWR to improve basin characterization and regional groundwater quality monitoring.
In addition to these structured efforts, the Butte Subbasin continues to engage in ongoing neighbor-to-neighbor inter-basin coordination discussions with adjacent subbasins as needed, sharing relevant data and maintaining direct working relationships at shared boundaries. Taken together, these activities reflect a multi-layered approach to regional coordination, spanning bilateral boundary discussions, issue-specific working groups, corridor-wide facilitated collaboration, and valley-wide information sharing that positions the Butte Subbasin as an active and constructive partner in regional groundwater sustainability.
The GSAs have also collaborated with or are aware of water management efforts led by the three Counties overlying the Subbasin regarding the following:
1. County well ordinances and effects on the GSP implementation
2. County Drought Resilience Plans Update in compliance with SB 552
3. Relevant updates to General Plans since 2022
These activities that have been led by the Counties within the Subbasin take into consideration the groundwater sustainability management and implementation goals.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 1 1 INTRODUCTION
This Periodic Evaluation (PE) documents the Butte Subbasin (Subbasin) Groundwater Sustainability Agencies’ (GSAs) first Periodic Evaluation of the implementation of its approved Groundwater Sustainability Plan (GSP or Plan) (Butte Subbasin GSAs, 2021). The Periodic Evaluation covers the evaluation cycle from Water Year (WY) 2021 to 2025 (evaluation cycle).
The Periodic Evaluation fulfills the requirements of the Sustainable Groundwater Management Act (SGMA) and the California Department of Water Resources (DWR) GSP Regulations.
The purpose of the PE is to evaluate whether GSP implementation is on track to achieve groundwater sustainability in the 20-year GSP implementation period from 2022 through 2042.
The Periodic Evaluation is summarized in this written assessment, which closely follows DWR Guidance (DWR, 2023b). If the evaluation identifies that changes to the Plan are needed, then the GSAs can prepare a GSP Amendment to document any revisions to the original Plan. Based on the information evaluated and presented in this Periodic Evaluation, the Subbasin continues to show sustainable groundwater conditions and is likely to maintain sustainability with the continued good stewardship of water resources by the GSAs, in particular, emphasizing conjunctive use of surface water and groundwater for irrigation needs. Since the GSP still accurately reflects the GSAs’ plan to maintain groundwater sustainability, a GSP Amendment is not necessary at this time. The GSAs have adopted a Resolution to document and approve this approach with a few modifications to initial SMC, as discussed throughout this Periodic Evaluation (Appendix 11A).
There are 11 independent GSAs in the Butte Subbasin who collectively manage groundwater and comply with SGMA under one approved GSP (Figure 1-1). The GSA collaborative approach is described in a Cooperation Agreement that was approved by all GSAs in the fall of 2019. The GSAs have continued to coordinate on various actions and enforcement authorities since the GSP was adopted and submitted. No governance changes have been implemented, and the 11 GSAs continue to operate and implement the GSP per the governance structures established during GSP development.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 2 Figure 1-1. Groundwater Sustainability Agencies in the Butte Subbasin Draft Butte Subbasin Periodic Evaluation, September 2026 Page 3 2 NEW INFORMATION COLLECTED
During GSP implementation, new information was collected to fill data gaps, evaluate groundwater conditions, and conduct IBC efforts. Table 2-1 summarizes the new information collected since the approval of the initial Butte Subbasin GSP. This new information is supplementary and does not necessitate a GSP Amendment.
Table 2-1. Summary of New Information Since the Last Periodic Evaluation (Initial GSP) Significant New
Information Collected
General Description of New
Information Aspects of Plan Affected
Reference to Section in
Periodic Evaluation Where
Information Is Discussed
Land Subsidence Data
Use available InSAR data
provided by DWR and refine
Sustainable Management Criteria
(SMC) based on new InSAR data
Land Subsidence
Recommended Corrective
Actions (RCA 2) and
Monitoring Network
Sections 4.5 and 7.4
Well Inventory Data
Compile well construction
records for analysis and mapping
within the Butte Subbasin and
complete a Dry Well Vulnerability
Analysis to evaluate potential
impacts to beneficial users of
groundwater
Groundwater Level SMC
(RCA 1)
Section 4.1.3.1
Appendix 3A
Groundwater Level Data
Groundwater level data reported
by DWR in the Water Data
Library database
Groundwater Level SMC Section 4.1
Groundwater Level MT/MO
Recalculated
The groundwater level
Sustainability Indicator Minimum
Threshold (MT) and Measurable
Objectives (MO) metrics updated
as of WY 2022 using corrected
groundwater level measurement
data reported by the DWR on
2/17/23
Groundwater Level SMC
Section 4.1
Appendix 4B
Inter-Basin Coordination
Report – Joint Evaluation
of GSPs
IBC Analysis by the Vina and
Wyandotte Creek Subbasins
Water Budgets and
Coordination Agreement
Sections 6.3, 6.4, 9.3, and
10.1 Airborne Electromagnetic
(AEM) Data
AEM data and supporting
analysis by DWR and Dr. Todd
Greene to evaluate RMS wells in
relation to subsurface geology
and to address data gaps and
refine the hydrogeologic
conceptual model (HCM)
Basin Setting (HCM
Updates)
Section 6.1.1
Appendix 6A
Groundwater Dependent
Ecosystem (GDE) Data
Normalized Difference
Vegetation Index (NDVI) data to
characterize and refine locations
of GDEs
ISW Monitoring Networks
Basin Setting
Section 6.2.2
Section 7.1 and 7.2
Groundwater Model
(BBGM) Data
Incorporate updated model
scenario designations,
hydrogeological, and land use
updates to refine BBGM
Water Use Changes and
Water Budgets Sections 6.3 and 6.4
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 4 In its July 27, 2023, review and approval of the Butte Subbasin GSP, the Department of Water Resources identified items that were missing in the GSP (DWR, 2023). Table 2-2 summarizes each of these missing items identified by DWR and where they can be found in this Periodic Evaluation.
Table 2-2. Missing Information in Initial GSP
Additional Information Compiled for
the Periodic Evaluation
Missing Information Identified in the
GSP (DWR, 2023)
Reference to Section in Periodic
Evaluation Where Information Is
Discussed
Description of Plan Area Elements
Wellhead protection
Well abandonment/destruction
Well construction policies
Replenishment of groundwater
Conjunctive use
Efficient water management practices
Relationships with state and federal
agencies
Sections 8.2, 8.5, 8.2, 9.3, 8.5
Map of Known Contamination Sites and
Plumes
Migration of contaminated groundwater
was identified as a GSP element that
was missing. Specifically, a map
identifying the locations of known
groundwater contamination sites using
publicly available data from the
SWRCB’s GeoTracker website
Section 6.2.1
Refine SGMA Portal Monitoring Network
Module (Groundwater Levels)
Rectify inconsistencies in the number of
monitoring wells and principal aquifer
designations in the SGMA Portal
Monitoring Network Module
Section 7.5
Tabular summary of Water Quality
Monitoring Network
Provide a tabular summary of the
monitoring site type and measurement
frequency
Section 7.3.2
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 5 3 STATUS OF RECOMMENDED CORRECTIVE ACTIONS
In its July 27, 2023, review and approval of the Butte Subbasin GSP, DWR identified three RCAs to guide plan implementation (DWR, 2023a). The following summarizes each RCA and progress made to address them.
3.1 Recommended Corrective Action 1
From DWR’s review:
Revise the sustainable management criteria for the chronic lowering of groundwater levels. a. Provide more information about how the proposed minimum thresholds for the chronic lowering groundwater levels may impact beneficial uses and users. Specifically, consider the impact of the selected minimum threshold levels on supply wells. The consideration should identify the degree/extent of potential impact including the percentage, number, and location of potentially impacted wells at the proposed minimum thresholds for chronic lowering of groundwater levels.
b. Evaluate how the proposed minimum thresholds for the chronic lowering of groundwater levels may impact other sustainability indicators (e.g., groundwater storage, depletion of interconnected surface water, etc.).
To address this RCA, the location and depth of existing supply wells was compiled into a well inventory. The data was then reviewed to assess the number and locations of supply wells at risk of going dry if groundwater levels fall below the established minimum thresholds. Methods and findings from this analysis are provided in Appendix 3A and summarized in Section 4.1.3.1.
Additional review was conducted on minimum thresholds for interconnected surface water and subsidence compared to the groundwater values, in Sections 7.2.4 and 4.5.3, respectively.
3.2 Recommended Corrective Action 2
From DWR’s review:
Revise the sustainable management criteria for land subsidence. a. Provide a description of how the InSAR data will be incorporated into identifying undesirable results for land subsidence.
b. Describe how undesirable results will be evaluated since it is unclear at what frequency data will be collected from the Sacramento Valley GPS Subsidence Monitoring Network.
If data is not collected at a minimum of every 5-years from the network, another method should be utilized to ensure undesirable results are not occurring on the Subbasin.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 6 In addition, the DWR letter mentioned that “The minimum thresholds for inelastic land subsidence were selected to represent conditions that are just above conditions that could collectively generate undesirable results in the Subbasin, and the GSP acknowledges that the sensitivity of local infrastructure to land subsidence is not well understood.” As described in Sections 4.5.3 and 7.4, the RCA was addressed by replacing the subsidence RMS identified in the GSP with Interferometric Synthetic Aperture Radar (InSAR) and extensometers, evaluating SMC, and implementing a proactive adaptive management approach.
3.3 Recommended Corrective Action 3
From DWR’s review:
Department staff understand that estimating the location, quantity, and timing of stream depletion due to ongoing, Subbasin-wide pumping is a complex task and that developing suitable tools may take additional time; however, it is critical for the Department’s ongoing and future evaluations of whether GSP implementation is on track to achieve sustainable groundwater management. The Department plans to provide guidance on methods and approaches to evaluate the rate, timing, and volume of depletions of interconnected surface water and support for establishing specific sustainable management criteria in the near future. This guidance is intended to assist GSAs to sustainably manage depletions of interconnected surface water.
In addition, the GSAs should work to address the following items by the first periodic evaluation:
a. Consider utilizing the interconnected surface water guidance, as appropriate, when issued by the Department to establish quantifiable minimum thresholds, measurable objectives, and management actions.
b. Continue to fill data gaps, collect additional monitoring data, and implement the current strategy to manage depletions of interconnected surface water and define segments of interconnectivity and timing.
c. Prioritize collaborating and coordinating with local, state, and federal regulatory agencies as well as interested parties to better understand the full suite of beneficial uses and users that may be impacted by pumping induced surface water depletion within the GSAs’ jurisdictional area.
To address the RCA, GSA managers partnered with DWR’s Technical Support Services (TSS) program staff to install five new shallow TSS wells in areas identified as spatial data gaps within the ISW and groundwater level RMS networks in the original GSP. As TSS monitoring data Draft Butte Subbasin Periodic Evaluation, September 2026 Page 7 becomes available, information will be used to better characterize the magnitude and timing of stream depletions and inform the future refinement to the ISW SMC in alignment with DWR’s guidance. Mapping and characterization of GDEs’ health were completed to support evaluation of potential impacts of current groundwater conditions on beneficial uses and users (Section
6.2.2). Information on the filling of data gaps and new well installation are provided in Sections
7.1.1 and 7.2.1.
In addition, Butte Subbasin GSAs are coordinating with other neighboring GSAs in the Northern Sacramento Valley on best approaches to evaluate ISWs in streams at shared boundaries. GSA staff and their technical consultants have worked across subbasins to develop a Workplan for Regional Coordination and Collaboration to Address the Depletions of Interconnected Surface Water Sustainability Indicator in the Sacramento Valley (ISW Workplan). This ISW Workplan was shared with DWR staff and efforts are underway to develop an approach to implement this workplan over the next implementation period. More information on Inter-basin Coordination activities is provided in Section 10.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 8 4 GROUNDWATER CONDITIONS RELATIVE TO SUSTAINABLE
MANAGEMENT CRITERIA
SGMA and GSP Regulations require the use of SMC as metrics for demonstrating that a groundwater basin is being sustainably managed. This section presents the SMC definitions developed for the GSP, followed by an assessment of the current conditions and evaluation of changes, as needed, to continue to manage the subbasin sustainably while implementing the GSP. The current status of groundwater conditions relative to SMC is summarized in Table 4-1.
The GSAs established a qualitative sustainability goal in the GSP to define locally sustainable conditions. Per the GSP, “The sustainability goal for the Butte Subbasin is to maintain, through a cooperative and partnered approach, locally managed sustainable groundwater resources to preserve and enhance the economic viability, social well‐being, and culture of all beneficial uses and users without experiencing undesirable results.” The sustainability goal is supported by locally defined quantitative sustainability metrics, or SMC, including undesirable results (UR), minimum thresholds (MT), measurable objectives (MO), and interim milestones (IM). This periodic evaluation demonstrates the absence of undesirable results throughout the evaluation period, which shows that the Subbasin continues to be managed sustainably. There are five applicable sustainability indicators in the Butte Subbasin:
Chronic lowering of groundwater levels
Depletion of interconnected surface water
Reduction in groundwater storage
Degraded groundwater quality
Land subsidence
Seawater intrusion is not an applicable sustainability indicator as Butte Subbasin is not located near the coast.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 9 Table 4-1. Current Groundwater Conditions Relative to Sustainable Management Criteria as Defined in the GSP Sustainability
Indicator Evaluation Status MT Definition Undesirable Result Identification MO Definition Chronic Lowering
of Groundwater
Levels and
Reduction of
Groundwater
Storage
No indication of undesirable results
There were no groundwater level
measurements below the MT between
WY 2021 and 2025
For each RMS well, the shallower
of either:
i. the shallowest 7th
percentile of nearby
domestic wells, or
ii. the range of measured
groundwater levels, or 20
feet (whichever is
greater), below the
observed historic low.
If the resulting value is shallower
than the observed historic low, the
MT is set 10 feet deeper than the
observed historic low.
When 25% of each aquifer’s
RMS wells fall below their MTs
for 24 consecutive months (11
of 41 wells in the Primary
Aquifer and 3 of 10 in the Very
Deep Aquifer)
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well
Depletion of
Interconnected
Surface Water
No indication of undesirable results
There are no groundwater level
measurements below the MT between
WY 2021 and 2025
10 feet below the measured
historical low groundwater
elevation at each RMS well
When 25% of the RMS wells (3
of 12) fall below their MTs for
24 consecutive months
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well
Degraded Water
Quality
No indication of undesirable results
One well routinely exceeds the MT due
to naturally occurring salinity. Otherwise,
there are no MT exceedances between
WY 2021 and 2025
The higher of either 900 μS/cm or
the measured historical high of EC
for each RMS well
When 25% of RMS wells
(2 of 8) exceed their MT for 24
consecutive months
Measured EC less than or equal to
700 μS/cm at each RMS well
Land Subsidence
[This has been
revised since a
different
monitoring
network is being
used to monitor
subsidence, as
discussed in
Section 4.5.3.]
No indication of undesirable results
There were no subsidence monitoring
sites with a subsidence rate greater than
the MT between WY 2021 and 202.
0.5 foot of subsidence over a 5-
year period at each site
When 25% of DWR’s
monitoring network (8 of 31
benchmarks) measure a
subsidence rate greater than
the MT
Up to 0.25 foot of subsidence per
5-year period at each site
(0.05 foot over one year; 1 foot
over 20 years)
μS/cm = microsiemens per centimeter
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 10
4.1 Chronic Lowering of Groundwater Levels
Groundwater levels are stable over time in most areas of the Subbasin, however there are some wells that demonstrate more variability and lower levels temporarily during drought years when groundwater is used to compensate for reduced surface water supplies. Groundwater levels typically rebound during wet years when surface water is available for irrigation. Groundwater levels are usually highest in spring and lowest during the irrigation season in summer. Fall groundwater measurements are typically collected after the primary irrigation season and usually before winter flood-up for rice decomposition and wetland habitat. In rice growing areas, summer groundwater levels can be relatively high compared to spring and fall levels due to field flooding using surface water supplies. Hydrographs showing historical groundwater levels at each RMS relative to the MT and MO are provided in Appendix 4A.
In most wells, groundwater levels are near the ground surface and do not fluctuate much over time. In many areas of the Subbasin the groundwater level remains within 20 feet below ground surface (bgs) year-round. The area on the boundary of the Butte and Vina Subbasins has orchards that are irrigated with groundwater and thus has more groundwater level fluctuation than surface water irrigated rice growing areas. Some wells along the Vina Subbasin boundary (Appendix 4A), have relatively deeper groundwater levels, fluctuating up to 20 feet between highs in spring and lows in fall, and in recent dry years falling to 60 feet bgs. These wells also demonstrate a gradual long-term declining groundwater level trend of about 1 foot per year since the early 1990s when the monitoring records began. Of note, the mid-90s had five consecutive wet years, and the last decade has included two of the worst droughts on record. Some of this sustained decline is likely due to climate variability. More information on current groundwater levels conditions is provided in Section 4.1.2.
Groundwater levels in the Subbasin are monitored at RMS wells described in detail in Section
7.1. 4.1.1 Chronic Lowering of Groundwater Levels SMC Defined in the 2022 GSP Groundwater level SMC are established to protect beneficial uses and users of groundwater. The SMC are defined as follows:
• Minimum Threshold (MT): For each RMS well, the shallower of either:
i. the shallowest 7th percentile of nearby domestic wells, or ii. the range of measured groundwater levels, or 20 feet (whichever is greater), below the observed historic low.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 11 If the resulting value is shallower than the observed historical low, the MT is set 10 feet deeper than the observed historical low.
• Undesirable Results (UR): When 25% of either Principal Aquifer’s RMS wells fall below their MTs for 24 consecutive months (11 of 41 wells in the Primary Aquifer or 3 of 10 in the Very Deep Aquifer).
• Measurable Objective (MO): Static 5-year average (generally 2012-2017) of measured groundwater level data for each RMS well.
Groundwater level measurements collected in spring and fall are compared to the MO and MT to evaluate sustainability. Since the MO is calculated based on a five-year average of all values, a well with fall measurements slightly below the MO and spring measurements above the MO demonstrates sustainable conditions. The GSP describes sustained groundwater level conditions below the MT for two consecutive years as constituting an undesirable result. Since spring groundwater level conditions are generally higher than fall conditions, spring groundwater levels observed below the MT for two consecutive years would likely trigger an undesirable result.
The MOs and MTs were recently recalculated in the WY 2023 Annual Report (Butte Subbasin GSAs, 2024) based on updated groundwater level data, which differ from the information (i.e., groundwater level data) used to calculate the original MOs/MTs shown in the GSP. The rationale and methodology used to calculate MOs/MTs described in the GSP did not change. Revised MO/MT values are generally higher elevations, or shallower groundwater levels, than documented in the GSP, and are therefore are more conservative metrics and protective of beneficial users, consistent with the intent of the GSP. The revised values were used for this evaluation and WY 2023, 2024, and 2025 Annual Reports. Additional information on the MO/MT recalculation can be found in Section 7.1.4 and Appendix 4B.
4.1.2 Current Chronic Lowering of Groundwater Levels Conditions Relative to SMC Current groundwater level conditions in the Subbasin are generally stable with some seasonal and annual variability with climate. Hydrographs and a summary table in Appendix 4A show all historical data collected at RMS relative to MTs and MOs.
Box and whisker plots are used to summarize spring and fall groundwater levels in RMS relative to the MO or MT. The MO/MT is at 0 on the chart, and the difference between the groundwater level and the MO/MT is plotted as a dot along a vertical line. A negative value means the measurement was below the MO/MT and a positive value means the measurement was above the MO/MT. The “box” on the plots denotes the range of values within ±1 standard deviation of the mean (average of all values), with the median represented by a line within the box. The “whiskers” extend to ±2 standard deviations, illustrating the overall spread and variability of the statistically significant dataset. The spring groundwater elevation from WY 2021-2025 is plotted Draft Butte Subbasin Periodic Evaluation, September 2026 Page 12 relative to the MO on Figure 4-1 and Figure 4-2 for the Primary and Very Deep Aquifer, respectively. Fall groundwater elevations are plotted relative to the MO on Figure 4-3 and Figure 4-4 and MT on Figure 4-5 and Figure 4-6, for the Primary and Very Deep Aquifer, respectively. Fall levels are generally lower than spring levels, which provides an indication of seasonal low conditions in relation to the MT, rather than seasonal highs.
Annual groundwater elevations generally meet or exceed the MO (Figure 4-1 through Figure 4-4). Temporary declining groundwater levels caused by increased groundwater extractions to offset curtailed surface water supplies is observed in dry years. In WYs 2021 and 2022, which were dry, average groundwater levels were close to the MO in the spring and approximately 5 to 10 feet below the MO in the fall. Groundwater levels generally recover after the period of increased groundwater extractions has ended due to wetter years. In WYs 2023 to 2025, which were wetter, average groundwater levels were about 5 feet above the MO in spring and close to the MO in fall. Maintaining groundwater levels near the MO provides operational flexibility to manage surface water shortages or other unexpected water supply challenges during GSP implementation.
Groundwater levels remained above the MT throughout the evaluation period. Seasonal low groundwater elevations in the fall are 10 to 94 feet above the MT in the Primary Aquifer (Figure 4-5) and 16 to 51 feet above the MT in the Very Deep Aquifers (Figure 4-6).
Groundwater elevations fluctuate more in some wells with lows in dry years closer to the MT than in wet years. There are two wells in particular in the Primary Aquifer network (20N02E15H001M and 21N01E08K002M; Appendix 4A) on the northern boundary with the Vina Subbasin that demonstrate a slight long-term declining trend of about 1 foot per year, but these wells still remain about 20 to 50 feet above the MT at their lowest levels in Fall 2022.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 13 Figure 4-1. Primary Aquifer Spring Groundwater Elevations Relative to Measurable Objective for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) Figure 4-2. Very Deep Aquifer Spring Groundwater Elevations Relative to Measurable Objective for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) Figure 4-3. Primary Aquifer Fall Groundwater Elevations Relative to Measurable Objectives for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) Draft Butte Subbasin Periodic Evaluation, September 2026 Page 14 Figure 4-4. Very Deep Aquifer Fall Groundwater Elevations Relative to Measurable Objectives for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) Figure 4-5. Primary Aquifer Fall Groundwater Elevations Relative to Minimum Thresholds for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025) Draft Butte Subbasin Periodic Evaluation, September 2026 Page 15 Figure 4-6. Very Deep Aquifer Fall Groundwater Elevations Relative to Minimum Thresholds for Chronic Lowering of Groundwater Level Representative Monitoring Points (2021-2025)
4.1.3 Evaluation of Chronic Lowering of Groundwater Levels SMC The chronic lowering of groundwater levels SMC were evaluated and no changes to the SMC are currently necessary. A dry well vulnerability assessment shows that some wells are at risk of going dry if groundwater levels lower to the MT. To adaptively manage groundwater conditions in the Subbasin, the GSAs have established trigger levels at historical spring minimum levels that will prompt an investigation into the causes and potential solutions or mitigations, as needed, to lowering of groundwater levels before the MT is reached. The vulnerability assessment and trigger levels are described in the following subsections.
4.1.3.1 Dry Well Vulnerability Analysis
A dry well vulnerability analysis was completed to better demonstrate the relationship between the groundwater level MT and beneficial users and uses of groundwater in the Subbasin. The analysis addresses DWR’s RCA #1, part a.:
Provide more information about how the proposed minimum thresholds for the chronic lowering groundwater levels may impact beneficial uses and users. Specifically, consider the impact of the selected minimum threshold levels on supply wells. The consideration should identify the degree/extent of potential impact including the percentage, number, and location of potentially impacted wells at the proposed minimum thresholds for chronic lowering of groundwater levels.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 16 The objective of the analysis is to identify existing vulnerable supply wells that may go dry at the groundwater level MT. The refined well inventory (Appendix 3A) is used to identify supply well locations and depths in the Subbasin. The groundwater elevation MTs at individual RMS (Section 7.1) were interpolated to create a continuous groundwater elevation MT surface across the Subbasin. The analysis was limited to MTs for wells completed in the primary aquifer, omitting the Very Deep Aquifer MTs, to better reflect conditions potentially affecting shallow supply wells. The groundwater elevation MT surface was converted to depth to groundwater using the ground surface elevation topographic surface.
The analysis evaluates the number and type of supply wells that may be vulnerable to going dry if groundwater levels decline to the MT. To account for uncertainty and to reflect the need to maintain groundwater levels above well pump intakes, a uniform 25-foot buffer was applied to the interpolated depth to MT surface. The resulting buffered depth to MT raster was combined with the well inventory supply well data to evaluate the number and percentage of wells at risk of losing access to groundwater under MT groundwater level conditions. The analysis excludes wells with missing depth and well screen interval information. Results are summarized separately for wells completed deeper than 50 feet (Table 4-2) and wells completed less than 50 feet (Table 4-3), reflecting the substantial differences in vulnerability between deeper and shallow well constructions. Wells older and newer than 1980 are also summarized in tables and discussed separately, because wells installed prior to 1980 are probably close to or beyond the expected lifespan of typical well casing materials.
Figure 4-7 shows the location of supply wells that might lose access to groundwater if groundwater levels reach the MT. The majority of at-risk wells are concentrated in the southeastern part of the Subbasin, near the cities of Biggs and Gridley. Many of these wells may have already been taken out of service and parcels added to the municipal water system. Another area with wells at risk is in the Glenn County portion of the Subbasin, where more groundwater pumping occurs as surface water is not available in this area.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 17 Figure 4-7. Location of Supply Wells that Could Lose Access to Groundwater if Water Levels Reach MT Draft Butte Subbasin Periodic Evaluation, September 2026 Page 18 Table 4-2. Summary of Wells Completed Deeper than 50 Feet At-risk to Be Dry Under MT Conditions Well Type Total Well
Count
Wells Dry at MT
with Safety Factor
of 25 feet
Number of Wells
Installed Since 1980
Wells Installed Since 1980 and
Dry at MT with Safety Factor
of 25 feet
Domestic 1,187 190 432 18
Industrial 39 6 16 0
Irrigation 1,004 25 450 2
Public Supply 21 0 11 0
Unknown 123 21 20 5
TOTAL 2,374 242 931 25
A total of 2,374 wells with completed depths greater than or equal to 50 feet are identified in the well inventory. Of these, 242 wells (10%) are projected to be dry at the MT when applying the 25-foot safety factor. Domestic wells represent the largest portion of the inventory and account for most of the vulnerable wells. Of the 1,187 domestic wells, 190 wells (16%) are projected to be dry at MT. Irrigation wells make up the second largest portion of the inventory with 1,004 wells deeper than 50 feet. However, irrigation wells are less vulnerable to lowering groundwater levels, with 25 wells (< 3%) projected to be dry, due to their tendency for deeper installation depths. Of the 39 industrial wells deeper than 50 feet, six wells (15%) are projected to be dry under MT conditions. There are no public supply wells projected to be dry at the MT.
When considering only wells installed since 1980, there are far fewer wells at risk of being dry at the MT due to deeper well construction depths following the 1977-1978 drought. There are 931 supply wells deeper than 50 feet installed in the Subbasin since 1980. Of these, 25 wells (< 3%) are projected to be dry at the MT. Nearly all deeper wells installed since 1980 that are projected to be dry are domestic or unknown well types, with only two irrigation wells and no public supply or industrial wells projected to be affected.
A total of 245 wells with completed depths of less than 50 feet were evaluated for dry well vulnerability. Under MT conditions with the 25-foot safety factor applied, all evaluated shallow wells are projected to be dry (Table 4-3). The supply wells less than 50 feet deep are dominated by domestic and unknown well types, including 95 domestic wells and 135 wells of unknown type. The shallow supply wells in the inventory include 84 wells installed since 1980. Per DWR well construction standards, no public supply wells are shallower than 50 feet.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 19 Table 4-3. Summary of Wells Completed Less than 50 Feet Projected to Be Dry Under MT Conditions Well Type Total Well Count
Wells Dry at MT with
Safety Factor of 25
feet
Number of Wells
Installed Since 1980
Wells Installed Since
1980 and Dry at MT
with Safety Factor of
25 feet
Domestic 95 95 6 6
Industrial 1 1 0 0
Irrigation 14 14 2 2
Public Supply 0 0 0 0
Unknown 135 135 76 76
TOTAL 245 245 84 84
Overall, the dry well vulnerability analysis shows that there are some shallow wells, particularly installed prior to 1980, that are at risk of going dry if groundwater levels decline to the MT.
These results indicate that projected impacts of groundwater level declines would be localized and depth dependent and do not represent a basin-wide risk to public supply or deeper production wells.
To validate this analysis, reported dry well information was reviewed to evaluate observed conditions within the Subbasin. A review of the DWR Dry Well Reporting System and publicly available county level dashboards indicates that no dry wells have been reported within the Butte Subbasin since January 1, 2014. While reported dry well data are based on voluntary submissions and may not capture all localized well impacts, the absence of reported dry wells within the Subbasin is consistent with groundwater level observations since 2014. Additionally, since no dry wells were reported when groundwater levels declined to historical low levels in 2022, some of the older, shallower wells in the inventory may no longer be in use.
Finally, this analysis of supply well vulnerability at the MT assumes that groundwater levels decline to the MT, which has not occurred since GSP implementation began in 2022 (Section
4.1.2). In general, groundwater levels are close to the MO and significantly higher than the MT, with temporary declines in the driest years such as 2014-2015 and 2021-2022. (even then maintaining levels well above the MT). Areas where rice is cultivated with surface water irrigation have very little groundwater level fluctuation, except when surface water availability is curtailed. Areas with crops that mainly use groundwater for irrigation demonstrate more pronounced seasonal and annual groundwater level change and some localized long-term declining groundwater level trends are noted along the Butte Subbasin’s northern boundary with the Vina Subbasin (RMPs 20N02E15H001M and 21N01E08K002M; Appendix 4A). Despite some inherent risk of shallower and older supply wells going dry during droughts, groundwater Draft Butte Subbasin Periodic Evaluation, September 2026 Page 20 levels have not approached the MT. The GSAs’ intend to maintain groundwater levels at the MO to provide operational flexibility during future droughts.
4.1.3.2 Tracking Conditions Relative to Historical Lows Potential impacts to shallow supply wells in some areas could occur if groundwater levels decline to the MT. New historically low spring groundwater levels observed in the future may prompt the GSAs to investigate whether shallow supply wells may be at risk of going dry due to lowering of groundwater levels. The investigation could entail reaching out to well owners in the at-risk areas and verifying if the wells are still in use and/or if an alternative water supply is available. Assisting state small water systems and domestic wells in drought periods is a component of county Drought Resilience Plans recently developed and/or updated in Butte, Colusa, and Glenn Counties per Senate Bill 552.
4.1.3.3 Groundwater Level SMC Evaluation Summary
Overall, groundwater levels are being managed in the operational range near the MO and far above the MT. Therefore, the groundwater level SMCs are being achieved, and groundwater levels are being managed sustainably. No changes are necessary for managing the chronic lowering of groundwater levels SMC; however, the GSAs recognize that tracking conditions relative to historical lows should help identify declining groundwater level trends before MT exceedances occur and thereby prompting appropriate action to avoid undesirable results.
4.2 Depletion of Interconnected Surface Water
The GSP states, “while information and understanding of interconnected surface waters is limited, groundwater levels that exceed the minimum threshold in the future for an extended period of time could impact beneficial uses of interconnected surface waters by reducing the volume and changing the timing of surface water availability, and potentially impacting the beneficial uses of groundwater by dewatering domestic wells and limiting groundwater supplies to groundwater dependent ecosystems. As additional data are collected during GSP implementation, minimum thresholds may change and the threshold calculations revised to reflect a better understanding of this complex interaction and the Subbasin’s unique conditions.” The GSAs continue to manage shallow groundwater levels as a proxy to prevent depletion of interconnected surface water from impacting beneficial uses and users of surface water and groundwater. Monitoring networks are being expanded to include more shallow wells for ISW evaluation, and DWR developed additional guidance for managing ISW depletion that will also be incorporated in future evaluations.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 21
4.2.1 Depletion of Interconnected Surface Water SMC Defined in the 2022 GSP Interconnected surface water SMC are established using groundwater levels as a proxy with a dedicated ISW RMS monitoring well network, which is a subset of the groundwater level RMS monitoring network (see Section 7). The interconnected surface water SMC include the following:
• Minimum thresholds (MT): 10 feet below the measured historical low level at each RMS well
• Undesirable results (UR): When 25% of RMS fall below their MTs for 24 consecutive months (3 of 12 RMS wells).
• Measurable Objectives (MO): Static 5-year average (generally 2012-2017) of measured groundwater level data for each RMS well.
The MO for depletion of interconnected surface water is the same as the chronic lowering of groundwater levels (Section 4.1). However, the ISW MTs are set using a different method. The ISW MT is 10 feet lower than the measured historical low groundwater level for each ISW RMS well. This approach provides operational flexibility for managing different conditions in the future. The ISW MTs are 9 to 14 feet shallower than the chronic lowering of groundwater level MTs in all but one RMS, 18N03E21G001M, which has a 2 foot deeper ISW MT than the groundwater level MT. A comparison of ISW and groundwater level monitoring network MT values are tabulated and discussed further in Section 7.2.4.
4.2.2 Current Depletion of Interconnected Surface Water Conditions Relative to SMC Spring and fall groundwater level measurements in the ISW RMS wells were compared to the MO/MT using box and whisker plots, with the method described in Section 4.1.1. The spring and fall groundwater elevations are plotted relative to the MO on Figure 4-8 and Figure 4-9, respectively. The fall groundwater elevation is plotted relative to the MT on Figure 4-10.
Groundwater levels were typically around the MO, and are therefore at levels that provide operational flexibility to protect beneficial users of groundwater and surface water during droughts or other changing conditions. Spring levels were above the MO except for the critically dry years in WY 2021-2022. As expected, seasonal low fall levels were slightly below the MO in many wells, since the MO is an average groundwater level between 2012-2017.
Groundwater levels were maintained more than 10 feet above the MTs during the evaluation period, which is protective of beneficial users of groundwater and surface water. On average, fall groundwater elevations were 17 feet above the MTs with higher elevations measured in the normal and wet years of 2024 and 2025, respectively.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 22 Figure 4-8. ISW Spring Groundwater Elevations Relative to MO Figure 4-9. ISW Fall Groundwater Elevations Relative to MO Draft Butte Subbasin Periodic Evaluation, September 2026 Page 23 Figure 4-10. ISW Fall Groundwater Elevations Relative to MT
4.2.3 Evaluation of Depletion of Interconnected Surface Water SMC Maintaining groundwater levels near the MO provides operational flexibility to manage changing conditions in the Subbasin and prevent ISW depletion. No changes are needed for the ISW SMC at this time, pending a more detailed review of the DWR guidance when it is published, and coordination with other GSAs in the Northern Sacramento Valley as discussed in Section 10.1.
4.3 Reduction of Groundwater in Storage
The reduction of groundwater in storage is evaluated using the chronic lowering of groundwater levels RMS network and SMC as a proxy. A review of the RMS well hydrographs (Appendix 4A) indicates that groundwater elevations are relatively stable over time. Groundwater level declines in some wells may be influenced by the significant percentage of water years since 2006 that have been dry (i.e., characterized as below normal, dry, or critical).
Changes in groundwater storage in large portions of the Subbasin remain stable in most years due to high surface water availability and usage. In contrast, other portions of the Subbasin without surface water supplies follow a more variable seasonal and long-term groundwater level pattern typically seen in the majority of the Sacramento Valley. For portions of the Subbasin without surface water supplies, in normal to wet years, groundwater is withdrawn during the summer for irrigation and replenished during the winter through recharge of precipitation and Draft Butte Subbasin Periodic Evaluation, September 2026 Page 24 surface water inflows, allowing groundwater storage to potentially rebound by the following spring. During dry years and drought conditions, this pattern is disrupted when more groundwater may be pumped to meet irrigation demand, and less recharge may occur due to reduced precipitation, diminished or curtailed surface water supplies, and lower stream levels.
During dry years and severe drought conditions, when surface water supplies are cut back, there is typically a combination of increased groundwater pumping and land fallowing (e.g., not planting a percentage of rice fields) to account for reduced surface water supply availability.
Additional information related to reduction of groundwater in storage at the Subbasin level and the sustainable yield is provided in Section 6.3.5.
4.4 Degraded Groundwater Quality
The degraded water quality SMC are established to protect beneficial uses and users of groundwater from water with relatively higher salinity known to exist in areas of the subbasin.
Specifically, the area near the Sutter Buttes is known to have naturally higher salinity concentrations which historically factored into the land and water uses in this area of the subbasin. Salinity is the main constituent of concern (COC) in the subbasin and is evaluated using electrical conductivity (EC) data collected at eight RMS.
4.4.1 Degraded Groundwater Quality SMC Defined in the 2022 GSP The SMC for degraded water quality in the GSP include the following:
• Minimum thresholds (MT): The higher of either 900 microsiemens per centimeter (μS/cm) or the measured historical high of EC for each RMS well • Undesirable results (UR): When 25% of RMS wells exceed their MT for 24 consecutive months (two of eight wells)
• Measurable Objectives (MO): Measured EC less than or equal to 700 μS/cm at each RMS well
4.4.2 Current Degraded Groundwater Quality Relative to SMC There are no undesirable results in the Subbasin for degraded water quality. Figure 4-11 shows the location of water quality RMS wells and their 2025 EC results. One RMS well 17N01W10A001M in the southern portion of the Subbasin near the Sutter Buttes has naturally elevated EC concentrations that routinely are close to or slightly exceed the MT but do not demonstrate a long-term increasing or decreasing trend. The MT at this well is set at the Draft Butte Subbasin Periodic Evaluation, September 2026 Page 25 maximum EC concentration prior to GSP implementation, so exceeding this value is expected as conditions fluctuate around the historical maximum concentration over time. No other wells exceeded their MTs for water quality in the evaluation period. Irrigation wells screened in the Very Deep Aquifer near the Sutter Buttes are at risk of elevated salinity levels, while shallower wells screened in the Primary Aquifer in the same area do not experience the same risk. A summary of groundwater quality monitoring data is available in Appendix 4C and discussed further in Section 7.3.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 26 Figure 4-11. 2025 EC Data for Butte Subbasin RMS Wells
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 27
4.4.3 Evaluation of Degraded Groundwater Quality SMC
Groundwater quality in the Subbasin is generally suitable for all beneficial uses and users.
However, there are areas with naturally occurring poorer quality water near the Sutter Buttes, a known area of volcanic geologic formations. This area has naturally occurring salinity accumulation due to groundwater flow impediment caused by the Sutter Buttes. Water quality challenges in this area are being addressed with regional coordination by the Sutter Buttes Water Quality Working Group, as discussed in Section 10.1.2. Since these water quality issues pre-date SGMA and are a naturally occurring condition, they are not considered water quality degradation due to groundwater management by the GSAs. GSAs are aware of these water quality conditions, as described in Section 6.2.1, and will continue to track conditions to prevent beneficial user impacts. No changes are currently needed for the degraded groundwater quality SMC.
4.5 Land Subsidence
Land subsidence SMC are designed to prevent significant and unreasonable impacts to critical infrastructure and aquifer storage capacity in the Subbasin. Per the GSP, “Inelastic land subsidence in areas of active groundwater extraction has the potential to be of major concern due to infrastructure damage, permanent reduction in the storage capacity of an aquifer, well casing collapse, and increased flood risk in low lying areas.” The GSP also states that “available data indicate that inelastic land subsidence due to groundwater withdrawal has not been observed in the Butte Subbasin.” During the evaluation period, little to no subsidence was observed because groundwater levels are generally stable during wet and normal water year types and recover quickly following short-term declines during dry years. The GSAs are not aware of any infrastructure impacts in the Subbasin caused by subsidence, particularly since there is no indication of measurable subsidence occurring at any time in the historical record.
4.5.1 Land Subsidence SMC Defined in the 2022 GSP
The GSP established land subsidence SMC to prevent significant and unreasonable impacts to critical infrastructure. The SMC in the 2022 GSP include the following:
• Minimum thresholds (MT): 0.5 foot of subsidence over a 5-year period • Undesirable results (UR): When 25% of subsidence RMS (8 of 31 benchmarks) measure a subsidence rate greater than the MT of 0.5 foot per five years • Measurable Objectives (MO): 0.25 foot of subsidence over a 5-year period Draft Butte Subbasin Periodic Evaluation, September 2026 Page 28 There are no IMs defined in the GSP since statistically significant subsidence is not observed; this approach is consistent with DWR guidance for subbasins without observed subsidence.
Essentially, the IM equals the MO.
The GSP identifies the Sacramento Valley Global Positioning System (GPS) Subsidence Monitoring Network as the RMS to evaluate cumulative subsidence, however this network has not been monitored since 2017. The GSAs have been using InSAR data provided by DWR to evaluate subsidence for GSP Annual Reports. The InSAR data is supplemented with three DWR extensometers installed in Very Deep Aquifer monitoring wells. The revised RMS for land subsidence are discussed further in Section 7.4.
4.5.2 Current Land Subsidence Conditions Relative to SMC Significant inelastic land subsidence did not occur in the Subbasin during the evaluation period.
Subsidence measured by InSAR and extensometers from October 2020 to October 2025 ranged from 0.049 foot of potentially elastic subsidence to 0.143 foot of uplift as reported in the WY 2025 Annual Report from available DWR dataset (Figure 4-12). Cumulative reported subsidence during the evaluation period is less than the InSAR measurement error of 0.1 foot, MO of 0.25 foot, and MT of 0.5 foot. Therefore, there are no significant and unreasonable impacts or undesirable results for land subsidence occurring in the Subbasin.
In WY 2025, new data filtering and interpolation methods expanded the InSAR dataset to areas of the Subbasin that had not previously been reported by DWR. The WY 2025, results ranged from 0.15 foot of subsidence to 0.094 foot of uplift (Figure 4-13). The maximum subsidence (0.15 foot) is less than the MO (0.25 foot over five years) but also includes a few values greater than the measurement error (0.1 foot) in isolated areas. Annual or cumulative subsidence measurements greater than 0.1 foot have not been observed in prior years. There are three discrete 2.5-acre pixels with measurable subsidence reported in WY 2025, shown as dark yellow colors on Figure 4-13. These are isolated measurements that could be anomalies that will be tracked as more data is collected for future Annual Reports.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 29 Figure 4-12. Cumulative Land Subsidence, WY 2020-2025 (from WY 2025 Annual Report) Draft Butte Subbasin Periodic Evaluation, September 2026 Page 30 Figure 4-13. Annual Land Subsidence for WY 2025 and Conveyance Canal Infrastructure Draft Butte Subbasin Periodic Evaluation, September 2026 Page 31 Extensometers provide high resolution groundwater level and clay compaction data to help understand the inter-relationship between subsidence and groundwater elevations. Little to no subsidence has been measured by the Subbasin extensometers since 2004 ((Figure 4-14 through Figure 4-16). Subsidence that occurs in dry seasons is elastic as it rebounds in wet seasons. For example, extensometer 19N01E35B002M (Figure 4-15) measured 0.03 foot (0.36 inches) of subsidence in Fall 2015 and Fall 2022 when groundwater levels in the accompanying Very Deep Aquifer well decreased by about 20 feet. By Fall 2017 and Fall 2024, groundwater levels recovered to pre-drought conditions, and the land surface rebounded elastically. Similar trends are observed between groundwater levels and subsidence at the other extensometer locations, although the magnitude of land surface change is smaller at 20N01E18L001M (Figure 4-16). It should be noted that the vertical scale on the extensometer graphs is less than 0.1 foot, representing very small (if not negligible) changes in land surface elevation; this amount is also the measurement error of the InSAR data. Even at this small scale, it is possible to see the elastic response corresponding to groundwater level changes.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 32 Figure 4-14. Land Subsidence and Groundwater Elevation at Extensometer 18N01E35L001M Draft Butte Subbasin Periodic Evaluation, September 2026 Page 33 Figure 4-15. Land Subsidence and Groundwater Elevation at Extensometer 19N01E35B002M Draft Butte Subbasin Periodic Evaluation, September 2026 Page 34 Figure 4-16. Land Subsidence and Groundwater Elevation at Extensometer 20N01E18L001M Draft Butte Subbasin Periodic Evaluation, September 2026 Page 35
4.5.3 Evaluation of Land Subsidence SMC
The land subsidence SMC are being achieved as there is no measurable inelastic subsidence occurring in the Subbasin. However, some changes to the SMC and monitoring network are needed to address RCA #2, align the management approach with DWR’s 2026 Subsidence Best Management Practice (Subsidence BMP; DWR, 2026), and proactively establish an adaptive management approach in case subsidence occurs in the future.
4.5.3.1 Revised Monitoring Network
As discussed in Section 7.4, InSAR and extensometers are the revised RMS network because the planned GPS benchmark network is no longer being monitored. The SMC approach for the original RMS network is transferable to the new RMS network.
4.5.3.2 Alignment with the Subsidence BMP
The GSA’s subsidence management approach was reviewed for alignment with the Subsidence BMP (DWR, 2026). The BMP classifies and provides specific recommendations for basins that fall into four different categories related to the historical amount of subsidence and groundwater level minimum thresholds (Figure 4-17). The Butte Subbasin is a “Scenario 2” basin because there has been no historical subsidence, but groundwater level minimum thresholds are below historical low levels. If groundwater levels were to decline to the minimum threshold over a prolonged period of time, the reduction in pressure in subsurface clays could theoretically cause subsidence. Other specific text in the BMP relevant to the Subbasin provided in the section below focuses on the subsidence monitoring approach, critical infrastructure analysis, and SMC identification. Key excerpts from the BMP are provided in italics followed by an explanation of how potential subsidence is being managed in the Subbasin.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 36 Figure 4-17. Subsidence BMP Management Scenarios (DWR, 2026) Subsidence Monitoring
BMP: The land surface monitoring network should be capable of identifying the spatial extent and magnitude of land subsidence, including the ability to evaluate temporal changes and efficacy of management strategies.
The revised subsidence monitoring network collects data at sufficient frequency and spatial coverage to effectively identify and address subsidence issues if they occur. Subsidence monitoring is performed near the majority of the critical infrastructure that could be impacted by subsidence including conveyance canals, major highways, and communities with supply wells and utilities, as shown on Figure 4-13. Data is available quarterly from DWR but reviewed annually after fall groundwater level measurements are collected to prepare GSP Annual Reports. Subsidence monitoring is concentrated more in the areas near the margins of the Subbasin that rely on groundwater pumping for water supply than in the center of the Subbasin where surface water supplies are sufficient. Therefore, despite some monitoring gaps, subsidence Draft Butte Subbasin Periodic Evaluation, September 2026 Page 37 data is being collected in areas of the Subbasin that are more likely at risk due to the increased potential for lowering of groundwater levels.
Critical Infrastructure
BMP: An essential part of subsidence management under SGMA is the review and identification of infrastructure within a basin as well as determining the amount of subsidence that may interfere with these surface land uses. Investigations should broadly encompass any infrastructure, land use, and property interest (current and future) that may be impaired by changes in land elevation and areas around them where groundwater pumping may affect subsidence.
The GSP identified critical infrastructure and developed SMC designed to limit infrastructure impacts in the Subbasin. Since there has been no observed inelastic land subsidence, no information is available regarding specific tolerances of local infrastructure to subsidence. If subsidence is observed in the future, the GSAs will coordinate with infrastructure owners or managers to adaptively manage the Subbasin and prevent infrastructure impacts. A comprehensive table of critical infrastructure and potential impacts in the Subbasin is included in Table 4-4.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 38 Table 4-4. Potential Risks to Critical Infrastructure in the Subbasin and Subsidence BMP Infrastructure or
Land Use Type
(Subsidence BMP)
Critical Infrastructure
Types
(Subsidence BMP)
Potential Impacts
(Subsidence BMP) Butte Subbasin Information
Cities and
Communities
Property drainage
Power systems
Municipal water systems
Sewer systems
Physical damage
Reduced service reliability
Increased maintenance
Communities are present with utilities,
transportation, and stormwater infrastructure
Larger communities are Biggs and Gridley
Smaller communities include Richvale (west of
Thermalito Afterbay) and Butte City (near
Sacramento River)
Pipelines and Other
Utilities
Natural gas
Water
Electrical
Cracking or joint failure
Over-pressurization
Reduced capacity
Pinch-points and unalignment of transmission
poles, towers, and lines
Numerous utilities in communities
Electrical transmission also in rural areas
Transportation Private rail
Drainage systems
Highways
Bridges
Rail deformation, instability,
or loss of use during
flooding
Surface cracking
Drainage failure
Structural instability
State Highways 99 and 162
Union Pacific Railroad used for freight transit,
passing through the Butte Subbasin between
Gridley and Chico
Numerous other local roads, bridges, and
drainage systems throughout the Subbasin
Canals Federal, local, and state
canal systems
Reduced conveyance,
freeboard
Lining damage
Altered slope
Erosion and sedimentation
of unlined channels
Local canals are present throughout Subbasin,
especially to the east
Numerous local canals sourced by Feather River
including Western, Cherokee, Main Drainage,
Biggs Extension, and Sutter Butte Canals
Flood Control &
Drainage
Federal, State, and Local
flood facilities (e.g., levees,
bypasses, and dams)
Loss of grade
Structural failure
Reduced flood capacity
Levee overtopping or breach
Levees, bypasses, and dams are present
Critical infrastructure includes Thermalito Afterbay
dam and levees on Sacramento and Feather
Rivers
Groundwater
Pumping Facilities
Domestic, agricultural, and
public supply wells
Casing collapse
Reduced capacity
Operational inefficiencies
Sand and gravel damaging
pump bowls
Critical infrastructure includes public supply wells
in Biggs and Gridley
Domestic and agricultural supply wells are
dispersed throughout the Subbasin and have
various subsidence tolerance depending on
construction and age
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 39 Sustainable Management Criteria
The Subsidence BMP recommends specific SMC for areas like the Butte Subbasin with no inelastic subsidence due to groundwater level decline. The intent of the BMP is to provide guidance for GSAs to prevent the onset of subsidence. The Subbasin’s subsidence SMC— specifically the measurable objective—allows for a small amount of subsidence to occur beyond the InSAR measurement error so the GSAs have the chance to monitor and adaptively manage if conditions change in the future. The SMC are set at levels that are thought to be protective and avoid significant and unreasonable impacts to critical infrastructure. The following further clarifies what the BMP recommends and how the GSAs intend to monitor and manage subsidence to maintain sustainability.
Undesirable Results
BMP: Each GSP should also include a quantitative description of undesirable results that represent a specific numerical value when subsidence that is substantially interfering with land uses becomes significant and unreasonable. Considering impacts to infrastructure often occur at a specific location, the quantitative definition of undesirable results may be based on as little as a single exceedance of a minimum threshold if an exceedance at that location leads to the significant and unreasonable conditions (e.g., damage to infrastructure, collapsed well casings, etc.) the GSA is managing the basin to avoid. The definition of undesirable results for land subsidence should be made in conjunction with entities responsible for the operation and maintenance of the infrastructure and the specific tolerance of that infrastructure or any mitigation efforts that are agreed upon by the GSA and the infrastructure manager.
The BMP recommends a more conservative approach to defining subsidence undesirable results than was used in the GSP. While the BMP focuses on preventing specific local infrastructure impacts, the GSP focuses on Subbasin-wide management. The BMP states that the quantitative definition of undesirable results may be based on as little as a single exceedance of a minimum threshold, while an undesirable result for subsidence is defined in the GSP as 25% of GPS monitoring benchmarks exceeding the MT of 0.5 foot over five years.
The subsidence undesirable results definition has been revised to better align with recommendations in the BMP. Significant and unreasonable subsidence could happen in localized areas if subsidence were to impact critical infrastructure. Therefore, undesirable results will occur when 25% of any PLSS Township (6 x 6 sections) exceeds the MT (0.5 foot of subsidence over five years) in two consecutive years within the same InSAR pixels.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 40 In other words, the definition of undesirable results states that:
• MT exceedance is defined as a cumulative subsidence of 0.5 foot over a 5-year period measured as reported from InSAR data in each pixel
• UR does not come into effect until the first year of MT exceedance and if it holds for an additional year
An example showing the WY 2025 subsidence monitoring, extensometers, PLSS Townships, and critical infrastructure is shown on Figure 4-18.
The occurrence of an undesirable result will be confirmed using detailed review of groundwater level and extensometer data and presence of critical infrastructure that could be damaged. The GSAs are not responsible for managing changes in land surface elevation due to other factors besides lowering of groundwater levels. If an undesirable result is determined to be due to the lowering of groundwater levels, the GSAs will review if there is critical infrastructure in the area that could be impacted.
The GSAs consider this change in management approach to be consistent with the intention of the GSP and not necessitate a GSP Amendment.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 41 Figure 4-18. Subsidence Monitoring Locations and PLSS Sections Draft Butte Subbasin Periodic Evaluation, September 2026 Page 42 Minimum Threshold
BMP: The minimum threshold for subsidence should be reflective of local conditions. In areas that have not recently experienced land subsidence, the minimum threshold should be set to disallow the onset of subsidence. This could be zero or the measurement error of the monitoring equipment.
The subsidence MTs are established in the GSP at 0.5 foot over five years, which is 5 times the measurement error, but is also likely protective of critical infrastructure in the Subbasin. The BMP recommends using zero or the measurement error for subsidence as the MT.
Measurable Objective
BMP: In basins that have not experienced land subsidence, the measurable objective should be set at zero.
The subsidence MO is 0.25 foot over five years, which is 2.5 times the measurement error. The MO is set at a level that will alert the GSAs if measurable subsidence starts to occur and is also likely protective of critical infrastructure in the Subbasin. The BMP recommends using zero subsidence as the MO.
Subsidence Management
BMP: In Scenario 2 areas without recent subsidence that are managing groundwater levels below recent lows, the GSA should monitor land subsidence and have PMAs ready to implement if subsidence is detected. Once subsidence is detected, the GSA should implement these PMAs and raise groundwater level thresholds and any associated sustainable management criteria, as the presence of new subsidence indicates groundwater levels have dropped below the critical head.
The GSAs are actively monitoring and managing groundwater levels and subsidence and have identified additional PMAs that would help manage groundwater level declines should they occur and cause subsidence. In addition, the GSAs are implementing an adaptive management approach for monitoring subsidence, discussed below.
4.5.3.3 Subsidence Adaptive Management
Through this evaluation, response to DWR’s RCA #2, and review of the Subsidence BMP, the GSAs recognized that establishing a subsidence adaptive management approach will help proactively manage subsidence if it is observed in the future. The following tiered adaptive management approach will be implemented if subsidence is observed:
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 43 • The InSAR measurement error of 0.1 foot is used as a first detection level in the GSP Annual Reports. Maps of the annual and cumulative subsidence totals over the previous five years are prepared to identify any areas with measurable subsidence. Subsidence detections greater than 0.1 foot will be investigated if it includes three or more contiguous InSAR 100 x 100 meter pixels (~3,280 square feet). InSAR data occasionally has anomalous measurements in areas with few fixed surfaces like paved roads to collect continuous measurements. Anomalous measurements will be considered false positives unless there are three or more contiguous points with greater than 0.1 foot of subsidence. • An exceedance of the MO (0.25 foot over five years) is a trigger level. If this value is exceeded in the annual or cumulative subsidence maps and analysis, the GSAs will perform a data review, collect specific information to understand the causes of subsidence, determine whether subsidence is currently or is projected to cause significant and unreasonable infrastructure impacts or risks, and identify specific PMAs that could be implemented in the area, if needed. Data review will include confirmation of InSAR monitoring data, review of groundwater levels in nearby wells in the GSP monitoring network, evaluation of aerial photography for land use disturbances that could be false positives, review of nearby extensometers to verify InSAR conditions are comparable with other available subsidence monitoring data, and identification of changes in land or water use that would lead to increased groundwater pumping. • An exceedance of the MT (0.5 foot over five years) will result in immediate action by the GSAs to adjust water use or implement PMAs to manage subsidence. Since the Subbasin has not historically experienced any measurable subsidence greater than 0.1 foot, experiencing 5 times that amount is a condition that will need to be addressed promptly.
Given that infrastructure damage could occur due to sustained subsidence at low rates over longer time periods than five years, a maximum subsidence limit could be necessary in the future if review of long-term InSAR and extensometer records show a change in conditions.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 44 5 STATUS OF PROJECTS AND MANAGEMENT ACTIONS
The GSP outlines several PMAs being implemented to ensure continued sustainable management of groundwater conditions in the Subbasin. This list also includes PMAs that are not necessary at this time, but may be needed if groundwater conditions worsen in the future due to unforeseen challenges. In general, this Subbasin is in good condition and therefore the need for PMAs is minimal.
This section describes the status of PMAs in the Butte Subbasin and includes a summary of the GSAs’ activities related to tracking, evaluating, and implementing PMAs since adoption of the GSP in 2022 through the most recently completed Annual Report (Butte Subbasin GSAs, 2026).
5.1 Background
This section provides background information and a review of GSP PMAs identification and approaches and defines categories and types of PMAs.
5.1.1 Overview of Butte Subbasin GSP and PMA Approach
As described in Section 5.2 of the GSP, the projected baseline water budgets show an estimated decline in groundwater storage of 2,000 acre-feet per year (AFY) on average. This simulation is based on estimated future projected conditions incorporating DWR’s 2070 climate change factors and without implementation of any PMAs. This very small volume relative to overall water use within the Subbasin is within the uncertainty of the model, especially considering uncertainty around future conditions and actual effects of climate change.
These results indicate that, depending on future conditions over the course of the GSP implementation period, implementation of PMAs may not be necessary to maintain sustainability and avoid undesirable results. However, as a conservative approach, the GSAs identified and planned a suite of PMAs that support the responsible stewardship and management of surface water and groundwater resources in the Butte Subbasin, including PMAs that directly influence groundwater sustainability by either reducing groundwater extractions or increasing groundwater recharge. In total, there were 25 PMAs included in the GSP. The implementation of even a subset of these PMAs would provide an annual average benefit sufficient to offset the decline in groundwater storage shown in the projected water budget.
The GSAs monitor Subbasin conditions and use adaptive management to guide PMA implementation, including expanding PMA implementation as necessary to address undesirable results and achieve long-term sustainability. Outside of SGMA requirements, the GSAs may also implement projects and programs based on other priorities, input from stakeholders, and Draft Butte Subbasin Periodic Evaluation, September 2026 Page 45 available funding. The implementation of these projects provides benefits to the Subbasin, even if they are not directly required for implementation of the GSP.
DWR has added all the original GSP PMAs into its new online module to track updates and for availability to the public. The DWR PMA Module was developed by DWR and released in spring 2025. It is a centralized, web-based platform designed to track and report PMA implementation status, costs, and benefits, and other information. It is available online within the SGMA Portal at: https://sgma.water.ca.gov/portal/
Based on the PMA updates described below, the DWR Module will be updated accordingly when the Periodic Evaluation (PE) is submitted.
5.1.2 Categories of Butte Subbasin PMAs (from GSP)
The GSP classifies PMAs into three distinct categories, related to status or necessity of implementation, as defined below.
1. Ongoing PMAs: PMAs underway at the time of GSP adoption and those the GSAs are committed to completing during the GSP implementation period to “achieve the sustainability goal for the basin… [and] respond to changing conditions in the basin” (23 CCR §354.44(a)). Three ongoing PMAs were identified and will support GSAs in meeting the interim milestones and measurable objectives set in the GSP and avoid minimum thresholds, even under future climate change conditions. These PMAs include level of detail required by 23 CCR §354.44(b).
2. Planned PMAs: Four PMAs in this category may be implemented at the discretion of GSAs to support ongoing sustainability, adapt to changing conditions in the Subbasin, or maintain other water management objectives. These PMAs were planned to the level of detail required by 23 CCR §354.44(b).
3. As Needed PMAs: PMAs not planned to the level of detail required by 23 CCR §354.44(b), but briefly summarized and discussed. Additional project development and planning will occur as projects become higher priorities or are determined to be needed by the GSAs. Their potential implementation will also support ongoing sustainability, adapt to changing conditions in the Subbasin, or advance other water management objectives.
In summary, of the 25 total PMAs, three (12%) were classified as Ongoing, four (16%) as Planned, and the remaining 18 (72%) As Needed. The GSAs are committed to tracking PMAs throughout GSP implementation to support continued stewardship of water resources in the Subbasin and to maintain groundwater sustainability over the long-term horizon.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 46
5.1.3 Types of Butte Subbasin PMAs and District Projects (introduced in PE) To further improve the distinction between different PMAs, they have been classified into different types. This new concept introduced here in the Periodic Evaluation is meant to clarify the objective and purpose of the various PMAs in the GSP. The four types of PMAs are defined and described below:
1. Improve Water Use Efficiency and Management: This is the most common type of PMA from the GSP. This PMA type focuses on the modernization of surface water distribution systems through infrastructure improvements, increased flow measurement or monitoring, or other means to increase water use efficiency and improve water management. These PMAs support groundwater sustainability through efficient use of surface water throughout the Subbasin. Water use efficiency improvements allow for more surface water to be delivered for irrigation, which can reduce groundwater pumping (especially in curtailment years) and increase groundwater recharge.
2. Increase In-Lieu Recharge: This PMA type provides surface water for irrigation to lands previously dependent on groundwater (i.e., providing surface water “in-lieu” of groundwater), which provides quantifiable benefits of reduced groundwater pumping.
The use of surface water for irrigation on these lands also contributes to increased groundwater recharge by deep percolation of irrigation water.
3. Increase Direct Recharge: This PMA type directly increases overall groundwater recharge, providing quantifiable benefits for groundwater sustainability. Examples of this PMA type include construction and operation of recharge basins, diverting surface water onto a working landscape for Managed Aquifer Recharge (MAR), or conducting multibenefit recharge (MBR) by flooding wildlife areas to improve habitat and increase recharge.
4. Improve Monitoring Network and Modeling: This PMA type includes efforts to improve availability and/or accessibility of data related to surface water and groundwater resources, as well as efforts to improve modeling of Subbasin conditions and related planning efforts. While these PMAs do not directly result in quantifiable increased recharge or reduced groundwater pumping, they provide additional data and improve understanding to support groundwater sustainability planning and potential implementation of other PMAs.
5.2 Summary and Status of PMAs
All 25 of the PMAs developed by the GSAs and included in the 2021 Butte Subbasin GSP are listed and summarized in Table 5-1, along with one new PMA that was developed after completion of the GSP. Table 5-1 provides a brief description of each PMA, the lead GSA, the Draft Butte Subbasin Periodic Evaluation, September 2026 Page 47 PMA category and type, the targeted sustainability indicator(s) from the GSP, its implementation status as of WY 2025 (the most recently completed Annual Report during this Periodic Evaluation cycle), the anticipated schedule for implementation, and a brief discussion of the PMA status and related activities during this Periodic Evaluation cycle. The current status of each PMA is defined as one of the following:
• Completed: The PMA has been completed, and benefits are currently being realized and accrued for the GSA and broader Subbasin.
• In Progress: Efforts to complete the PMA are underway, although quantifiable benefits may or may not have been achieved to date (dependent on status and potential partial implementation). This stage of PMA implementation could vary from planning and design, to permitting and construction, to partial implementation of some aspects of PMA are operational while construction and implementation of others has not yet been completed.
Other PMAs, that were previously listed in the “As Needed” category are not considered highpriority for groundwater sustainability at this time. Efforts to complete these PMAs may still be active (and in various stages), or the PMAs may not be actively advanced. If the PMA is actively being advanced, it is reflected in the status update in the table below.
These are other projects the Districts are already completing for good stewardship of water management, and this is a category of projects that is not SGMA-specific and will not be included in the DWR Module as this is something that occurs regardless of SGMA implementation.
Table 5-1 was developed using materials available in the GSP Annual Reports prepared during this Periodic Evaluation cycle. The estimated benefits of PMAs at the current level of implementation are described in Section 5.3 and summarized in Table 5-2. Updates on relevant PMAs will continue to be provided each year in the GSP Annual Report and tracked in the DWR Module.
The updated list of PMAs, that is tracked in the DWR Module as of this Periodic Evaluation, is included in Table 5-4 in Section 5.4.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 48 Table 5-1. Summary and Status of Projects and Management Actions Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_1
Project - System
modernization
(Biggs-West
Gridley Water
District [BWGWD]
GSA)
BWGWD will upgrade and modernize system infrastructure to improve system operability and efficiency, reduce operational spillage, and enhance the timing of farm deliveries. Modernization improvements will include:
1. Improvements at canal headings to improve water level control, flow control, flow measurement, SCADA, and automation/measurement
2. Improvements at customer delivery turnouts to improve delivery flexibility and steadiness
3. Implementation of a canal operations decision support system (CODSS)
Biggs-West
Gridley
Water
District GSA
Ongoing
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
In Progress
This project was nearly complete at the end of WY 2025, and was anticipated to be completed before the 2026
irrigation season. Portions of the project were already completed and used by WY 2025. The SCADA system has
been integrated, and data for six operational water control sites are available in near real-time to District operators and other staff. Full project benefits are
expected to be realized following the 2026 water season.
WY 2026
5-021.70_PMA_2
Project - System
modernization
(Butte Water
District [BWD]
GSA)
BWD will modernize its irrigation distribution system as part of its comprehensive plan for system modernization and boundary flow monitoring developed and reported in the Feather River Regional Agricultural Water Management Plan (FRRAWMP). The project is similar to the system modernization projects currently being implemented by BWGWD and Richvale Irrigation District (RID). System modernization improvements will help system operators to strategically manage surface water diversions from the Feather River, supporting their ability to increase system efficiency, reduce operational spillage, and/or reduce excess farm deliveries. The system modernization projects generally include improvements to three site categories: heading structures, upstream water level control structures, and spill structures. Table 5‐6 in GSP Section 5.4 identifies the modernization objectives of improvements to each site category, and the measurable objectives that are expected to benefit from these improvements. Each project is expected to promote the ongoing maintenance of sustainable conditions in the Butte Subbasin.
Butte Water
District GSA
- Butte
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs and BWD has completed modernization projects
along the Sutter Butte Main Canal since GSP adoption, and operators are utilizing improvements to quickly respond to flow changes and match supplies more precisely with
demands, thereby enhancing the beneficial use of surface water supplies delivered to both BWD and Sutter Extension Water District (SEWD). BWD also engaged in planning for other modernization efforts, including improved control and measurement at lateral headings and turnouts.
N/A
5-021.70_PMA_3
Project - System
modernization
(Richvale Irrigation
District GSA)
RID will upgrade and modernize system infrastructure to improve system operability and efficiency, reduce operational spillage, and enhance the timing of farm deliveries. Modernization improvements will include:
1. Improvements at canal headings to improve water level control, flow control, flow measurement, SCADA, and automation/measurement
2. Improvements at control structures to improve water level control, flow control, flow measurement, SCADA, and automation/measurement Richvale
Irrigation
District GSA
Ongoing
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
Completed
This project was completed at the beginning of WY 2025
with benefits fully realized during the 2025 irrigation season.
Infrastructure improvements to 10 Main Canal level control check structure gates, construction of four flow
measurement stations on the Main Canal, and SCADA
integration are complete and associated water conservation benefits are being realized from improved infrastructure and operational data availability.
WY 2025
5-021.70_PMA_4
Project - System
Modernization
(Western Canal
Water District
[WCWD] GSA)
Western Canal Water District (WCWD) will modernize its irrigation distribution system as part of its comprehensive plan for system modernization and boundary flow monitoring developed and reported in the FRRAWMP. Detailed information about the project is included in the FRRAWMP and in project documentation included in GSP Appendix
5.D.1l. The project is similar to the system modernization projects currently being implemented by BWGWD and RID, described in GSP Section 5‐4. System modernization improvements will help system operators to strategically manage surface water diversions from the Feather River, supporting their ability to increase system efficiency, reduce operational spillage, and/or reduce excess farm deliveries. The system modernization projects generally include improvements to three site categories: heading structures, upstream water level control structures, and spill structures.
Western
Canal Water
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs and WCWD is continually searching for opportunities to modernize its system to enhance its water management, water use efficiency, and delivery to growers. Some of the other PMAs in the table with WCWD as the lead GSA that
have been implemented are examples of system
modernization.
N/A
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 49 Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_5
Project - Boundary
flow and primary
spill measurement
(Western Canal
Water District
[WCWD] GSA)
WCWD will implement a boundary flow and primary spill measurement project at three key outflow sites in its distribution system. The boundary flow and primary spill measurement project is part of the district’s comprehensive plan for system modernization and boundary flow monitoring developed as part of the FRRAWMP. The three boundary outflow sites identified for this project represent approximately 70 percent of total district outflow. Without these improvements, WCWD water operators are unable to remotely monitor key boundary outflows, making it difficult to identify when system inflows do not meet irrigation demands or conversely, when system inflows exceed irrigation demands. As a result, significant operational spillage has historically occurred at the end of laterals. The time required for operators to manually measure outflows at the boundary outflow sites further diminishes the operators’ ability to make changes and adjustments to customer deliveries and upstream water control structures in the canal system, further increasing operational spillage.
Western
Canal Water
District GSA
Ongoing
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
Completed
This project was completed during WY 2023, and the
monitoring and SCADA equipment has been used by
WCWD in the time since to monitor boundary outflows,
resulting in surface water conservation benefits.
WY 2023
5-021.70_PMA_6
Project - Dual
source irrigation
systems (Butte
Water District
GSA)
BWD will incentivize the use of irrigation systems capable of using both surface water and groundwater. These systems will increase use of surface water, increase on-farm recharge of surface water, and reduce groundwater pumping.
Butte Water
District GSA
- Butte
Planned Increase InLieu Recharge
Groundwater Storage;
Groundwater Levels;
Interconnected Surface
Waters
In Progress
BWD is currently implementing a dual source irrigation
project within the Sutter Subbasin portion of its service area (funded through Proposition 68), which is anticipated to result in benefits to adjacent areas, but it
has not moved forward to the same extent on this project within the Butte Subbasin portion of its service area.
However, since GSP adoption, during WYs 2022-2023,
BWD did complete a conceptual evaluation of dual source irrigation systems, including conceptual designs, cost
estimates, and estimates of increased surface water use.
TBD
5-021.70_PMA_7
Project - Multibenefit recharge
(RD 2106 GSA)
This PMA has shifted in scope since GSP adoption with a new focus on Rancho Llano Seco (within the RD 2106 GSA), although other GSAs may consider MBR in the future as opportunities arise.
This project, with guidance from conservation groups, will develop and implement a MBR program that will provide groundwater recharge through normal farming operations while also providing critical wetland habitat for waterbirds migrating along the Pacific Flyway. Fields with soil and cropping conditions conducive to groundwater recharge will be flooded and maintained with shallow depths. Sloughs and lakes with subsurface geology conducive to groundwater recharge will be used to store and transport water. Water will be sourced from existing water rights contracts, depending on availability. GSA may also consider financial compensation for participation to offset field preparation, irrigation, water costs, infrastructure and habitat restoration.
Reclamation
District
No. 2106
GSA
Planned Direct
Recharge
Groundwater Levels;
Groundwater Storage In Progress
This PMA has shifted in scope since GSP adoption with a new focus on Rancho Llano Seco (within the RD 2106
GSA), although other GSAs may consider MBR in the future as opportunities arise.
Rancho Llano Seco implemented a project in WY 2025
within a conservation easement on their land to fallow
approximately 400 acres of agricultural land, transition it into wildlife habitat and refuge area, increasing total refuge area in the Subbasin and MBR. Additionally, a related Llano Seco project in the 2025 WY with notable progress involved initial planning and design for restoration of a siphon structure to deliver surface water to an additional 1,0000 to 2,000 acres of wildlife habitat (wetlands and sloughs), which will provide additional MBR within the Subbasin.
TBD
5-021.70_PMA_8 Outreach Activities
- Grower education
A grower education and outreach program is proposed as a management action for the Butte Subbasin. The program will provide growers with educational resources that help them to plan and implement on-farm practices that simultaneously support groundwater sustainability and maintain or improve agricultural productivity.
County of
Butte GSA -
Butte 1
Planned
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters; Water Quality
No longer
considered a
PMA – remove
from Module
Although this PMA has shifted in scope from a specific
program as described in the GSP, ongoing grower education has occurred from applicable entities such as the Farm
Bureau, Agricultural Groundwater Users of Butte County, the USDA Natural Resource Conservation Service, and CSU
Chico's Regenerative Agricultural program. Additionally, Richvale Irrigation District, Biggs-West Gridley Water
District; Butte Water District; and Western Canal Water District included grower education in their annual grower meetings since GSP adoption.
N/A
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 50 Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_9
Management
Action - Installation
of additional
shallow monitoring
wells
GSAs will install 10 shallow monitoring wells in key areas of the Subbasin to support monitoring of interconnected surface water and groundwater dependent ecosystems.
Western
Canal Water
District GSA
Planned
Improve
Monitoring
Network and
Modeling
Interconnected Surface
Waters; Groundwater
Levels; Groundwater
Storage
In Progress
This PMA has been advanced and is nearing completion.
Through continued coordination with TSS, site locations were identified, agreements between landowners and GSAs were coordinated, and well construction is expected to be completed soon, allowing monitoring to begin.
WY 2026
5-021.70_PMA_10
Project - Alternate
delivery to RID
Secondary via
Kelleher Dam
This project will provide an alternate delivery route to RID’s secondary service area using Kelleher Dam, a privately owned dam located on the east side of the Cherokee Canal. The objective of this project is to convey water to the secondary service area, enhancing RID’s water management efforts by avoiding several key constraints of current service. First, delivering water to secondary users currently requires that the entire distribution system be charged, resulting in operational spillage. Second, the secondary users often require water during periods when rice fields are being drained, increasing the accretion to fields and preventing complete drainage. Use of Kelleher Dam would alleviate these challenges and reduce water loss through spillage. This project is proposed as part of the “Potential Projects to Enhance RID Water Management Capabilities” in the FRRAWMP.
Richvale
Irrigation
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Storage;
Groundwater Levels;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. RID has not moved forward with this PMA at this
time, although they periodically re-evaluate the project against other district needs and priorities and evaluate funding opportunities for project implementation as they arise.
N/A
5-021.70_PMA_11
Project - Boundary
flow and primary
spill measurement
(Butte Water
District GSA)
BWD will implement a boundary flow and primary spill measurement project for its irrigation distribution system as part of its comprehensive plan for system modernization and boundary flow monitoring developed and reported in the FRRAWMP. The project is similar to the boundary flow and primary spill measurement project currently being implemented by WCWD, described in GSP Section 5.5. The project will support water operators in remote monitoring of system outflows and help them to quickly respond to any excess or shortages by moving water within the system and by changing diversions at Thermalito Afterbay. Enhanced monitoring afforded by this project will help operators to reduce operational spillage and leave water in the Feather River system to meet the needs of threatened and endangered aquatic species, including Chinook Salmon and Steelhead Trout.
Butte Water
District GSA
- Butte
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. BWD has not moved forward with this PMA at this
time. Through their normal operations they minimize spills and boundary outflows to the extent possible (and boundary outflows are recoverable and serve as a source of water supply for other water users downstream).
N/A
5-021.70_PMA_12
Project - Boundary
flow and primary
spill measurement
(Richvale Irrigation
District GSA)
RID will implement a boundary flow and primary spill measurement project for its irrigation distribution system as part of its comprehensive plan for system modernization and boundary flow monitoring developed and reported in the FRRAWMP. The project is similar to the boundary flow and primary spill measurement project currently being implemented by WCWD, described in GSP Section 5.5. Additional information about the RID project is described in GSP Appendix 5.C.1. The project will support water operators in remote monitoring of system outflows and help them to quickly respond to any excess or shortages by moving water within the system and by changing diversions at Thermalito Afterbay.
Enhanced monitoring afforded by this project will help operators to reduce operational spillage and leave water in the Feather River system to meet the needs of threatened and endangered aquatic species, including Chinook Salmon and Steelhead Trout.
Richvale
Irrigation
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. RID has not moved forward with this PMA at this
time. Through their normal operations they minimize spills and boundary outflows to the extent possible (and boundary outflows are recoverable and serve as a source of water supply for other water users downstream).
N/A
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 51 Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_13
Coordination
Activities - Develop
partnerships to
implement project
addressing
regional water
management
This project will develop and foster partnerships between agencies in the Butte Subbasin to support regional water management. Such partnerships may support regional water management planning or project development. This project was proposed in the 2014 FRRAWMP, and is aimed at improving water management and operational enhancement among the various entities in the Subbasin.
County of
Butte GSA -
Butte 1
As Needed
Improve
Monitoring
Network and
Modeling
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
No longer
considered a
PMA – remove
from Module
As described in Section 5.4, the Butte Subbasin GSAs meet periodically as part of the Butte Advisory Board (BAB) to discuss regional water management, among other topics.
Additionally, the GSAs have been engaged in additional
broader regional coordination efforts. One example is the North Sacramento River Corridor Interbasin Coordination and Sutter Buttes Water Quality Working Group, which
initiated discussions in 2025 that will continue into 2026 where they are developing work plans describing regional approaches to water management.
N/A
5-021.70_PMA_14
Outreach Activities
- Facilitate
financing of capital
improvements for
on-farm irrigation
systems
This project would facilitate financing of capital improvements for on‐farm irrigation systems. This project will support improved agricultural water conservation and on‐farm water management by helping growers to upgrade their irrigation system to potentially improve water use efficiency and improve precision in nutrient application. This project directly aligns with the efficient water management practice identified in California Water Code §10608.48 (c)(3). Agricultural water suppliers that are required to submit agricultural water management plans must implement this practice if locally cost‐effective and technically‐feasible; therefore, this project also supports agricultural water management planning requirements.
County of
Butte GSA -
Butte 1
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
No longer
considered a
PMA – remove
from Module
The Butte Farm Bureau and the Agricultural Groundwater
Users of Butte County have provided opportunities through CDFA SWEEP funding to implement improvements for onfarm irrigation systems. Additionally, many agricultural water suppliers in the Subbasin provide at-cost labor and materials to assist landowners in improving on-farm irrigation systems, subject to resource availability. Glenn RCD also administers SWEEP and other funding opportunities.
N/A
5-021.70_PMA_15
Facilitate use of
available recycled
water that
otherwise would
not be used
beneficially, meets
all health and
safety criteria, and
does not harm
crops or soils
This project would facilitate use of available recycled water of suitable quality for various agricultural purposes, directly utilizing recycled water that otherwise would not be used beneficially within the Subbasin. This project will support improved water management and water use efficiency by identifying additional water supply sources to meet agricultural demand in the Subbasin and to supply groundwater recharge to the underlying aquifer. This project directly aligns with the efficient water management practice identified in California Water Code §10608.48 (c)(2).
County of
Butte GSA -
Butte 1
As Needed
Improve Water
Use Efficiency
and
Management;
Increase InLieu Recharge
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. Butte County (and other Butte Subbasin GSAs) have not moved forward with this PMA at this time. They evaluate opportunities to facilitate use of available recycled water on a case-by-case basis, as they arise.
N/A
5-021.70_PMA_16
Improve
monitoring of
surface water
outflows and refine
water balances to
improve
understanding of
surface
water/groundwater
interactions/net
recharge
This project would improve monitoring of surface water outflows and refine water balances in the Subbasin, with the goal of improving understanding of interactions between surface water and groundwater, and of the precise balance of net recharge from the surface water system in the Subbasin. Greater knowledge of surface water and groundwater interactions would enhance ongoing refinement of the basin setting and the evaluation of conjunctive management opportunities to increase local water supplies to meet local and regional water management objectives.
This project was proposed in the FRRAWMP (2014).
County of
Butte GSA -
Butte 1
As Needed
Improve
Monitoring
Network and
Modeling
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. On a broad Subbasin-wide scale, the Butte Subbasin GSAs have not moved forward with this PMA at this time.
GSAs evaluate opportunities to improve monitoring and
water balances as they arise and against other needs and priorities.
One example of an individual GSA advancing improved
outflow monitoring is WCWD’s implementation of their
boundary flow and primary spill measurement PMA (5-
021.70_PMA_5).
N/A
5-021.70_PMA_17
Project - Improved
delivery service to
pressurized
irrigation systems
BWD will improve delivery service to pressurized irrigation systems as part of its comprehensive plan for system modernization and boundary flow monitoring developed and reported in the FRRAWMP. The project is directly related to and supportive of the District’s dual source irrigation system project described in GSP Section 5.6. The project will also be supported by the District’s planned system modernization project described above. This project will help supply surface water to irrigators that use pressurized irrigation systems, offsetting groundwater use with in‐lieu groundwater recharge benefits to the Subbasin.
Butte Water
District GSA
- Butte
As Needed Increase InLieu Recharge
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. BWD has not moved forward with this PMA at this
time, although they periodically re-evaluate the project against other district needs and priorities, and evaluate funding opportunities for project implementation as they arise. As described in the GSP, this PMA is directly related to the dual source irrigation system project, and its
implementation will help supply surface water to irrigators with dual source systems.
N/A
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 52 Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_18
Increase planned
conjunctive use of
surface water and
groundwater within
the supplier
service area
WCWD is planning to increase conjunctive use of surface water and groundwater within the district’s service area. In this water management strategy, WCWD will continue to use surface water when it is available, providing in‐lieu and direct groundwater recharge benefits of surface water use to the Subbasin. During periods of shortage, WCWD will use both surface water and groundwater to meet demand. This project directly aligns with the efficient water management practice identified in California Water Code §10608.48 (c)(8). As an agricultural water supplier subject to agricultural water management planning regulations, WCWD must implement this practice if locally cost‐effective and technically‐feasible; therefore, this project also supports WCWD’s agricultural water management planning efforts.
Western
Canal Water
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
Conjunctive use, as described in the GSP and WCWD's
AWMP, is part of their long-term water management and
sustainability strategy, and this Project is implemented as needed in an ongoing manner. During typical water supply conditions, little to no groundwater is used within WCWD, but its use increases during times of drought and surface water curtailments.
N/A
5-021.70_PMA_19
Little Butte Creek
Reservoir main
canal bypass
project
WCWD would construct a bypass canal from a reservoir on Little Butte Creek and provide unrestricted supply via siphons to the Main and Ward Canals. The reservoir is formed in an intentionally impounded section of Little Butte Creek that accepts inflows from the east via the Western Canal and allows for delivery on the west side to the Main and Ward Laterals. This project will increase conveyance from the reservoir, reduce fluctuations in supply, and add measurement to deliveries made to the Main and Ward Canals. These improvements are expected to increase operational efficiency and surface water utilization in WCWD, helping to reduce groundwater reliance and depletion in case of failure. The project will also provide additional data to support water use monitoring. The Little Butte Creek Reservoir Main Canal bypass project is part of the district’s comprehensive plan for system modernization and boundary flow monitoring developed as part of the FRRAWMP.
Western
Canal Water
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage
As Needed –
remove from
Module
WCWD has been advancing the project since GSP adoption
and anticipates completing it in the coming years. As of the end of WY 2025, designs and permitting were advanced
from 60% designs and are currently nearing completion, and WCWD was seeking additional funding for construction and is anticipating construction activities beginning during WY 2026.
WY 2027
5-021.70_PMA_20 M&T - Llano Seco
fish screen project
The M&T – Llano Seco Fish Screen Project will modify the M&T Chico Ranch Pump Station intake structures at the Sacramento River to clear inflows and incorporate fish screens. This project will reduce groundwater reliance and potential depletion, in case of failure of the diversion.
County of
Butte GSA -
Butte 1
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage Completed
Llano Seco Fish Screen Project was completed in fall 2021, has been maintained in subsequent years, and has provided benefits to the Subbasin since completion. The substantial volume of surface water that M&T Ranch and Rancho Llano Seco divert from the Sacramento River continues to offset this volume of groundwater from being pumped from the
Subbasin.
WY 2021
5-021.70_PMA_21 Monitoring well
installation
This project will install an additional monitoring well in the southwestern portion of the Butte Subbasin, providing GSAs with additional data related to groundwater levels.
County of
Butte GSA -
Butte 1
As Needed
Improve
Monitoring
Network and
Modeling
Groundwater Levels;
Groundwater Storage
As Needed –
remove from
Module
Butte County has not moved forward with this PMA at this time. Butte County and other Butte Subbasin GSAs evaluate opportunities to improve monitoring as they arise and
against other needs and priorities.
N/A
5-021.70_PMA_22
Parrot Phelan
Diversion
restoration project
This project will result in reestablishment of the original Butte Creek channel and provide a flood bypass to handle excess flood flows to protect the Parrot Phelan Diversion from channel realignment in the future. The diversion and fish screening project that had been completed in 1995 were in jeopardy, resulting in a cooperative project between a variety of agencies for the reestablishment of the original channel and a flood bypass. The flood bypass channel contains rock slope protection along Butte Creek just past the entrance to the overflow. Shotcrete and rock slope protection was utilized to stabilize the entrance to the overflow channel to prevent additional erosion.
County of
Butte GSA -
Butte 1
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage
As Needed –
remove from
Module
Although this PMA is categorized as In Reserve, it has been advanced in the time since GSP adoption. Environmental
permitting for the project was completed in WY 2023, and ongoing sediment removal and bank stabilization work has been conducted up through WY 2025.Additionally, M&T
Ranch, Rancho Llano Seco, and project partners continued project development to restore the Parrott Phelan Diversion and provide a reliable surface water supply, improve delivery flexibility, reduce groundwater use in curtailment years, and support direct recharge from Comanche Creek.
N/A
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 53 Project or
Management
Action (PMA) ID
PMA Name PMA Description Lead
GSA(s)
PMA
Category
(from GSP)
PMA Type Targeted Sustainability
Indicator(s)
PMA Status
(as of WY 2025) Summary of PMA Status
Anticipated
Completion
Date
5-021.70_PMA_23
Re-establish
historical
monitoring
locations and
identify new sites
This project would re‐establish historical monitoring locations and identify new monitoring sites, with the goal of improving data and monitoring in the Subbasin. Greater knowledge of surface water flows, hydrologic conditions, and groundwater system responses to water management activities would support GSAs as they implement the GSP and work to maintain groundwater sustainability. This project was proposed in the 2014 FRRAWMP.
County of
Butte GSA -
Butte 1
As Needed
Improve
Monitoring
Network and
Modeling
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
Butte County has not moved forward with this PMA at this time. Butte County and other Butte Subbasin GSAs evaluate opportunities to improve monitoring as they arise and
against other needs and priorities.
N/A
5-021.70_PMA_24
Removal of
bottlenecks on the
Sutter-Butte main
canal
BWD would remove bottlenecks on the Sutter‐Butte Main Canal, improving delivery service to irrigation customers. This project is proposed as part of the district’s comprehensive plan for system modernization and boundary flow monitoring developed and reported in the FRRAWMP. The project will be supported by the District’s planned system modernization project and improved delivery service to pressurized irrigation systems project, described above.
Butte Water
District GSA
- Butte
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. BWD has not moved forward with this PMA at this
time, although they periodically re-evaluate the project against other district needs and priorities, and evaluate funding opportunities for project implementation as they arise.
N/A
5-021.70_PMA_25 Shu Fly/Cherokee
Project
This project would modernize the Shu Fly/Cherokee location within RID’s distribution system, resulting in Increased water supply, supply reliability, delivery flexibility, and/or instream flow; improved water quality, improved water management and reduced groundwater depletion in curtailment years
Richvale
Irrigation
District GSA
As Needed
Improve Water
Use Efficiency
and
Management
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
As Needed –
remove from
Module
This project is developed regardless of SGMA compliance needs. RID has not moved forward at this time, although they periodically re-evaluate the project against other district needs and priorities, and evaluate funding opportunities for project implementation as they arise.
N/A
5-021.70_PMA_26
(NEW)
M&T Ranch
Pipeline
This project will install a new pipeline to convey surface water to a 375- acre portion of M&T Ranch that was previously dependent on groundwater as its sole irrigation supply.
County of
Butte GSA -
Butte 1
New Increase InLieu Recharge
Groundwater Levels;
Groundwater Storage;
Interconnected Surface
Waters
Completed
This pipeline (which was completed in WY 2024) was
utilized in WY 2025 to convey surface water to a 375-acre portion of M&T Ranch that was previously dependent on
groundwater as its sole irrigation supply, providing in-lieu recharge. It is planned to be continually used in future years as well.
WY 2024
Note: Estimated benefits for PMAs that have been realized due to partial or full completion are described in Section 5.3.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 54
5.2.1 Ongoing PMAs
Of the three PMAs classified as Ongoing in the GSP, two have been implemented and are fully complete and the third is expected to be implemented and fully complete in WY 2026 ( Table 5-1):
• Project - System modernization (Richvale Irrigation District [RID] GSA) - complete • Project - Boundary flow and primary spill measurement (Western Canal Water District [WCWD] GSA) – complete
• Project - Multi-benefit recharge (RD 2106 GSA) – in progress All three of these PMAs improve water use efficiency and management, which (as described above) do not directly result in quantifiable increased recharge or reduced groundwater pumping.
However, they support groundwater sustainability through efficient use of surface water throughout the Subbasin, allowing for more surface water to be delivered, which can reduce groundwater pumping—especially in curtailment years—and increase groundwater recharge.
5.2.2 Planned PMAs
For the four PMAs classified as Planned in the GSP, all of them are currently in progress and currently being implemented although none of them are fully implemented as described in the GSP. The PMA referred to as grower education (5-021.70_PMA_8) will be removed from the DWR Module as it is not a SGMA project with quantifiable benefits, and occurs at the District level, as needed. The PMA on grower education has been implemented through ongoing activities by GSAs and other stakeholders and agencies within the Subbasin, although the grower education program described in the GSP has not been developed at this time. The Management Action to install 10 shallow monitoring wells 5-021.70_PMA_9 has been advanced through DWR TSS funds and is nearly fully implemented, with construction and the initiation of monitoring anticipated to occur during WY 2026.
The other two Planned PMAs will remain in the DWR PMA Module; these are dual source irrigation (5-021.70_PMA_6) and MBR (5-021.70_PMA_7). They have each been partially implemented and are described in their own subsections below and shown in Table 5-2. These PMAs will receive increased focus on implementation in the coming years relative to the other PMAs in the GSP. At this time and their current level of implementation, quantifiable benefits have not been achieved or estimated. Additionally, the quantifiable benefits will be dependent on the level of implementation of each of these PMAs and the estimated costs have not been updated from those in the GSP at this time.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 55
5.2.2.1 Dual Source Irrigation Systems (5-021.70_PMA_6) The dual source irrigation system PMA is still planned as described in the GSP. During WYs 2022-2023, Butte Water District (Butte WD) advanced planning for these systems by completing a conceptual evaluation that included conceptual designs, cost estimates, and estimates of increased surface water use. Butte WD’s service area is divided between the Butte and Sutter Subbasins, and it serves as the GSA for its service area within each Subbasin. Butte WD applied for grant funding from the SGM grant program for the implementation of this PMA. It received funding for the Sutter Subbasin portion of its service area, but not the Butte Subbasin portion.
With the funding, it is currently implementing a dual source irrigation project within the Sutter Subbasin portion of its service area, which will result in groundwater recharge benefits and is anticipated to result in benefits to adjacent areas. Butte WD has not moved forward to the same extent on this project within the Butte Subbasin portion of its service area, although it does coordinate directly with growers and provide support for the installation of private dual source irrigation systems. Butte WD aims to continue planning and implementation of dual source irrigation systems across its service area, with the increased surface water use resulting from the project reducing groundwater extractions through in-lieu recharge.
5.2.2.2 Multi-benefit Recharge (5-021.70_PMA_7)
The multi-benefit recharge PMA has shifted in scope from what was planned and described in the GSP. The original PMA in the GSP planned for multiple GSAs in the Subbasin to develop an MBR program based on guidance from The Nature Conservancy (TNC). The program would use water sourced from existing water rights contracts and provide groundwater recharge and critical wetland habitat for migrating birds through normal farming operations and on working farmed lands.
In the time since GSP adoption, this PMA has shifted to a new focus on Rancho Llano Seco, which is located within the Reclamation District (RD) 2106 GSA, although other GSAs may still consider MBR in the future as opportunities arise. Rancho Llano Seco includes managed wetlands and refuge areas that utilize surface water and provide both environmental habitat values and groundwater recharge benefits. In WY 2025, they implemented a project within a conservation easement to fallow approximately 600 acres of agricultural land and transition it into wildlife habitat and managed wetlands, increased the total refuge area in the Subbasin and increasing MBR. Another 300 acres are planned to be fallowed in the upcoming year, for a total of 900 acres fallowed for conservation. Additionally, in WY 2025 Rancho Llano Seco completed initial planning and design for restoration of a siphon that will deliver surface water to an additional 1,000 to 2,000 acres of managed wetlands and provide additional MBR in the Subbasin after completion. Rancho Llano Seco will continue to search for opportunities to plan and implement MBR projects on its lands.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 56 Table 5-2. Planned Projects and Management Actions
Project or Management
Action Name
(PMA ID - Type)
Lead GSA Brief Description
Targeted
Sustainability
Indicator(s)
Project Status
Estimated
Groundwater
Recharge Benefits
(AFY)
Anticipated
Completion Date
Dual Source Irrigation
Systems
(5-021.70_PMA_6 –
Increase In-lieu Recharge)
Butte Water
District GSA -
Butte
Incentivize the use of irrigation systems
capable of using both surface water and
groundwater. These systems will increase
use of surface water and on-farm recharge
of surface water, and reduce groundwater
extractions through in-lieu recharge.
Groundwater Levels,
Groundwater Storage,
Surface Water
Depletion
In Progress Dependent on level of
implementation TBD
Multi-benefit Recharge
(5-021.70_PMA_7 –
Direct Recharge)
Reclamation
District
No. 2106 GSA
Complete projects to increase MBR on
Rancho Llano Seco lands with anticipated
benefits including increased direct
groundwater recharge and developing
additional managed wetlands and refuge
areas to provide habitat for migrating birds
and other species, among other
environmental benefits.
Groundwater Levels,
Groundwater Storage,
Surface Water
Depletion
In Progress Dependent on level of
implementation TBD
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 57
5.2.3 As Needed PMAs
Of the 19 PMAs classified as “As Needed” in the GSP, one was completed since its inclusion in the GSP: the M&T - Llano Seco fish screen project (5-021.70_PMA_7) was completed in fall 2021, before adoption and submittal of the completed GSP to DWR. This PMA provides increased surface water reliability and reduces dependence on groundwater in the Butte Subbasin.
No other As Needed PMAs have been completed in the time since GSP adoption. Some of them have not been pursued beyond the planning effort required for inclusion in the GSP (e.g., 5- 021.70_PMA_10), others have been partially implemented through ongoing District activities and operations (although a major effort to implement them has not been undertaken) (e.g., 5- 021.70_PMA_2), and others are actively being advanced and expected to be completed (e.g., 5- 021.70_PMA_19). The large majority of these PMAs improve water use efficiency and management, which supports groundwater sustainability, but does not result in direct measurable increased recharge and reduced groundwater pumping benefits.
As described above, the implementation of these PMAs are not anticipated to be required to achieve sustainability. However, if Subbasin conditions change over time and undesirable results are detected, they can be re-evaluated by the GSAs as a means to address undesirable results and achieve sustainability. Districts may also choose to implement them regardless of Subbasin conditions to support water resources stewardship. Therefore, these PMAs are considered other District water management projects that have ancillary benefits for groundwater sustainability, with no direct need for SGMA implementation. These projects will be removed from the DWR PMA Module.
5.2.4 New PMAs
One new PMA was identified in the time since GSP adoption, and it has been fully implemented and was completed in WY 2024 (5-021.70_PMA_26 on PMA module). The conjunctive use project installed a new pipeline on M&T Ranch (part of RD 2106 GSA) to convey surface water to 375 irrigated acres that were previously dependent on groundwater as their sole source of water supply. This pipeline was used in WY 2025 and is expected to continue to be used into the future, accruing quantifiable in-lieu recharge benefits through reduced groundwater pumping and increasing direct groundwater recharge. This project was developed as part of an existing PMA approach identified in the GSP, but elevated to a more specific PMA. New wells were installed as part of this project that may be used for future monitoring in the Subbasin.
5.3 Estimated Benefits of Active PMAs
Benefits from PMA implementation can begin to accrue even at a partial level of implementation, and definitely will accrue after PMAs are fully implemented. Table 5-3 presents Draft Butte Subbasin Periodic Evaluation, September 2026 Page 58 a summary of estimated benefits of PMAs that have been fully completed since GSP adoption, or that are nearing completion and anticipated to have quantifiable benefits. In total, there are five PMAs included in Table 5-3. Two types of quantifiable benefits are included, along with other benefits (which are not directly related to water conservation or groundwater sustainability). The two quantifiable benefits are surface water conservation benefits and groundwater recharge benefits.
Surface water conservation benefits are those resulting from improved surface water use efficiency and management (which constitute a majority of the PMAs for the Butte Subbasin); the conserved surface water from these PMAs is available to support additional irrigated demands or other purposes. Surface water conservation benefits do not directly correspond to groundwater recharge benefits, but they can help support groundwater sustainability through both decreased groundwater pumping (if conserved surface water is used to meet irrigation demands that would otherwise be met by groundwater pumping) and increased groundwater recharge (if conserved surface water is used to irrigate lands that otherwise would have remained fallow or been irrigated with groundwater). The groundwater recharge benefits from surface water conservation have not been estimated at this time, but are anticipated to only be a portion of surface water conservation volumes.
Groundwater recharge benefits are either an overall increase in volume of groundwater recharge or decrease in the volume of groundwater pumping, or a combination of the two. The estimated groundwater recharge benefits from the M&T Ranch Pipeline PMA were based on an estimated surface water delivery of 2 acre-feet per acre to 375 acres to meet a substantial portion of irrigation demands on these lands. The additional groundwater recharge benefits from increased recharge of surface water from these lands has not been estimated at this time.
The total estimated surface water conservation benefits from the Richvale Irrigation District (RID) and Western Canal Water District (WCWD) PMAs are estimated to be 5,600 AF; the estimated volume for each project is based on the surface water conservation volumes calculated during project planning, coordination with GSA staff, and post-project monitoring data. The Biggs-West Gridley Water District (BWGWD) PMA is achieving surface water conservation benefits at its partial level of implementation, but these benefits have not been estimated at this time. The Butte County M&T – Llano Seco Fish Screen Project allows for continued diversion of surface water to meet demands that would otherwise be met—at least in part—by increasing groundwater pumping. As outlined in recent annual reports, groundwater users in the Subbasin benefit from generally stable and shallow groundwater levels supported by the substantial recharge resulting from large volumes of surface water supplied throughout the Subbasin.
Ongoing access to surface water supplies is crucial to maintaining groundwater sustainability in the Subbasin.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 59 Table 5-3. Estimated Benefits of Completed or In Progress Projects and Management Actions Lead GSA
Project or
Management Action
Name
(PMA ID - Type)
Targeted
Sustainability
Indicator(s)
Project
Status
(Completion
Date)
Estimated
Surface Water
Conservation
Benefits
(AFY)
Estimated
Groundwater
Recharge
Benefits
(AFY)
Explanation of Estimated
AFY Benefits Other Benefits
Biggs-West
Gridley
Water
District GSA
System Modernization
(5-021.70_PMA_1 –
Improve Water Use
Efficiency and
Management)
Groundwater
Levels,
Groundwater
Storage, Surface
Water Depletion
In Progress
(WY 2026)
See Other
Benefits
Surface water benefits are
achieved through increased
operational flexibility and water use
efficiency.
This PMA is nearing completion and is
partially implemented, although surface
water conservation benefits have not been
quantified at this level of partial
implementation.
Richvale
Irrigation
District GSA
System Modernization
(5-021.70_PMA_3 –
Improve Water Use
Efficiency and
Management)
Groundwater
Levels,
Groundwater
Storage, Surface
Water Depletion
Completed
(Fall 2024) 3,800
Surface water benefits are
achieved through increased
operational flexibility and water use
efficiency.
Less field and travel time for operators,
increased field safety, and additional
operational data.
Western
Canal Water
District GSA
Boundary flow and
primary spill
measurement (5-
021.70_PMA_5 –
Improve Water Use
Efficiency and
Management)
Groundwater
Levels,
Groundwater
Storage, Surface
Water Depletion
Completed
(WY 2023) 1,800
Surface water benefits are
achieved through increased
operational data and associated
increased water use efficiency.
Less field and travel time for operators,
increased field safety, and additional
operational data.
County of
Butte GSA -
Butte 1
M&T - Llano Seco fish
screen project Pipeline
(5-021.70_PMA_20–
Improve Water Use
Efficiency and
Management)
Groundwater
Levels,
Groundwater
Storage, Surface
Water Depletion
Completed
(Fall 2021
See Other
Benefits
The continued use of this diversion facility
through this PMA reduces groundwater
reliance and pumping by roughly the
volume of water diverted, which can range
from 20,000 to 40,000 AFY.
County of
Butte GSA -
Butte 1
M&T Ranch Pipeline
(5-021.70_PMA_26 -
Increase In-Lieu
Recharge)
Groundwater
Levels,
Groundwater
Storage, Surface
Water Depletion
Completed
(Spring 2024) 750
In-lieu groundwater recharge
benefits are achieved through
providing surface water to irrigated
acres previously dependent on
groundwater.
Increased recharge from additional surface
water irrigation, reduced energy usage
corresponding to reduced groundwater
pumping.
Totals (AFY) 5,600 750
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 60
5.4 Tracking, Administering, and Reporting of PMAs
PMAs are tracked and reported at the Subbasin-level through coordination between all GSAs.
This evaluation cycle allowed for a more detailed review of PMAs that were initially included in the GSP and establish the ones that are needed for SGMA implementation.
5.4.1 Revised PMA List for DWR Module
All PMAs included in the 2022 GSP remain applicable for the Subbasin. However, as described above, the majority of them were classified as “As Needed” and many of them have not been substantially advanced in the time since GSP adoption. During this period, no undesirable results occurred, and conditions in the Subbasin were generally around the measurable objectives (as of the end of WY 2025). Additionally, many of the PMAs are focused on efficient use and management of surface water supplies or on increasing data availability within the Subbasin, and thus, do not have a direct effect on groundwater sustainability (although they influence and support groundwater sustainability).
In summary, based on the description and project status updates provided in the preceding sections, the revised list contains eight remaining Butte Subbasin projects and management actions that are being implemented for SGMA compliance, as shown in Table 5.4.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 61 Table 5-4. Revised List of PMAs for the Butte Subbasin
Project or
Management Action
(PMA) ID
PMA Name PMA Description Lead GSA(s) PMA Status
(as of WY 2025) Summary of PMA Status Anticipated Completion Date 5-021.70_PMA_1 Project - System modernization
(Biggs-West Gridley Water District
[BWGWD] GSA)
BWGWD will upgrade and modernize system infrastructure to improve system operability and efficiency, reduce operational spillage, and enhance the timing of farm deliveries. Modernization improvements will include:
1. Improvements at canal headings to improve water level control, flow control, flow measurement, SCADA, and automation/measurement
2. Improvements at customer delivery turnouts to improve delivery flexibility and steadiness
3. Implementation of a CODSS
Biggs-West Gridley
Water District GSA
In Progress This project was nearly complete at the end of WY 2025, and was anticipated to be completed before the 2026 irrigation season.
Portions of the project were already completed and used by WY 2025.
The SCADA system has been integrated, and data for six operational water control sites are available in near real-time to District operators and other staff. Full project benefits are
expected to be realized following the 2026 water season.
WY 2026
5-021.70_PMA_3 Project - System modernization
(Richvale Irrigation District [RID]
GSA)
RID will upgrade and modernize system infrastructure to improve system operability and efficiency, reduce operational spillage, and enhance the timing of farm deliveries. Modernization improvements will include:
1. Improvements at canal headings to improve water level control, flow control, flow measurement, SCADA, and automation/measurement
2. Improvements at control structures to improve water level control, flow control, flow measurement, SCADA, and automation/measurement Richvale Irrigation
District GSA
Completed This project was completed at the beginning of WY 2025 with benefits fully realized during the 2025 irrigation season. Infrastructure improvements to 10 Main Canal level control check structure gates, construction of four flow measurement stations on the Main Canal, and SCADA integration are complete and associated water conservation benefits are being realized from improved infrastructure and operational data availability.
WY 2025
5-021.70_PMA_5 Project - Boundary flow and primary
spill measurement (Western Canal
Water District [WCWD] GSA)
WCWD will implement a boundary flow and primary spill measurement project at three key outflow sites in its distribution system. The boundary flow and primary spill measurement project is part of the district’s comprehensive plan for system modernization and boundary flow monitoring developed as part of the FRRAWMP. The three boundary outflow sites identified for this project represent approximately 70 percent of total district outflow. Without these improvements, WCWD water operators are unable to remotely monitor key boundary outflows, making it difficult to identify when system inflows do not meet irrigation demands or conversely, when system inflows exceed irrigation demands. As a result, significant operational spillage has historically occurred at the end of laterals. The time required for operators to manually measure outflows at the boundary outflow sites further diminishes the operators’ ability to make changes and adjustments to customer deliveries and upstream water control structures in the canal system, further increasing operational spillage.
Western Canal Water
District GSA
Completed This project was completed during WY 2023, and the monitoring and SCADA equipment has been used by WCWD in the time since to monitor boundary outflows, resulting in surface water conservation benefits.
WY 2023
5-021.70_PMA_6 Project - Dual source irrigation
systems (Butte Water District GSA)
BWD will incentivize the use of irrigation systems capable of using both surface water and groundwater. These systems will increase use of surface water, increase on-farm recharge of surface water, and reduce groundwater pumping.
Butte Water District
GSA - Butte
In Progress BWD is currently implementing a dual source irrigation project within the Sutter Subbasin portion of its service area (funded through Proposition 68), which is anticipated to result in benefits to adjacent areas, but it has not moved forward to the same extent on this project within the Butte Subbasin portion of its service area. However, since GSP adoption, during WYs 2022-2023, BWD did complete a
conceptual evaluation of dual source irrigation systems, including conceptual designs, cost estimates, and estimates of increased surface water use.
TBD
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 62 Project or
Management Action
(PMA) ID
PMA Name PMA Description Lead GSA(s) PMA Status
(as of WY 2025) Summary of PMA Status Anticipated Completion Date 5-021.70_PMA_7 Project - Multi-benefit recharge (RD
2106 GSA)
This PMA has shifted in scope since GSP adoption with a new focus on Rancho Llano Seco (within the RD 2106 GSA), although other GSAs may consider MBR in the future as opportunities arise.
The project, with guidance from conservation groups, will develop and implement an MBR program that will provide groundwater recharge through normal farming operations while also providing critical wetland habitat for waterbirds migrating along the Pacific Flyway. Fields with soil and cropping conditions conducive to groundwater recharge will be flooded and maintained with shallow depths. Sloughs and lakes with subsurface geology conducive to groundwater recharge will be used to store and transport water. Water will be sourced from existing water rights contracts, depending on availability. GSAs may also consider financial compensation for participation to offset field preparation, irrigation, and water costs, infrastructure and habitat restoration.
Reclamation District
No. 2106 GSA
In Progress This PMA has shifted in scope since GSP adoption with a new focus on Rancho Llano Seco (within the RD 2106 GSA), although other GSAs may consider MBR in the future as opportunities arise.
Rancho Llano Seco implemented a project in WY 2025 within a conservation easement on their land to fallow approximately 400 acres of agricultural land, transition it into wildlife habitat and refuge area, increasing total refuge area in the Subbasin and MBR.
Additionally, a related Llano Seco project in the 2025 WY with notable progress involved initial planning and design for restoration of a siphon structure to deliver surface water to an additional 1,0000 to 2,000 acres of wildlife habitat (wetlands and sloughs), which will provide additional MBR within the Subbasin.
TBD
5-021.70_PMA_9 Management Action - Installation of
additional shallow monitoring wells
GSAs will install 10 shallow monitoring wells in key areas of the Subbasin to support monitoring of interconnected surface water and groundwater dependent ecosystems.
County of Butte GSA In Progress This PMA has been advanced and is nearing completion. Through continued coordination with TSS, site locations were identified, agreements between landowners and GSAs were coordinated, and well construction is expected to be completed soon, allowing monitoring to begin.
WY 2026
5-021.70_PMA_20 M&T - Llano Seco fish screen
project
The M&T – Llano Seco Fish Screen Project will modify the M&T Chico Ranch Pump Station intake structures at the Sacramento River to clear inflows and incorporate fish screens. This project will reduce groundwater reliance and potential depletion, in case of failure of the diversion.
County of Butte GSA -
Butte 1
Completed Llano Seco Fish Screen Project was completed in fall 2021, has been maintained in subsequent years, and has provided benefits to the Subbasin since completion. The substantial volume of surface water that M&T Ranch and Rancho Llano Seco divert from the Sacramento River continues to offset this volume of groundwater from being pumped from the Subbasin.
WY 2021
5-021.70_PMA_26
(NEW)
M&T Ranch Pipeline This project will install a new pipeline to convey surface water to a 375- acre portion of M&T Ranch that was previously dependent on groundwater as its sole irrigation supply.
County of Butte GSA -
Butte 1
Completed This pipeline (which was completed in WY 2024) was utilized in WY 2025 to convey surface water to a 375-acre portion of M&T Ranch that was previously dependent on groundwater as its sole irrigation supply, providing in-lieu recharge. It is planned for it to be continually used in future years as well.
WY 2024
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 63
5.4.2 Process of Reviewing and Adding PMAs
The implementation and administration of specific PMAs is the responsibility of the lead GSA for each PMA (see Table 5-1). Implementation may be carried out directly by the lead GSA or through coordination with partnering agencies or other project proponents (potentially including other GSAs). GSAs regularly track and evaluate the implementation of Ongoing PMAs, frequently track and evaluate Planned PMAs, and periodically evaluate As Needed PMAs and their potential implementation. Individual GSAs are responsible for maintaining communications with project proponents, conducting public outreach associated with projects, and reporting PMA status updates to other Subbasin GSAs.
The GSAs’ managers generally meet monthly to review GSP implementation activities and discuss any PMA or other project implementation needs. The GSAs also meet periodically (2 to 3 times a year) throughout the year as part of the Butte Advisory Board (BAB).
Projects can be started by any GSA on a continuous basis. An annual roundtable meeting of GSA Managers will provide the forum for discussion on the need for PMA additions to decide whether new projects should be described in the Annual Reports as PMAs and added to the DWR PMA Module – any changes to the PMAs will be reported out to the BAB for concurrence, as needed. A more in-depth discussion will be held every five years during PE development.
As required under SGMA, the coordination on PMAs between GSAs and the BAB results in status updates and reporting at least annually as part of the Butte Subbasin’s GSP Annual Report.
The GSAs plan to focus implementation of PMAs in coming years on those directly related to groundwater sustainability, with the flexibility of implementing additional projects dependent on GSA priorities, available funding, and other opportunities and factors related to overall subbasin water management approaches.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 64 6 BASIN SETTING BASED ON NEW INFORMATION OR CHANGES IN
WATER USE
The Basin Setting described in the GSP remains relevant and no new substantial changes to the understanding of Basin Setting have been uncovered through the review of new information. The key new information that led to refinements in the Basin Setting description relate to improved HCM through review of DWR-provided AEM data, additional groundwater quality data review and analysis, a review of GDE health based on the original GSP footprint, and clarifications to the groundwater budget assumptions based on new modeling and refined analysis. Land use and water use in the subbasin remain similar to when the GSP was prepared.
6.1 Hydrogeologic Conceptual Model
The GSP summarized the HCM succinctly as follows: “the Butte Subbasin is comprised of a diverse mix of geologic units, including the Tuscan, Tehama, and Laguna Formations and a collection of younger formations and deposits referred to as the Quaternary Alluvium. These formations are comprised of volcanic sediments and layers of tuff breccias and clays (Tuscan); overlain by moderately consolidated sandstone, siltstone, sand, gravel, silt, and clay (Tehama and Laguna); overlain by unconsolidated gravel, sand, silt, and clay (Quaternary Alluvium).” The GSP also stated, “better understanding the hydrogeology, aquifer dynamics, and recharge paths of the aquifer systems in the Northern Sacramento Valley region is an area of active research.” Some of this research to address data gaps has been conducted through partnerships with Chico State, Stanford University, and partnerships with the Vina and Wyandotte Creek Subbasins. There are other grant funded project concepts that would further help evaluate groundwater recharge water sources and rates in the Subbasin. However, the GSAs were unable to allocate limited funds to this research without SGMA grant funding assistance from DWR.
There are still some areas of uncertainty in hydrogeologic understanding that, if filled, would help the GSAs better understand how to facilitate groundwater recharge. However, groundwater levels are relatively shallow and stable in most areas of the Subbasin from natural recharge and conjunctive use of surface water and groundwater. Therefore, understanding recharge mechanisms is not critical to the sustainable management of the Subbasin at this time and are not considered a data gap per SGMA. In addition, DWR is undertaking a large Basin Characterization Program with Sacramento Valley as one of the regional study areas, and the GSAs in Butte Subbasin will track these efforts for incorporation into their GSP implementation during the next evaluation period, as described below.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 65
6.1.1 Butte Subbasin Analysis
The Butte Subbasin HCM was re-analyzed during the evaluation cycle by CSU Chico Professor Dr. Todd Greene. The analysis reviewed the geologic layering in the subbasin based on DWR’s 2022 aerial electromagnetic (AEM) geophysical surveys and supporting lithology and geophysical data (Appendix 6A). The analysis verifies the HCM presented in the GSP, provides aquifer stratigraphy for the existing RMS monitoring wells, and was envisioned as a guide for selecting the location and screen intervals for future monitoring well sites.
The study confirmed that there are two distinct hydrogeologic units that comprise the principal aquifers in the Subbasin. The Tehama Formation was not identified as a principal aquifer in the analysis as it’s mostly on the western side of the Sacramento River and outside the Subbasin.
The main findings as they pertain to the GSP are summarized below:
• The northern area Principal and Very Deep Aquifer is mainly composed of the Tuscan Formation. This volcaniclastic formation (i.e., sourced by volcanic activity and lahars) consists of volcanic flows, consolidated tuff breccia, tuffaceous sandstone, and volcanic ash from the Cascade Range. The GSP describes this aquifer as deepening towards the center of the Sacramento Valley, with the deepest areas in the northeast portion of the subbasin. The aquifer is highly permeable in layers and hydraulically confined into four distinct water-bearing zones by tuff breccias and clay interbeds. The interbeds create physical separation between the Primary and Very Deep principal aquifers. • The southern area Principal Aquifer is mainly composed of the Laguna Formation. This fluviatile formation (i.e., deposited by rivers and streams), consists of a heterogeneous mixture of interbedded alluvial gravel, fine sand, silt, and clay of granitic and metamorphic origin from the Sierra Nevada Range. Like the Tuscan Formation, the aquifer also thickens to the west and center of the valley, but thins in the southern part of the Subbasin near the Sutter Buttes. The GSP describes the Laguna Formation as having low to moderate permeability, except in scattered gravels in the upper portion, and yields moderate quantities of water to wells along the eastern margin of the valley. There is not as clear of a physical separation between the Primary and Very Deep principal aquifers in the Laguna Formation as noted in the Tuscan Formation.
• Volcanic sediments from the Sutter Buttes Rampart formation influence the southern area of the Subbasin. This formation is volcaniclastic debris derived from the Sutter Buttes volcanic complex. The GSP notes that the Sutter Buttes are not a principal aquifer as they impede groundwater movement, causing groundwater to move upward and result in a shallow groundwater table and the formation of wetlands. Groundwater from the Sutter Buttes Rampart is known to have higher arsenic concentrations than the Laguna and Tuscan Formations.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 66 • It is still uncertain how the Tuscan and Laguna formations are laterally connected given the depth of investigation of the AEM data. Toward the east side of the Butte Subbasin, the Laguna Formation and the Lower Tuscan appear to be a continuous coarse-grained unit. However, this connection fades to the west both in AEM resistivity response as well as an apparent mismatch of aquifer depths. This is due to either deepening of the aquifers below the AEM survey limits, or more fine-grained materials on the west side leading to a lower resistivity response in the AEM data.
The new HCM information verifies that the Tuscan and Laguna Formations comprise the primary water supply aquifers in the Subbasin. Both formations are found at the ground surface to the east of the Subbasin where they receive direct recharge from overlying creeks and precipitation and deepen to the west. The depth and thickness of the formations is variable throughout the subbasin. In the southern portion of the subbasin the Sutter Buttes and subsurface Colusa Dome create an impediment to groundwater flow and potentially a source of poor-quality groundwater due to naturally occurring arsenic.
6.1.2 DWR Sacramento Valley Regional Investigation
DWR continues to develop tools and collect new information to investigate hydrogeologic settings throughout the state. DWR’s ongoing Sacramento Valley Regional Investigation focuses on providing data digitization, texture model development and analysis to support aquifer recharge potential mapping.2 DWR is also collaborating with the United States Geological Survey (USGS) to develop a refined salinity and base of freshwater map for the Sacramento Valley, which is expected to be released in late 2028. All of this new information will culminate in the development of a revised regional HCM for the Sacramento Valley. DWR has indicated a commitment to continue working with the GSAs in the Sacramento Valley to prioritize data collection and developing information relevant for GSP implementation to support GSAs with better planning of management actions. In particular, the GSAs within the Subbasins surrounding the Sutter Buttes are interested in better understanding the geologic formations responsible for degradation of water quality.
6.2 Groundwater Conditions
New groundwater conditions information has been gathered during GSP implementation, as described in Section 2. The subsections below describe new water quality and GDE information reviewed during the evaluation period to supplement information in the GSP. 2 https://data.cnra.ca.gov/dataset/dwr-basin-characterization/resource/fed9f9dd-fd0d-4ef6-a21b-a8835404b3c6 Draft Butte Subbasin Periodic Evaluation, September 2026 Page 67
6.2.1 New Water Quality Information
Butte County, with support from Glenn County for one well, currently monitors and reports salinity (measured as EC) at eight RMS in the Subbasin (Section 4.4, Section 7.3, and Appendix 4C). The documentation of potential migration of other contaminants in groundwater was identified as a missing element in DWR’s 2023 determination letter for the Subbasin’s initial GSP (Table 2-2). Specifically, DWR noted that a map identifying the locations of known groundwater contamination sites using publicly available data from the SWRCB’s GeoTracker website was missing from the GSP. Additionally, the State Water Resources Control Board provided a comment letter that identified nitrate, arsenic, and 1,2,3 – Trichloropropane (1,2,3- TCP) as potential water quality COC in the Subbasin (SWRCB, 2022). To address these comments, the GSAs reviewed historical and recent data related to these potential COCs and reviewed the presence of contaminant plumes in the Subbasin. The evaluation indicated that nitrate, arsenic, and 1,2,3-TCP are not constituents of major concern to be monitored and managed by the GSAs. The following subsections identify the locations of known degraded groundwater and contamination plumes, and summarize the results of the assessment using SWRCB publicly available data.
In general, this additional information and mapping confirmed information described in the GSP.
Consultation with DWR leading up to submittal of this Periodic Evaluation helped to clarify the continued approach for managing known water quality issues in the subbasin. The naturally occurring water quality issues must be managed at the basin scale through these actions:
1. Salinity: avoid pumping from deep wells below the base of freshwater to avoid upwelling of connate water
2. Arsenic: avoid locations known to have high arsenic for installation of new wells, install wellhead treatment or identify alternate source of water if drinking water wells are affected
6.2.1.1 Nitrate
Nitrate was assessed using the 25-year maximum concentration from 2000 to 2025 across the Butte Subbasin. Figure 6-1 shows the locations of 107 wells that have been monitored for nitrate.
In the 25-year period, only two wells had maximum concentrations at or above the MCL of 10 milligrams per Liter (mg/L); one is southeast of Gridley and the other in the northwest portion of the Subbasin near Angel Slough in Butte County (Figure 6-1). Additionally, nitrate monitoring in the Subbasin is currently conducted through the Irrigated Lands Regulatory Program (ILRP), as administered by the two Water Quality Coalitions in the Sacramento Valley: California Rice Commission and Sacramento Valley Water Quality Coalition. Therefore, at this time, GSA monitoring and management of nitrate in the Subbasin is not needed.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 68 Figure 6-1. Maximum Nitrate Concentration (mg/L) from 2000 to 2025 Draft Butte Subbasin Periodic Evaluation, September 2026 Page 69
6.2.1.2 Arsenic
Groundwater quality degradation due to the presence of high levels of arsenic is known to occur near the Sutter Buttes due to the regional geology. To reassess arsenic in the Subbasin, the 25- year maximum arsenic concentration from 2000 to 2025 for 107 supply wells was evaluated.
Figure 6-2 shows the locations of these wells. Of the 107 wells evaluated, 42% had maximum concentrations at or above the MCL of 10 micrograms per Liter (µg/L), many of which are clustered toward the south of the subbasin near the Sutter Buttes (Figure 6-2). A 2008 USGS study documented this area as having elevated levels of arsenic as well as other constituents, including total dissolved solids, chloride, and boron (USGS, 2008). As described in Section 4.4, poor water quality conditions near the Sutter Buttes are due to natural geologic conditions due to the volcanic formations of the Buttes.
Efforts to further characterize groundwater quality degradation due to arsenic in this area is underway and are being addressed through the Sutter Buttes Water Quality Working Group.
Under this working group, the GSAs of the Butte, Sutter, Yolo, North Yuba, and South Yuba Subbasins are coordinating on developing a plan to address the geologically-influenced groundwater quality issues associated with the Sutter Buttes volcanic formation and underlying Ramparts. Additional information of the working group’s approach and meetings is provided in Section 10.1.2.
Following a meeting with DWR SGMO technical staff with the Sutter Buttes Water Quality Working Group, it is clear that SGMA implementation should avoid exacerbating water quality degradation in areas with naturally occurring high levels of arsenic and salinity, due to the management actions of GSAs. Therefore, from a GSA perspective, the focus for arsenic will be on working with DWR to improve monitoring and understanding of the Sutter Buttes HCM, as well as education and outreach to well users in the affected areas.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 70 Figure 6-2. Maximum Arsenic Concentration (ug/L) from 2000 to 2025 Draft Butte Subbasin Periodic Evaluation, September 2026 Page 71
6.2.1.3 1,2,3 – Trichloropropane
1,2,3-TCP has historically been used as an additive to soil fumigants, making it an increasingly common issue in agricultural areas of the Central Valley. 1,2,3-TCP was evaluated by mapping the maximum historical concentration from 1989 to 2025 for 31 wells in the Subbasin (Figure 6-3). During this period, seven of the 31 wells near the City of Gridley had maximum concentrations at or above the MCL of 0.005 ug/L.
The seven wells with elevated concentrations of 1,2,3-TCP are located in an area with active cleanup program sites. Using publicly available data from SWRCB’s GeoTracker website, four active cleanup sites were identified in the subbasin that are undergoing site assessment, verification monitoring, or interim remedial actions. Details on these four cleanup sites, including the site name and their cleanup program status, are summarized in Table 6-1.
Figure 6-4 maps the location of these four sites, all of which are found south and east of the City of Gridley.
Based on review of case files, several wells identified with elevated 1,2,3-TCP levels may continue to be monitored under the active cleanup site programs. These cleanup site programs are regulated under the Central Valley Regional Water Quality Control Board and include specific monitoring and reporting requirements. While the GSAs are not responsible for monitoring or managing 1,2,3-TCP in the subbasin, awareness of the locations of elevated concentrations will help inform future groundwater projects and management actives in the subbasin.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 72 Figure 6-3. Maximum 1,2,3-TCP Concentration (ug/L) from 1989 to 2025 Draft Butte Subbasin Periodic Evaluation, September 2026 Page 73 Table 6-1. Summary of Active Cleanup Program Sites in the Butte Subbasin Site Name Cleanup Status Media Constituents of
Potential Concern Latitude Longitude
WILSON FAMILY FARM
RANCH LLC
Open -
Assessment &
Interim Remedial
Action
Under Investigation Tetrachloroethylene
(PCE), Diesel 39.335182 -121.656822
WILBUR-ELLIS 557
ORD RANCH RD
(FORMERLY JTF)
Open - Site
Assessment Soil
1,2,3-
Trichloropropane
(TCP), Other
Insecticides /
Pesticide /
Fumigants /
Herbicides
39.381111 -121.660885
WILBUR-ELLIS CO. Open - Site
Assessment Soil
1,2,3-
Trichloropropane
(TCP), Nitrate
39.3244 -121.67912
WILBUR-ELLIS CO,
FORMERLY JOHN
TAYLOR FERTILIZER
VNP SITE
Open - Verification
Monitoring Soil
1,2,3-
Trichloropropane
(TCP)
39.33422 -121.68304
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 74 Figure 6-4. Active Contamination Sites in the Butte Subbasin Draft Butte Subbasin Periodic Evaluation, September 2026 Page 75
6.2.2 Groundwater Dependent Ecosystem Analysis
The GSP identifies depletion of ISW as one of the sustainability indicators for which undesirable results must be avoided. Section ES-3.1.6 of the GSP explains that significant reductions in groundwater contributions to streams could adversely affect beneficial uses, including riparian habitat and GDEs. The GSP uses groundwater levels near streams as a proxy for monitoring potential depletions and sets minimum thresholds to protect shallower groundwater resources that support GDEs.
Mapped GDE polygons in the GSP were used as the spatial basis for this analysis. These polygons represent areas classified as “Likely a GDE” in Appendix 2E of the GSP and were developed using TNC’s Natural Communities Commonly Associated with Groundwater (NCCAG) dataset, with refinements by GSA managers that were based on local hydrologic knowledge, vegetation indicators, and drought year imagery.
An assessment of vegetation health conditions using Normalized Difference Vegetation Index (NDVI) data within mapped GDE regions in the subbasin was conducted over the 2020–2025 period (Appendix 6B). Results of this evaluation indicate that vegetation health across most GDE regions was generally healthy throughout the evaluation cycle. Table 6-2 summarizes the percentage of mapped “Likely GDE” acreage within the Butte Subbasin falling into each NDVI-based vegetation health category for each water year from 2020 through 2025. The table illustrates a temporary shift toward lower GDE vegetation health categories during the dry years of 2021–2022, followed by an increase in the proportion of acreage classified as very healthy during wetter conditions beginning in 2023.
Observed reductions in GDE vegetation health during the critically dry and below normal water years of 2020–2022 were moderate and were followed by recovery during wetter conditions beginning in 2023. These trends are consistent with expected ecological responses to climate variability and do not indicate widespread health declines or significant and unreasonable impacts to GDEs related to groundwater conditions in the Subbasin. The GDE health results summarized below describe temporal and regional patterns observed during the evaluation cycle.
6.2.2.1 GDE Health Regions
The GDE polygons in the Butte Subbasin are grouped into four regions: Big Chico Creek to Glenn County, Glenn County to Colusa County, South of Glenn County, and the Feather River corridor (Figure 6-5). Regionalization helps assess GDE vegetation health relative to local conditions.
GDE health conditions were evaluated using the NDVI and summarized using vegetation health categories based on commonly used NDVI thresholds by TNC and others (Table 6-2). These categories range from standing water or bare ground to very healthy GDE vegetation and are Draft Butte Subbasin Periodic Evaluation, September 2026 Page 76 used throughout this section and the associated figures to describe relative changes in GDE vegetation health over time.
Table 6-2. Summary of Butte Subbasin NDVI‑Based Vegetation Health Categories for Likely GDE Areas (2020 –2025)
Vegetation
Health
Category
NDVI
Range
Percent of Likely GDE Area
WY 2020 WY 2021 WY 2022 WY 2023 WY 2024 WY 2025
Standing Water
or Bare Ground < 0 0% 0% 0% 0% 0% 0%
Unhealthy 0 – 0.3 4% 5% 5% 4% 4% 3%
Poor Health 0.3 to 0.45 6% 12% 11% 5% 5% 6%
Moderately
Healthy 0.45 to 0.6 17% 26% 26% 13% 14% 12%
Very Healthy 0.6 to 1 73% 57% 57% 78% 77% 78%
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 77 Figure 6-5. Butte Subbasin GDE Regions
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 78
6.2.2.2 Overview of GDE Vegetation Health Conditions
Average NDVI values for each of the four GDE regions within the Butte Subbasin for the 2020– 2025 analysis period are shown on Figure 6-6, illustrating regional GDE health response to recent dry and wet conditions.
Figure 6-6. Regional NDVI Averages and Health (2020-2025) The Big Chico Creek to Glenn County, Glenn County to Colusa, and South of Glenn County regions display similar temporal patterns, with NDVI values recovering quickly following the dry period and stabilizing within the upper ‘Moderately Healthy’ to ‘Very Healthy’ range during 2023–2025 (Figure 6-6). The Feather River corridor region NDVI values fall within the ‘Moderately Healthy’ range throughout the 6-year period (Figure 6-6).
Figure 6-6 shows that during the critically dry and below normal WYs of 2020–2022, average NDVI values in all regions experienced a drop in health. During this drought, most regional values clustered near the upper portion of the ‘Moderately Healthy’ range, reflecting sustained but somewhat reduced GDE vegetation vigor. Beginning in WY2023, which was classified as a wet year, average NDVI values increased across all regions into the ‘Very Healthy’ category in Draft Butte Subbasin Periodic Evaluation, September 2026 Page 79 most regions. This improvement persisted into 2024 and 2025, two consecutive above normal years, illustrating strong GDE vegetation response to increased precipitation, soil moisture, and likely improved shallow groundwater availability.
6.2.2.3 Regional Vegetation Health Patterns
Examining each region reveals how GDE health changed during the 2022 drought, as well as its recovery by 2025.
Big Chico Creek to Glenn County, representing the northern region of the Subbasin, experienced only a slight reduction in NDVI during 2022. By 2025, GDE vegetation vigor had recovered, with average NDVI returning to ‘Very Healthy’ levels. This region continues to have the most stable GDE vegetation conditions in the Subbasin.
The trend observed in the Glenn to Colusa County region is very similar: health dipped slightly in 2022, then improved noticeably in 2023, and NDVI values remained high through 2025. Even in dry years, this region maintains some of the highest NDVI levels overall.
The region South of Glenn County experienced the most noticeable drought effect, with the largest NDVI decline among the four regions in 2022. However, it also shows the strongest rebound where NDVI increased substantially during 2023–2025, indicating robust GDE vegetation recovery following the multiyear drought.
In contrast, the Feather River corridor region has a more muted response, with NDVI values increasing steadily from the drought years through 2024 but remaining generally lower than the other three regions. Even so, average NDVI values in this region remain within the ‘Moderately Healthy’ range and show no abrupt or concerning declines.
Taken together, these regional patterns show that most GDEs in the Subbasin responded to drought as expected, experiencing moderate declines in 2022 followed by recovery during the wetter 2023 to 2025 period.
To support the regional trends described above, Table 6-3 provides a summary of key vegetation-health statistics for each GDE region, including total acreage, average NDVI ranges from 2020 to 2025, and the percent of vegetation falling within each health category. These statistics support the observed patterns on Figure 6-6 and show how each region responded to recent climate changes.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 80 Table 6-3. Summary of GDE Regional Range of 2020 to 2025 NDVI Annual Averages GDE Region Approximate GDE Area
(Acres)
Range of 2020 to 2025
Average NDVI
Big Chico Creek to
Glenn County 4,978 0.59 - 0.65
Moderately to Very Healthy
Glenn County to
Colusa County 2,678 0.59 - 0.67
Very Healthy
South of Glenn
County 1,785 0.56 - 0.68
Moderately to Very Healthy
Feather River
Corridor 754 0.45 - 0.55
Moderately to Very Healthy
Overall, the results indicate that GDE vegetation health across most of the Butte Subbasin remained generally moderate during dry conditions, with slight reductions in NDVI observed during drought years, followed by improvement during above normal and wet years. Vegetation health recovered to conditions comparable to or exceeding those observed in Water Year 2020, which followed a wet year, and currently reflects healthy ecological conditions. These findings are consistent with the expected hydrologic response to recent climate patterns and indicate no widespread GDE vegetation decline suggestive of significant or unreasonable impacts related to depletion of ISW or other groundwater conditions.
6.3 Water Use Changes and Associated Water Budget
This section provides an overall summary of hydrologic conditions and water use during this evaluation period, covering WYs 2021-2025 and compares these years to the historical, current, and projected water budgets in the GSP to assess potential water use changes. Water budgets for the evaluation period were developed using the BBGM, which was updated with recent land use, surface water flows and diversions, and climate inputs for WYs 2021-20243 . The section also
discusses sustainable yield and changes in storage for the Butte Subbasin. 3 Model input data were not available for WY 2025 for the BBGM at the time it was updated for the Periodic Evaluation, leading to the update through WY 2024 only. However, groundwater extraction estimates for WY 2025 were developed using the GEEEO model (see Appendix 6C for more method details), measured data, and other estimates as described in the WY 2025 Annual Report and are used here in the Periodic Evaluation for WY 2025 only. The results indicate comparable water use estimate for WYs 2024 and 2025, consistent with the same abovenormal hydrologic year type for those years.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 81
6.3.1 Water Use for WYs 2021-2025
This section compares water use (i.e., surface water and groundwater) during WYs 2021-2025 (Figure 6-7). BBGM groundwater extraction estimates were used for WYs 2021-2024, and Annual Report groundwater extraction estimates were used for WY 2025. Surface water use from the WYs 2021-2025 Annual Reports were summarized, based on volumes reported directly from water supplier records or collected from publicly available sources (e.g., water rights diversion records) where available. Missing surface water supply data was estimated based on available historical diversions data in similar water years. The BBGM relies on similar data for surface water use.
During the evaluation period, the annual average groundwater extraction was 198,100 AF, which is higher than groundwater extraction estimates in the GSP’s historical (142,200 AF) and current (162,800 AF) water budgets, but within the range of the projected water budgets (162,600- 210,500 AF). This elevated average is primarily attributed to two consecutive critical hydrologic years in WYs 2021 and 2022 following a dry year in WY 2020. During these critical years, surface water availability was reduced due to curtailment on the Feather and Sacramento Rivers, resulting in increased groundwater extractions to meet water demands.
Annual surface water use within the Subbasin in WY 2022 was 386,000 AF4 (less than half of
the average annual volume diverted without curtailment), while groundwater extractions for WYs 2021-2022 averaged approximately 242,900 AF. As hydrologic conditions improved in subsequent wet and above‑normal years, full surface water allocations became available, and annual surface water diversions into and application within the Subbasin during WYs 2023-2025 averaged 871,200 and 809,000 AF, respectively. Groundwater extractions correspondingly declined substantially to an average of 168,200 AF for WYs 2023-2025. These post‑curtailment groundwater extraction volumes are slightly higher but comparable to the GSP’s current water budget and lower than this PE’s current water budget (see Section 6.3.4).
Overall water use during this evaluation period is reflective of drastic changes in hydrology from the severe drought of WYs 2021-2022 to extremely wet conditions in WY 2023 and above average precipitation and conditions in WYs 2024-2025. Figure 6-7 shows a calculation and percentages of surface water and groundwater use relative to total water use for each Water Year from 2021 to 20255
.
4 During Periodic Evaluation development, an issue was identified with the surface water accounting for WY 2021, so it is not included here for comparison. See Section 6.3.1.2 for more details. Also, during WY 2022, only one volume was reported for surface water use in the Annual Report, but in WYs 2023-2025, both diversion and application volumes were reported.
5 During Periodic Evaluation development, an issue was identified with the surface water accounting for WY 2021.
The volume shown here is consistent with what was included in the WY 2021 Annual Report, but actual surface water diversions this year were lower. See Section 6.3.1.2 for more details.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 82 Figure 6-7. Annual Surface Water Diversions and Groundwater Extractions for WYs 2021-2025
6.3.1.1 Comparison of Periodic Evaluation and Annual Report Groundwater Extraction Estimates Two different water budget models were used for estimating water use in the Subbasin. The BBGM, as described above and in Section 6.4, was originally developed prior to the GSP, used in GSP development, and serves as the primary groundwater-surface water simulation model for the Subbasin. However, due to time and cost constraints associated with updating the BBGM annually, the Groundwater Extraction Estimates from Earth Observations (GEEEO) model was utilized as a complementary, cost-effective partial water budget approach for estimating agricultural groundwater extraction for the WYs 2022-2025 Annual Reports. GEEEO relies on OpenET6 data to define consumptive water demands, or evapotranspiration (ET), across irrigated areas within the Subbasin. GEEEO uses similar inputs and applies assumptions consistent with those used in the BBGM framework to estimate groundwater extractions. Additional details of GEEEO water use analysis methodology are provided in Appendix 6C. For the annual reports, municipal groundwater extractions were directly measured and reported, and rural residential groundwater extractions from private domestic wells were calculated based on an assumed per capita water use and estimated population.
6 OpenET produces geospatial ET data from satellite-based remote sensing analyses. The OpenET modeling approaches are also similar to the approaches used to quantify ET in the BBGM. Additional information about the OpenET team, data sources, and methodologies are available at: https://openetdata.org/. 79%
58%
83%
83%
85%
21%
42%
17% 17% 15%
0 100
200
300
400
500
600
700
800
900
1,000
2021 2022 2023 2024 2025
Water Volume (TAF)
Surface Water Diversions Groundwater Extractions
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 83 Overall, when comparing groundwater extraction estimates, no consistent bias is observed between the methods, although the GEEEO model estimates are slightly higher total volumes over the overlapping period from WYs 2021-2024. While differences between the methods are greater in individual years, the average annual difference is approximately 15,800 AF, which represents 8% of the average annual groundwater extraction (207,600 AF) simulated by the BBGM during WYs 2021-2024. This difference is relatively small, and both methods capture similar temporal trends, indicating that the results from the two approaches are generally consistent and reasonably comparable. When comparing average annual water budgets over a longer time frame, between 2015 and 2024, the two methods show a much smaller difference:
175,400 AF on average for the annual report method, and 178,700 AF on average with the updated BBGM. The reason for the greater similarity is that the GEEEO method was only used for 2021-2024 Annual Reports and associated data, while the values in the Annual Reports prior to 2018 are from the BBGM. The longer timeframe includes more similar data to compare.
6.3.1.2 Surface Water Use Discrepancy in 2021 Annual Report Surface water use reported in the Annual Report for 2021—which was a critical year—is notably higher than surface water use observed in 2022, also a critical year (Butte Subbasin GSAs, 2022). This discrepancy is attributed to large surface water diversions during the early portion of WY 2021, (i.e., winter water), as well as methodological differences in data compilation for that year. Review of the 2021 dataset indicates that the Subbasin had normal fall diversions between October and December 2020 before the curtailment in Feather and Sacramento Rivers took place for the 2021 irrigation season. At the start of WY 2022, fall diversions were severely reduced due to surface water curtailments and continuing drought conditions, and surface water supplies were curtailed again for the 2022 irrigation season. Although both WYs 2021 and 2022 are classified as critical years, the larger diversions during the early months of WY 2021 (i.e., October to December 2020) resulted in higher reported surface water use relative to the subsequent critical year.
In addition, methodological differences also contributed to the discrepancy. Specifically, the WY 2021 Annual Report presented diverted surface water volumes, whereas subsequent Annual Reports reported applied surface water volumes (Butte Subbasin GSAs, 2022). Finally, estimated surface water use in areas outside irrigation districts (i.e., white areas) was higher for the WY 2021 report and refined beginning with the WY 2022 report to better represent actual surface water availability in the Subbasin’s white areas (Butte Subbasin GSAs, 2023).
6.3.2 Land Use or Cropping Pattern Changes
Land use remained generally consistent during the evaluation period, with agricultural lands comprising approximately 80-82%, native vegetation accounting for 16-17%, and urban areas representing 2-3% of the total 264,500 acres in the Subbasin (Figure 6-8). During years affected Draft Butte Subbasin Periodic Evaluation, September 2026 Page 84 by surface water curtailments, fallowed agricultural lands increased substantially, as evidenced in 2022. The increase in fallowed land primarily corresponds to reductions in rice-growing acreage, reflecting constraints on surface water availability and fallowing of annual crops during curtailment years. These cropping patterns and trends are consistent with historical conditions, and no land use or cropping pattern changes occurred during these years that would substantially change Subbasin conditions or management approaches.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 85 Figure 6-8. 2024 Land Use in the Butte Subbasin
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 86
6.3.3 Hydrology and Surface Water Supply Reliability
The subbasin relies primarily on surface water to meet water demands when supplies are available and supplemented with groundwater during periods of reduced surface water availability (Figure 6-7, Figure 6-9). Surface water is crucial for maintaining groundwater sustainability in the Butte Subbasin.
During this evaluation period, surface water accounted for approximately 58-85% of total water use in the Subbasin, depending on hydrologic conditions and curtailment. In the critical hydrologic years of WYs 2021 and 2022, surface water deliveries from the Feather and Sacramento Rivers were curtailed and surface water contributed to a relatively smaller share of total water use (approximately 58% in WY 2022). In contrast, during years with full surface water allocations, surface water supplies provided a relatively larger share of total water use (approximately 83%-85% in WYs 2023-2025).
In general, in years characterized by drought and low precipitation and streamflow, reduced surface water supplies led to increased groundwater extraction and reduced recharge, which can cause a decline in groundwater storage. Contrastingly, in wet and above normal years such as WYs 2023-2025, substantial surface water supplies are used and less extraction occurs, resulting in stable or increasing groundwater storage. Therefore, the availability and reliability of surface water play an important role in maintaining groundwater sustainability within the Subbasin.
Although surface water supply availability varies hydrologically, it has generally been reliable over the long term. Over the past 20 years (four dry years and five critical years), surface water curtailments affecting the Subbasin occurred only during three critical hydrologic years (WYs 2015, 2021, and 2022) associated with two prolonged multi‑year drought periods. While two of the five years during this evaluation cycle included surface water curtailments, the longterm surface water supply reliability in the Subbasin is not anticipated to change or impact water budget assumptions, as curtailment on the Feather River cannot exceed 50% in a given year or 100% in a 7-year consecutive period based on the Settlement Agreement.
One state-wide program that may threaten the surface water supply reliability in the Subbasin in the future, is a series of agreements, to be approved by the State Water Resources Control Board (State Board) as an implementation pathway for an updated Bay-Delta Plan. Currently, the Healthy Rivers and Landscapes approach is the preferred alternative developed by the Sacramento Valley water rights holders. This approach would have little effect on recharge and groundwater levels, and ensure continued groundwater sustainability. However, if the unimpaired flow approach put forth by the State Board gets implemented, then reservoir levels will be impacted causing potential shortages for both fall and future irrigation seasons. These impacts will likely affect subbasin recharge and sustainability overall.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 87 Figure 6-9. Water Use Type in the Butte Subbasin (updated 2025) Draft Butte Subbasin Periodic Evaluation, September 2026 Page 88
6.3.4 Updates to Current and Projected Water Budgets
In the GSP, the current conditions water budget was developed using the BBGM-2020 (BBGM version 1.0, Section 6.4, Butte County, 2021) with 2015-2016 land use and surface water diversions, 2016-2018 average urban water demands, and 1971-2018 hydrology. While this approach appropriately represented current Subbasin conditions at the time of GSP development, it does not necessarily reflect the most recent conditions. Therefore, the current water budget in the periodic evaluation represents the average conditions over the most recent timeframe. The periodic evaluation Current Water Budget was developed using available data for this evaluation cycle as estimated for the last five years, for WY 2021-2025. WYs 2021-2024 rely on BBGM2025 (BBGM version 1.3; Woodard & Curran, 2025) results, which incorporated land use, surface water diversions, and urban water demands from the corresponding years. As described previously, groundwater extractions from WY 2025 Annual Report were used, along with Annual Report surface water use for WYs 2022-2025 (which rely on similar data as BBGM version 1.3).
Table 6-4 presents surface water diversions and groundwater extractions from the current water budgets of the GSP and PE. The PE’s current water budget includes average values for a time period that included surface water curtailment and full surface water supplies. Overall, groundwater extractions estimated in the PE’s current water budget are higher than that reported in the GSP’s current water budget. As discussed in Section 6.3.1, this difference is largely driven by groundwater extractions during the two critical years (WYs 2021-2022) with surface water curtailments. In the post-curtailment period with full surface water supply (WYs 2023-2025), surface water supplies and groundwater extractions in the PE’s current water budget are slightly higher but comparable to the GSP’s current water budget.
Table 6-4. GSP and PE Current Surface Water and Groundwater Use for WYs 1971-2018 and 2021-2025 Water Budget Scenario
Surface Water
Diversions
(AF)
Groundwater
Extractions
(AF)
Total Water Use
(AF)
GSP Current Water Budget
(Average of WYs 1971-2018) 837,500 162,800 1,000,300
PE Current Water Budget
(Average of WYs 2021-2025) 763,100 199,200 962,300
PE: Curtailment (WY 2022) 386,000 274,800 660,800
PE: Full Surface Water Supply
(WYs 2023-2025) 871,200 168,200 1,039,400
The PE’s current water budget includes average values during a five year period that included surface water curtailment and full surface water supplies. For context and comparison, the third and fourth scenarios in Table 6-4 provide recent examples of curtailment and full water supply Draft Butte Subbasin Periodic Evaluation, September 2026 Page 89 for comparison of contrasting conditions between water year types. This further shows the importance of surface water supply availability and management by the water district GSAs to maintain healthy groundwater levels and overall sustainable groundwater conditions in the Subbasin.
The full groundwater budget for the Butte Subbasin, as simulated by the updated BBGM is shown on Figure 6-10 for water years 1996 through 2024.
The cumulative change in storage shows a slight decline in the recent 10 years, stabilizing around -50 thousand acre-feet (TAF), after the 2012-2016 drought. The annual report method shows a better rebound of the change in storage with the cumulative change in storage closer to zero in 2025 (Figure 6-11).
Figure 6-10. Butte Subbasin Groundwater Budget, as Simulated by BBGM -1,000
-800
-600
-400
-200
0 200
400
600
800
1,000
-1,000
-800
-600
-400
-200
0 200
400
600
800
1,000
19961997199819992000200120022003200420052006200720082009201020112012201320142015201620172018201920202021202220232024 Cumulative Change in Groundwater Storage (thousand acre-feet) Annual Volume (thousand acre-feet)
Water Year
BBGM Version 1.2 Through WY 2024 Groundwater Budget for Butte Subbasin Change in Storage Interaction with Stream Deep Percolation Gain from Thermalito Reservoir Subsurface Flow from Outside BBGM Canal and Reservoir Recharge Groundwater Pumping Root Water Uptake Subsurface Flow with Adjacent Subbasins Cumulative Change in Storage
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 90 Figure 6-11. Groundwater Pumping and Change in Storage from WY 2025 Annual Report The three projected water budgets in the GSP representing no climate change, 2030 climate change, and 2070 climate changes conditions remain unchanged for this PE. These projections continue to represent the future projected conditions based on the best available information at the time of this PE. Notably, no changes to future land use and surface water supply are anticipated, and the Subbasin will continue to be managed similarly to today.
6.3.5 Updates to Sustainable Yield and Changes in Storage In the GSP, four sustainable yield values were calculated based on estimated groundwater extractions and groundwater storage change under current conditions and three projected water budget scenarios (Table 6-5). DWR commented in the GSP determination letter that a single sustainable yield value would be preferred to better describe the management approach for the Subbasin. To better reflect actual sustainable conditions based on observations from the most recent 10-year period, this Periodic Evaluation updates and establishes the subbasin’s sustainable yield at 182,300 AF using groundwater extractions and storage change estimates from WY 2015- 2024 from the BBGM. This 10-year period includes three years of surface water curtailment (2015, 2021, and 2022) and seven years of full surface water supplies. The resulting value is within the range of projected conditions from the GSP water budgets being higher than the future Draft Butte Subbasin Periodic Evaluation, September 2026 Page 91 scenario with no climate change impacts and lower than the future condition with 2030 climate change conditions.
Table 6-5. Estimated Groundwater Extractions, Storage Change, and Sustainable Yield for Four Water Budget Scenarios
Water Budget Scenario
Groundwater
Extractions
(AF)
Storage Change
(AF)
Sustainable Yield
(AF)
GSP (Current) 162,800 -1,200 161,600
GSP (Future, No Climate Change) 162,600 -1,300 161,300
GSP (Future, 2030 Climate Change) 189,400 -1,500 187,900 GSP (Future, 2070 Climate Change) 210,500 -2,000 208,500 GSP (Average) 181,300 -1,500 179,800
Periodic Evaluation (2015-2024) 178,700 +3,600 182,300
Groundwater in storage for the Periodic Evaluation is estimated as an average annual increase of approximately 3,600 AF between WY 2015-2024 (Table 6-5). In comparison, the GSP estimated a reduction in storage of between -1,200 and -2,000 AF for the various scenarios, resulting in an average reduction of 1,500 AF across all scenarios. The WYs 2021-2025 Annual Reports also use a separate method for estimating changes in groundwater storage as a complementary, costeffective alternative to annual updates to the BBGM7 . This method calculated a comparable but
slightly higher average annual increase of approximately 5,300 AF between 2015 and 2024, indicating the BBGM change in storage estimate is a conservative one for calculating sustainable yield.
6.3.6 Subbasin Conditions and Assessment of Overdraft
During the evaluation cycle, the Subbasin ’s groundwater conditions are considered sustainable.
The cumulative change in groundwater storage between WYs 2021 and 2024, as simulated by BBGM, is approximately -10,300 AF (an annual average decrease of 2,600 AF). This relatively small net decrease in storage is largely attributed to two consecutive critical hydrologic years with higher groundwater extractions. During the critical years (WY 2021-2022), annual changes in groundwater storage ranged from approximately -89,700 to -52,800 AF. As hydrologic conditions improved, groundwater storage rebounded quickly, with increases of approximately 111,800 AF in the wet year (WY 2023) and 20,400 AF in the above‑normal year (WY 2024).
These storage recoveries balance the earlier drought period declines and support sustainable conditions. As described above, if the period were expanded to include the most recent 10 years from 2015-2024, the calculated net storage change in the Subbasin is positive. Finally, no 7 This method is based on changes in groundwater level at RMS wells, storage coefficients based on BBGM assumptions and inputs, and representative areas associated with each RMS well. It is described and documented in the Annual Reports.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 92 overdraft conditions or undesirable results were observed in the Subbasin during this evaluation cycle.
6.4 Model Updates
The BBGM includes the Vina, Wyandotte Creek, and Butte Subbasin. The Vina and Wyandotte Creek GSAs received DWR SGMA grant funding to complete an update to the BBGM, the integrated groundwater surface water hydrologic model that was used during the initial GSP development for all three Subbasins.
The goal of this effort was to ensure that BBGM continues to serve as a reliable and practical tool for local analysis and for supporting SGMA implementation activities in the three Subbasins. The BBGM Version 1.0 (BBGM-2020; Butte County, 2021) used for GSP development was updated to:
• Revise model based on recommended improvements documented in the BBGM-2020 Report to create Version 1.2
• Enhance the model’s simulation of inter-basin flows and stream-aquifer interaction o The Sacramento River model boundary was identified as the primary area of focus for inter-basin flows and stream-aquifer interaction
• Update model code, IWFM, to most recent version available from DWR • Extend the timeseries of the revised model through WY 2024 to create Version 1.3 • Support 2027 GSP Periodic Evaluations in the three Butte County Subbasins • Encourage proactive coordination among technical teams working with other modeling tools in the region and with DWR to support future efficiencies and consistencies in model updates across basin boundaries
The resulting model, BBGM Version 1.3, was used for the water budget evaluation and estimates described in Section 6.3.4.
6.4.1 Key Model Update Findings and Recommendations for Future Modeling Efforts BBGM Version 1.3 is a useful tool that can be used as a baseline model for developing groundwater budgets and predictive scenarios for each of the Vina, Wyandotte Creek, and Butte Subbasin Periodic Evaluations.
The model timeframe extension requires substantial effort due to the large number of surface water diversions and pumping wells that are specified in the model on a daily time step. This requires thorough record keeping and developing assumptions when water districts do not collect Draft Butte Subbasin Periodic Evaluation, September 2026 Page 93 or provide comprehensive data required to create the model input files. Creating a more streamlined process for model updates is necessary for improved efficiency in updating the model in the future.
BBGM Version 1.3 performs better than BBGM Version 1.0 at stream boundaries without substantially modifying the calibration. However, compared to a regional model like C2VSimFG, stream depletion values are quite different. This shows that the complexity and level of detail of the inputs and the assumptions for parameters make a big difference, even if using the same overall code, IWFM.
Recommendations for future improvements include:
• Developing a streamlined model update workflow for efficiency improvement • Revisiting BBGM updates related to specific findings in model files, such as:
o Update the representation of boundary conditions for subsurface inflows from the foothills
o Reevaluate model layering with further processed AEM data, with DWR support o Refine pumping data after establishing a more streamlined process to receive accurate pumping data from agencies
o Update the boundary condition along Deer Creek in collaboration with Los Molinos Subbasin GSA
o The representation of the streams in BBGM should continue to be evaluated in future updates to the model.
• Future BBGM applications should continue to adopt the most current IWFM version to take advantage of ongoing support from DWR, improved runtime efficiency, and more reliable representation of groundwater and surface water processes. • Regional coordination on modeling and collaboration with DWR for future model updates
A few challenges were identified in using the BBGM for inter-basin flow evaluation:
• Sacramento River is a subbasin boundary, model boundary, and river boundary • Inter-basin flows versus stream-aquifer interaction flows are difficult to assess • Uncertainties regarding regional modeling and data updates • Continued IWFM code revisions complicate the decision process on model code selection
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 94 • Future C2VSim-FG updates from DWR will include major revisions incorporating new GSP and AEM data
• Unpredictable timing of model updates by DWR and neighboring subbasins In light of these identified challenges and the regional challenges of characterizing interconnected surface waters throughout the Sacramento Valley, it is anticipated that modeling efforts over the next implementation cycle (2027-2032) will focus on refinement of a regional model, such as C2VSim-FG, and close coordination with DWR to improve representation of groundwater conditions along river systems. This will be important for supporting regional coordination between GSAs related to interconnected surface water sustainability indicator and increasing confidence in estimates of inter-basin groundwater flows between subbasins.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 95 7 MONITORING NETWORKS
This section reviews all monitoring networks associated with the five sustainability indicators identified in the Butte Subbasin GSP including, groundwater levels and storage, groundwater quality, land subsidence, and interconnected surface water. Seawater intrusion does not apply to the Butte Subbasin and is therefore not described in this section. Groundwater storage is monitored by proxy through the groundwater level RMS network. A subset of shallow RMS wells in the Primary Aquifer groundwater level network are also used as a proxy to evaluate ISW as described in Section 7.2, however, not all ISW RMS are included in the groundwater level network. Since GSP approval, some of the Subbasin’s monitoring networks have changed through data gap filling and to address DWR’s RCAs. The following subsections describe these changes in detail, including an evaluation of remaining data gaps and SMC updates.
7.1 Groundwater Level and Groundwater Storage
Groundwater levels are monitored using a network of wells for evaluating three of the sustainability indicators. As defined in the GSP, the broad GSP groundwater level monitoring network consists of 89 wells, of which 51 were identified as RMS to evaluate groundwater conditions relative to the chronic lowering of groundwater levels SMC and as a proxy to evaluate the reduction of groundwater in storage (See Appendix 7A). A subset of wells from the groundwater level RMS network is used for evaluating ISW and therefore defines the ISW RMS network, as described in Section 7.2. The GSP classifies the 51 groundwater level RMS into two aquifer zones relative to the well screen or total depth – 41 wells in the Primary Aquifer are shallower than 700 feet deep and 10 wells in the Very Deep Aquifer are screened deeper than 700 feet. The wells are monitored by staff from DWR, various water districts, and Butte County.
Per GSP Regulations, RMS are monitored at least twice a year in the spring and fall, though some wells are monitored more frequently by the GSAs and others are equipped with transducers for hourly measurements. Figure 7-1 shows the locations of groundwater level RMS wells in the Primary and Very Deep Aquifer networks identified in the GSP.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 96 Figure 7-1. 2022 GSP Primary and Very Deep Aquifer Groundwater Level Representative Monitoring Site Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 97
7.1.1 Status of GSP Data Gaps for Groundwater Level and Groundwater Storage No data gaps were explicitly identified in the GSP for evaluating the chronic lowering of groundwater level or change in storage sustainability indicators. However, a lack of shallow monitoring wells was identified as a data gap for monitoring ISW and shallow groundwater that may support GDEs. Ten shallow well installation zones were identified in the GSP (Figure 7-2).
Figure 7-2. GSP-identified Primary Aquifer Water Level Monitoring Data Gaps Draft Butte Subbasin Periodic Evaluation, September 2026 Page 98 The Butte Subbasin received TSS from DWR to install shallow wells within the Primary Aquifer to fill the GSP-identified data gaps (Figure 7-2). Phase 1 of data gap filling, occurring through early 2026, addressed six data gaps. Four additional locations will be filled during a Phase 2 well installation, as discussed further in Section 7.2.1. Figure 7-3 Figure 7-3 shows the two phases of data gap filling and identifies the monitoring network data gap the wells are addressing. The following steps are being taken to fill the 10 data gap locations:
Phase 1 Existing Well Identification and Monitoring – One existing well was identified to address a GDE and groundwater level monitoring data gap in the Gray Lodge Wildlife Area identified as Zone 9 on Figure 7-3. The new monitoring well, 17N01E24A006M, is the shallowest well in an existing telemetered nested well. The screen interval is 30 to 75 feet below the top of the well casing.
• Phase 1 Monitoring Well Installations – Five new TSS monitoring well locations have been identified by DWR and will be installed after final construction logistics are addressed. The additional wells include three new wells for ISW monitoring and two new wells for GDE and groundwater level monitoring. It is anticipated these will be incorporated into the broad network for groundwater levels and will be considered in the future as designated ISW RMS.
• Phase 2 Monitoring Well Installations – Four remaining ISW monitoring data gaps will be filled during a planned second phase of well installations with DWR TSS support.
New wells to address data gaps will be monitored as part of the GSP monitoring network, but they will not yet be added to the RMS network since there is not enough data to develop SMC.
During the next Periodic Evaluation, the GSAs will assess the new well data, add the wells to the RMS network as appropriate, and assign relevant SMC.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 99 Figure 7-3. Primary Aquifer Groundwater Level Representative Monitoring Network Data Gap Filling Efforts Draft Butte Subbasin Periodic Evaluation, September 2026 Page 100
7.1.2 Monitoring Network Assessment for Groundwater Level and Groundwater Storage
An evaluation of the spatial and depth distribution of groundwater level monitoring wells shows that well coverage remains sufficient across the Subbasin and no additional data gaps are identified for this network. The only large areas with few monitoring wells are in the center of the Subbasin where rice is primarily grown. The relatively sparse monitoring well distribution in the center portion of the Subbasin is appropriate given that areas with rice growing have minimal groundwater pumping, surface water for irrigation and incidental recharge, and shallow and stable observed groundwater levels.
The groundwater level RMS network was assessed to determine each well’s suitability for evaluating groundwater conditions. Factors considered in the assessment include availability of construction information such as well depth or screen intervals, a history of access issues, and the reliability and quality of measurements. Six wells in the groundwater level Primary Aquifer network did not meet one or more of these quality objectives (Table 7-1). However, despite their limitations, the wells still provide useful groundwater level monitoring data as they fill a spatial data gap or have been continuously monitored for decades. Four of the wells flagged in the assessment will be kept in the RMS network as they continue to be evaluated for well functionality or replacement (Figure 7-4). Wells 18N03E18F001M and 20N02E16P001M will be removed from the RMS network but will continue to be monitored with the other non-RMS wells in the broad groundwater level monitoring network.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 101 Table 7-1. Groundwater Level RMS Assessment Summary
State Well Number Total Depth
(bgs)
Screen
Interval
(feet bgs)
Well Assessment Notes Assessment Results
17N03E08K002M 100 Unavailable Missing perforation data with a total depth of 100 feet bgs
Retain as RMS. The well is
sufficiently shallow so that the
unknown screen interval does not
affect the usefulness of the data.
18N03E18F001M 462 100-150,
190-220
Presence of oil has resulted
in frequent questionable
measurements (QMs).
Remove from groundwater level
Primary Aquifer RMS network.
18N03E21G001M 125 Unavailable
Missing perforation data and
intermittent presence of oil
has resulted in QMs.
However, there is a long
historical record of data and
removal of well would create
data gaps in both
groundwater level and ISW
networks.
Retain as RMS. Continue to
assess oil impact on future
measurements and re-evaluate
well functionality.
The well is sufficiently shallow so
that the unknown screen interval
does not affect the usefulness of
the data.
19N01E09Q001M 200 140-200
Intermittent presence of oil in
water has resulted in QMs.
Removal of well would
create spatial data gap in
network.
Retain as RMS and continue to
assess oil impact on future
measurements and re-evaluate
well functionality.
20N02E16P001M 466 Unavailable Missing perforation data with a total depth of 466 feet bgs.
Remove from groundwater level
Primary Aquifer RMS network.
21N01W23J001M 54 Unavailable
Missing perforation data, but
total depth is shallow (54
feet bgs). The well is close to
where TSS well, Zone 1
M&T Ranch will be installed.
Retain as RMS and potentially be
replaced by a new TSS well in the
future.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 102 Figure 7-4. Revised Primary Aquifer Groundwater Level Representative Monitoring Site Network with Identified Wells to be Removed
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 103
7.1.3 2027 Representative Monitoring Networks
The representative monitoring networks were revised to remove two wells in the Primary Aquifer, and now include a total of 49 groundwater level RMS wells. Figure 7-5 and Figure 7-6 show the 2027 Primary Aquifer and Very Deep Aquifer groundwater level RMS networks, respectively. Table 7-2 summarizes well construction information for each groundwater level RMS well. The full Subbasin-wide broad groundwater level monitoring network is provided in tabular format in Appendix 7A and includes a total of 81 groundwater level monitoring network wells.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 104 Figure 7-5. 2027 Primary Aquifer Groundwater Level Representative Monitoring Site Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 105 Figure 7-6. 2027 Very Deep Aquifer Groundwater Level Representative Monitoring Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 106 Table 7-2. 2027 Groundwater Level Representative Monitoring Network Well Construction Information State Well Number Principal Aquifer Total Well Depth
(feet bgs)
Top of Screen
Interval
(feet bgs)
Bottom of Screen
Interval
(feet bgs)
17N01E06D001M Primary 500 110 500
17N01E10A001M Primary 110 66 110
17N01E17F001M Primary 160 130 150
17N01E24A006M Primary 75 45 55
17N01W10A004M Primary 117 88 98
17N01W27A003M Primary 203 160 170
17N02E14A001M Primary 102 70 102
17N02E14H001M Primary 102 60 102
17N03E08K002M Primary 100 --- ---
18N01E13A002M Primary 317 80 317
18N01E15D002M Primary 112 56 112
18N01W02E003M Primary 200 110 120
18N01W14B001M Primary 204 59 173
18N01W17G001M Primary 108 48 108
18N01W22L001M Primary 124 76 124
18N02E16F001M Primary 80 20 60
18N02E25M001M Primary 220 61 220
18N03E08B003M Primary 463 156 463
18N03E21G001M Primary 125 --- ---
19N01E09Q001M Primary 200 140 200
19N01E27Q001M Primary 548 260 280
19N01E35B001M Primary 156 85 135
19N01W15D002M Primary 300 250 295
19N01W22D007M Primary 120 80 90
19N01W27R001M Primary 465 68 108
19N02E07K004M Primary 192 140 150
19N02E13Q001M Primary 221 130 210
19N03E05N002M Primary 180 24 48
20N01E18L003M Primary 173 100 110
20N01E35C001M Primary 92 50 92
20N01W11N002M Primary 170 84 170
20N02E15H001M Primary 180 170 180
20N02E28N001M Primary 277 160 277
21N01E08K002M Primary 181 19 181
21N01W11A002M Primary 200 120 185
21N01W13J003M Primary 400 355 385
21N01W23J001M Primary 54 --- ---
21N01W35K002M Primary 270 75 135
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 107 State Well Number Principal Aquifer Total Well Depth
(feet bgs)
Top of Screen
Interval
(feet bgs)
Bottom of Screen
Interval
(feet bgs)
22N01E32E004M Primary 160 85 160
17N01E24A003M Very Deep 833 770 790
17N01W10A001M Very Deep 828 770 800
18N01E35L001M Very Deep 899 816 836
18N01W02E001M Very Deep 739 719 729
19N01E35B002M Very Deep 980 930 950
19N01W22D004M Very Deep 820 780 790
19N02E13Q003M Very Deep 690 670 680
20N01E18L001M Very Deep 940 767 894
21N01W11A001M Very Deep 1311 810 1280
21N01W13J001M Very Deep 830 780 820
--- Screen interval unavailable
7.1.4 Sustainable Management Criteria Updates
With the revision to MOs and MTs discussed in Section 4.1.1, most MO/MT values only changed slightly, although a few had more significant changes. The majority of the revised MOs /MTs are shallower (i.e., more protective of beneficial users) than those reported in the GSP. On average, the MTs increased 3 feet in the Primary Aquifer and decreased 1 foot in the Very Deep Aquifer. MOs increased in both aquifers on average, increasing 5 feet in the Primary Aquifer and 1 foot in the Very Deep Aquifer. A summary table of MO/MT revisions from the GSP to 2027 values is compiled in Table 7-3. This summary from the 2023 analysis includes two wells that are being removed from the RMS network following this Periodic Evaluation.
Table 7-3. Summary of MO/MT Revisions for Groundwater Level Representative Monitoring Network Primary Aquifer Very Deep Aquifer
MO MT MO MT
RMS Wells in GSP
(Number of wells) 41 41 10 10
RMS Wells Revised
(Number of wells) 34 30 1 4
Average Change (feet) 5 3 1 -1
Minimum Change (feet) -1 -36 0 -7
Maximum Change (feet) 12 27 1 1
Although groundwater elevation MTs are now on average 3 feet shallower in the Primary Aquifer and only 1 foot deeper in the deep aquifer, there are some individual wells with greater downward adjustments that were reviewed to ensure that beneficial users are being protected.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 108 The wells with MT decreases greater than 10 feet are all along the Vina Subbasin boundary and are calculated using the range of measured groundwater levels below the observed historic low method. Specific wells with a greater than 10-foot change in MT include the following:
• 20N02E16P001M (-36 feet) is no longer an RMS well due to history of QMs which likely contributes to measurements used for recalculating the MT. Therefore, the change is no longer applicable.
• 20N02E15H001M (-25 feet; Appendix 4B) near Little Dry Creek on the Vina Subbasin boundary.
• 21N01E08K002M (-17 feet; Appendix 4B) and 21N01W13J003M (-11 feet; Appendix 4B) are in close proximity on the Vina Subbasin boundary in the northeast corner of the Butte Subbasin.
Common observations for wells with a lower revised MT include:
• Wells with seasonal and temporal observations that cover a relatively long time period can have a wider range of fluctuation than newer wells. Therefore, the range of measured groundwater levels below the observed historic low method is used to estimate the MT, which results in a relatively large downward adjustment. • The wells are on the Vina Subbasin border, aligning a bit better with the Vina Subbasin’s current MTs.
• Wells with the largest decrease in MTs are in areas with very few, if any, domestic wells.
Therefore, lower groundwater elevation MTs in these areas are not calculated using the shallowest 7th percentile of nearby domestic wells and significant impacts to domestic wells are not likely.
• Finally, lowering the groundwater level MT in these areas is not expected to impact the Subbasin’s ability to achieve sustainability. No other undesirable results or impacts to beneficial users have been observed in these areas for the other sustainability indicators.
In summary, the groundwater level SMC were revised with GSP adopted methods using a refined dataset, are not worsening potential impacts to beneficial users (Section 4.1.3.1), align with other sustainability indicators, and do not prevent the neighboring subbasins from achieving sustainability. The revised MO/MT values are included in the WY 2023-2025 Butte Subbasin Annual Reports, reported out to the GSA Advisory Board, and have replaced the SMC presented in the GSP (Butte Subbasin GSAs, 2024; Butte Subbasin GSAs, 2025; Butte Subbasin GSAs, 2026). Revised values are provided in Table 7-4, and shown on Figure 7-7 through Figure 7-10.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 109 Table 7-4. Summary of 2027 Measurable Objectives and Minimum Thresholds for Groundwater Levels RMS State Well Number Principal Aquifer MO
(feet amsl)
MT
(feet amsl)
17N01E06D001M Primary 59 28
17N01E10A001M Primary 58 31
17N01E17F001M Primary 52 25
17N01E24A006M Primary 67 44
17N01W10A004M Primary 60 34
17N01W27A003M Primary 54 27
17N02E14A001M Primary 78 50
17N02E14H001M Primary 75 42
17N03E08K002M Primary 83 58
18N01E13A002M Primary 75 39
18N01E15D002M Primary 69 41
18N01W02E003M Primary 60 19
18N01W14B001M Primary 58 19
18N01W17G001M Primary 61 30
18N01W22L001M Primary 62 33
18N02E16F001M Primary 77 53
18N02E25M001M Primary 81 51
18N03E08B003M Primary 93 60
18N03E21G001M Primary 84 70
19N01E09Q001M Primary 87 58
19N01E27Q001M Primary 82 33
19N01E35B001M Primary 82 52
19N01W15D002M Primary 72 40
19N01W22D007M Primary 68 33
19N01W27R001M Primary 62 22
19N02E07K004M Primary 99 68
19N02E13Q001M Primary 114 88
19N03E05N002M Primary 114 65
20N01E18L003M Primary 102 79
20N01E35C001M Primary 99 75
20N01W11N002M Primary 93 70
20N02E15H001M Primary 100 41
20N02E28N001M Primary 117 93
21N01E08K002M Primary 104 50
21N01W11A002M Primary 114 92
21N01W13J003M Primary 103 57
21N01W23J001M Primary 106 80
21N01W35K002M Primary 98 74
22N01E32E004M Primary 113 78
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 110 State Well Number Principal Aquifer MO
(feet amsl)
MT
(feet amsl)
17N01E24A003M Very Deep 70 39
17N01W10A001M Very Deep 55 0
18N01E35L001M Very Deep 69 34
18N01W02E001M Very Deep 68 20
19N01E35B002M Very Deep 87 54
19N01W22D004M Very Deep 76 40
19N02E13Q003M Very Deep 114 86
20N01E18L001M Very Deep 96 45
21N01W11A001M Very Deep 107 75
21N01W13J001M Very Deep 102 66
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 111 Figure 7-7. 2027 Measurable Objectives for Groundwater Level Primary Aquifer RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 112 Figure 7-8. 2027 Minimum Thresholds for Groundwater Level Primary Aquifer RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 113 Figure 7-9. 2027 Measurable Objectives for Groundwater Level Very Deep Aquifer RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 114 Figure 7-10. 2027 Minimum Thresholds for Groundwater Level Very Deep Aquifer RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 115
7.2 Interconnected Surface Water
The ISW RMS network described in the GSP consisted of 12 shallow wells within 1 mile of an interconnected stream and four stream gages. Interconnected streams with monitoring include the Sacramento River, Butte Creek, Little Dry Creek, Dry Creek, Angel Slough, and the Feather River. Table 7-5 summarizes the four different surface water gages for collecting stream stage data reported on the California Data Exchange Center (CDEC)8. The ISW RMS includes a subset of eight wells in the Groundwater Level Primary Aquifer RMS network, discussed in Section 7.1, and four other shallow wells that are unique to the ISW network. Figure 7-11 shows the locations of the ISW RMS identified in the GSP.
Table 7-5. Butte Subbasin Surface Water Gages
Stream Monitored Gage ID Gage Network Measurement
Frequency
Cherokee Canal Near Richvale CHC CDEC hourly
Feather River Near Gridley GRL CDEC hourly
Sacramento River at Butte City BTC CDEC hourly
Sacramento River at Ord Ferry ORD CDEC hourly
8 https://cdec.water.ca.gov/riv_flows.html
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 116 Figure 7-11. 2022 GSP Interconnected Surface Water RMS Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 117
7.2.1 Status of GSP Data Gaps for Interconnected Surface Water The spatial distribution of available shallow monitoring wells in the GSP was not adequate and some areas were identified as data gaps as shown on Figure 7-2. The data gaps include seven proposed locations that would improve monitoring within 1 mile of an interconnected stream. As the first phase of filling these data gaps, five locations are planned to be installed by DWR in WY 2026 through the TSS program. The new wells will be equipped with transducers for monitoring groundwater levels daily. The final four planned wells will be installed in a second phase once site access has been acquired and coordination completed with DWR. Figure 7-12 shows the locations of the planned TSS wells (Phase 1 and Phase 2) to fill ISW data gaps.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 118 Figure 7-12. Revised Interconnected Surface Water RMS Network with Remaining Data Gaps Draft Butte Subbasin Periodic Evaluation, September 2026 Page 119
7.2.2 Monitoring Network Assessment for Interconnected Surface Water The ISW RMS well network was assessed by reviewing available construction information, QMs, and other factors that preclude a well from being a suitable monitoring location. The analysis performed is similar to Section 7.1.2 for evaluating the groundwater level RMS network. Table 7-6 summarizes the results from the ISW RMS well assessment for three wells.
Since GSP development, one ISW RMS well, 18N01E05D002M, has been dropped from the network due to safety and accessibility issues. The well is located near the planned TSS well “Zone 5 Howard Slough Unit UBBWA.” Therefore, the new TSS well is anticipated to replace 18N01E05D002M in the RMS network once it has been installed and sufficient data has been collected to establish SMC. in the future. At least five years of data is desirable before assigning SMC to a new monitoring well.
Two ISW RMS wells, 18N03E21G001M and 17N03E05C003M, have missing perforation data with total depths between 120 and 125 feet bgs. These two wells are located along the eastern border of the Subbasin and remain the only two wells in the RMS network suitable for measuring shallow groundwater levels along the Feather River. Therefore, these two wells are retained in the ISW RMS network.
Well construction information may be collected through outreach or video logging; however, as explained in the groundwater level section this missing data does not prevent these shallow monitoring network wells from being useful ISW RMS locations.
Table 7-6. Butte Subbasin Interconnected Surface Water RMS Assessment Summary State Well
Number
Total
Depth
(feet bgs)
Screen
Interval
(feet bgs)
Well Assessment Notes Potential Action
17N03E05C003M 120 Unavailable Missing perforation data with a total well depth of 120 ft bgs.
Retain as RMS. The well is
sufficiently shallow so that the
unknown screen interval does
not affect the usefulness of the
data.
18N01E05D002M 240 120-240
No longer accessible. Well is
located near a planned TSS
well, which can effectively
replace this well, resulting in no
new data gaps.
Remove from ISW RMS network
18N03E21G001M 125 Unavailable
Missing perforation data and
historical QMs due to oil in
water. There is a long historical
record of data. Removal of well
would create data gaps in both
groundwater level and ISW
networks.
Retain as RMS. Continue to
assess oil impact on future
measurements and re-evaluate
well functionality.
The well is sufficiently shallow
so that the unknown screen
interval does not affect the
usefulness of the data.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 120
7.2.3 2027 Representative Monitoring Network for Interconnected Surface Water The ISW monitoring networks are being revised through data gap filling and assessment of existing locations. Table 7-7 summarizes well construction information for the 2027 ISW RMS network. Figure 7-13 shows the 2027 ISW RMS network, removing one location that is no longer accessible.
Table 7-7. 2027 ISW Representative Monitoring Network Well Construction Information State Well Number Total Well Depth
(feet)
Depth of Screened Zone(s)
(feet bgs)
Included in Groundwater
Level RMS Network?
17N03E05C003M 120 Unavailable No
17N03E16N001M 178 48-178 No
18N01W14B001M 204 59-83,
145-173 Yes
18N01W17G001M 108 48-108 Yes
18N02E16F001M 80 20-60 Yes
18N03E21G001M 125 Unavailable Yes
19N01E35B001M 156 85-95,
125-135 Yes
19N01W22D007M 120 80-90 Yes
19N01W27R001M 465 68-108 Yes
20N01E35C001M 92 50-92 Yes
20N02E15H002M 75 55-65 No
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 121 Figure 7-13. 2027 Interconnected Surface Water RMS Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 122
7.2.4 Sustainable Management Criteria Updates for Interconnected Surface Water As discussed in Section 7.1.4, the MOs/MTs were re-calculated for some of the groundwater level RMS wells, some of which are also part of the ISW network (Appendix 4B). The MO decreased by 1 foot at 19N01E35B001M and the MT decreased by 1 foot at 19N01W22D007, otherwise the recalculation of MOs and MTs resulted in the same or higher groundwater elevations. Therefore, the revised MO/MT are more protective of beneficial users than the values in the GSP. The MO/MT revisions are summarized in Table 7-8 and the revised SMC are listed for each RMS in Table 7-9.
Table 7-8. Summary of MO/MT Revisions for ISW Representative Monitoring Network MO MT
RMS Wells in GSP
(Number of wells) 12 12
RMS Wells Revised
(Number of wells) 12 12
Average Change
(feet) 6 6
Minimum Change (feet) -1 -1
Maximum Change (feet) 20 12
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 123 Table 7-9. Summary of 2027 Measurable Objectives and Minimum Thresholds for ISW RMS State Well Number
Total Well
Depth
(feet)
Depth of
Screened
Zone(s)
(feet bgs)
Included in
Groundwater Level
RMS Network?
MO
(ft amsl)
MT
(ft amsl)
17N03E05C003M 120 Unavailable No 80 47
17N03E16N001M 178 48-178 No 71 42
18N01W14B001M 204 59-83,
145-173 Yes 58 28
18N01W17G001M 108 48-108 Yes 61 44
18N02E16F001M 80 20-60 Yes 77 63
18N03E21G001M 125 Unavailable Yes 84 68
19N01E35B001M 156 85-95,
125-135 Yes 82 62
19N01W22D007M 120 80-90 Yes 68 47
19N01W27R001M 465 68-108 Yes 62 34
20N01E35C001M 92 50-92 Yes 99 85
20N02E15H002M 75 55-65 No 121 105
Figure 7-14 and Figure 7-15 map the MOs and MTs for the 2027 11 ISW RMS wells.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 124 Figure 7-14. Measurable Objectives for Interconnected Surface Water RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 125 Figure 7-15. Minimum Thresholds for Interconnected Surface Water RMS Wells Draft Butte Subbasin Periodic Evaluation, September 2026 Page 126 Out of the 11 2027 ISW RMS wells, eight are also part of the groundwater level Primary Aquifer RMS network. As described in Section 4.2.1, the ISW MO methodology is the same as the groundwater level sustainability indicator, however the MTs are set using different methods. The ISW MTs are generally 10 feet higher in elevation than the groundwater level RMS MTs. This means that ISW MTs would be exceeded before groundwater level MTs, making the ISW approach more conservative. Table 7-10 compares the MO and MT for the eight ISW wells that are also included in the groundwater level RMS network.
Table 7-10. Summary of ISW and Groundwater Level SMC Comparison State Well Number
ISW
Groundwater
Level MO
(feet amsl)
ISW MT
(feet amsl)
Groundwater
Level MT
(feet amsl)
MT
Difference
(feet)
18N01W14B001M 58 28 19 9
18N01W17G001M 61 44 30 14
18N02E16F001M 77 63 53 10
18N03E21G001M 84 68 70 -2
19N01E35B001M 82 62 52 10
19N01W22D007M 68 47 33 14
19N01W27R001M 62 34 22 12
20N01E35C001M 99 85 75 10
7.3 Groundwater Quality
A total of eight wells were identified in the GSP as the Subbasin’s water quality RMS network.
The eight RMS wells are all dedicated monitoring wells except for one well, 19N01W28A001M, which is an agricultural production well. The GSP identifies salinity as the main constituent of concern and EC as the analytical method for evaluating salinity conditions. The RMS wells are monitored annually in August by Butte and Glenn County staff using conductivity meters.
One minor change to the monitoring network has been made since the GSP. In 2024, Butte County measured well 20N01E18L002M, a shallower well in the same nested well cluster as the original RMS well, 20N01E18L001M, due to obstructions preventing field sampling at appropriate depths within the well screened intervals (Appendix 4C). Since 20N01E18L001, which was screened from 767 to 894 feet, is not accessible for proper sampling, it is now replaced with 20N01E18L002, which is screened from 510 to 560 feet bgs in the same nested well cluster. This modification to the monitoring network is acceptable as the locations both Draft Butte Subbasin Periodic Evaluation, September 2026 Page 127 access groundwater in an area that has demonstrated no evidence of EC degradation though the new well is located within the Primary instead of the Very Deep Principal Aquifer. Table 7-11 summarizes the revised network of water quality RMS wells. Figure 7-16 shows the 2027 monitoring network described in the GSP with the location of the well that was removed and replaced (20N01E18L001M replaced by 20N01E18L002M).
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 128 Table 7-11. Summary of Groundwater Quality Monitoring Network State Well Number Total Well Depth
(feet bgs)
Screened Interval
(feet bgs) Type of Well Notes
17N01E24A003M 833 770-790 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network
17N01W10A001M 828 770-780
790-800 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network 18N01E35L001M* 899 816-836 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network
19N01E35B002M* 980 930-950 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network
19N01W28A001M 140 120-140 Production
(Irrigation)
Original GSP well only in the Water Quality Representative Monitoring Network
19N02E13Q003M 690 670-680 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network
20N01E18L002M 581 510-530
550-560 Observation
Added to network in 2024 as an alternative to 20N01E18L001M. Well is in the Primary Aquifer and not in the Groundwater Level Representative Monitoring Network.
21N01W13J001M 830 780-820 Observation Original GSP well included in the Very Deep Aquifer Groundwater Level Representative Monitoring Network
*Also extensometer site in the subsidence GSP monitoring network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 129 Figure 7-16. Water Quality Representative Monitoring Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 130
7.3.1 Status of GSP Data Gaps for Groundwater Quality
There are no data gaps associated with water quality monitoring identified in the GSP.
7.3.2 Monitoring Network Assessment for Groundwater Quality The 2027 water quality RMS network is sufficient for monitoring salinity as EC in the Subbasin and no new data gaps have been identified. Ongoing monitoring will include assessments on network performance, and any future modifications will be documented in subsequent annual reports and periodic evaluations.
There are two wells that could potentially be replaced in the future if a viable option was identified. Original well 20N01E18L001M has extensometer equipment preventing monitoring access, so it was replaced by 20N01E18L002M, which is a shallower well with a screen interval from 510 to 560 feet. This replacement well also appears to have some obstructions preventing the monitoring equipment from being lowered to the bottom of the well. The GSAs have been notified that DWR intends to perform extensometer retrofitting in the Sacramento Valley, so the obstructions in the original or replacement well could potentially be addressed by DWR’s work.
As an alternative, well 19N01W15D002M from the Primary Principal Aquifer RMS network could be used as a replacement EC sampling well.
Glenn County well 19N01W28A001M is screened from 120 to 140 feet deep, which is much shallower than the other locations in the network. Ideally a deeper well would be identified in this area of the Subbasin for monitoring potential upwelling of higher salinity connate water from beneath the freshwater aquifers. An initial review showed that there are two potential wells from the Very Deep Principal Aquifer RMS network that could be used as a replacement:
• Well 18N01W02E001M – Total Depth:739 feet; screened interval: 719-729 feet, effective screen depth 680-760 feet; there was one record found of EC at 1,094 µS/cm. • Well 19N01W22D004M – Total Depth: 820 feet, screened interval: 780-790 feet, effective screen depth 710-820 feet; there were four records found of EC all below 600 µS/cm.
The GSAs will investigate these proposed alternative wells for both of these locations during this next evaluation cycle. If there are better wells available that can be reliably sampled, these two problematic wells could be replaced in the RMS monitoring network, and the change would be documented in annual reports. A revised full network evaluation regarding RMS replacements will be completed for the next Periodic Evaluation.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 131
7.3.3 Sustainable Management Criteria Update for Groundwater Quality There are no changes needed to the SMC for degraded water quality other than a clarification of the undesirable result definition to better align with the current monitoring protocols. As discussed in Section 5.4.2, one RMS well 17N01W10A001M near the Sutter Buttes has naturally elevated EC concentrations. This well, which is also screened in the Very Deep Aquifer extent of the Subbasin, has exceeded its MT in the past three years of GSP implementation, but undesirable results were not triggered because it is the only well with an MT exceedance in the Subbasin. Although elevated salinity levels are known to occur at deeper depths near the Sutter Buttes, this well will continue to be monitored as part of the water quality SMC. The undesirable result for water quality was defined in the GSP as occurring “when 25% of RMS wells exceed the MT for 24 consecutive months,” however, since wells are not monitored monthly, but annually in the fall, this undesirable result is clarified to occur “when 25% of RMS wells exceed the MT for two consecutive years.” This clarification uses the same timeframe as defined in the GSP but is more in line with actual monitoring approaches and provides a better quantifiable definition.
7.4 Land Subsidence
The GSP basin setting and monitoring sections identify three data sources used to historically monitor subsidence in the Subbasin:
1. Sacramento Valley Subsidence Network – 29 monitoring benchmarks periodically surveyed, and most recently by DWR in 2017 (29 total)
2. InSAR monthly data provided by DWR since June 2015. DWR releases data quarterly with about three months lag time for data processing.
3. Extensometer data are reported daily and provided by DWR on the Water Data Library website annually since 2004 (three extensometer locations are available in Butte Subbasin)
The 29 survey benchmarks were identified as the subsidence monitoring RMS in the GSP but are no longer being surveyed routinely. DWR recommended in their approval of the GSP (RCA #2) that the GSAs consider establishing a monitoring network with sufficient temporal data collection frequency to evaluate subsidence conditions relative to SMC (DWR, 2023a). For that reason, the GSAs have elected to replace the surveyed benchmark RMS network with InSAR data and extensometer sites. The data collected from InSAR, and the extensometers will be used going forward as the subsidence RMS network to evaluate SMC, as described in Section 4.5. The Draft Butte Subbasin Periodic Evaluation, September 2026 Page 132 GPS benchmarks will be retained in the land subsidence broad monitoring network in case future surveys of the network are conducted, but will no longer be relied on to evaluate sustainability.
Figure 7-17 shows the three monitoring networks identified in the GSP for land subsidence, with InSAR cumulative subsidence from 2015 to 2025 depicted.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 133 Figure 7-17. GSP Land Subsidence Monitoring Networks with Cumulative Subsidence Measured by InSAR Draft Butte Subbasin Periodic Evaluation, September 2026 Page 134
7.4.1 Status of GSP Data Gaps for Land Subsidence
There are no land subsidence monitoring data gaps explicitly identified in the GSP. However, as the survey benchmark RMS network was not surveyed since 2017, the temporal frequency was a data gap during GSP implementation. The survey benchmarks are being replaced with InSAR and extensometers per this Periodic Evaluation. The revised subsidence RMS network has the temporal data collection frequency suitable for evaluating conditions relative to SMC, which addresses the data gap identified in RCA #2.
The revised subsidence RMS network has the spatial coverage to evaluate subsidence conditions.
While there are some spatial gaps in the InSAR network in rice growing areas, these are areas that are not prone to subsidence because there is little groundwater pumping. InSAR coverage improved in WY 2025 in the areas without coverage in prior years, and is expected to maintain or continue to improve the coverage going forward with refinements to remote sensing technology. Observed InSAR measurements and extensometers in rice growing areas also shows no inelastic subsidence has occurred since 2004 with extensometers and since 2015 with InSAR.
7.4.2 Monitoring Network Assessment for Land Subsidence A monitoring network consisting of InSAR data and extensometers provides sufficient monitoring data to evaluate land subsidence during GSP implementation. Subsidence calculated using InSAR remote sensing methods is provided by DWR at monthly intervals. The InSAR data is released by DWR for all areas with fixed land surfaces such as roads and untilled lands.
InSAR data is not released for rice growing areas since the land surface fluctuates as it is tilled and graded and exceeds land surface data quality objective tolerance. The InSAR remote sensing technique has a margin of error of about 0.1 foot, which is within the margin of error for identifying significant and unreasonable levels of subsidence and evaluating SMC defined in the GSP. InSAR data has been used to evaluate subsidence in the GSP Annual Reports since WY 2023.
Three DWR extensometers are used to collect high resolution subsidence data near critical infrastructure in rice growing areas in the center of the Subbasin. Extensometer measurements are collected daily, which is more frequent than monthly InSAR. Also, the extensometer measurement error is 0.01 foot, which is an order of magnitude less than InSAR’s margin of error of 0.1 feet. The extensometers were presented in the GSP but were not identified as a component of the RMS network or evaluated in GSP Annual Reports. The extensometers will be used to supplement the lower resolution, but more spatially robust InSAR in the subsidence RMS monitoring network going forward.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 135 Combined, the subsidence monitoring network provides sufficient data to evaluate if subsidence starts to occur in the Subbasin. The network covers most of the areas with critical infrastructure that could be impacted by subsidence including conveyance canals, flood control levees on the major rivers and creeks, and state highways. If subsidence monitoring indicates that significant subsidence starts to occur then the GSAs will attempt to estimate the sensitivity of critical infrastructure to subsidence. More discussion about potential subsidence impacts to critical infrastructure is included in Section 4.5.3.
Finally, should InSAR or extensometer monitoring networks detect measurable and significant land subsidence in the future, the GSAs may collaborate with DWR and other regional partners to perform elevations surveys using the inactive benchmarks. The Sacramento Valley survey benchmark locations are retained in the GSP broad monitoring network though these survey benchmarks are no longer RMS as their future monitoring is uncertain.
7.4.3 2027 Representative Monitoring Network for Land Subsidence Table 7-12 summarizes the well construction and measurement frequency information for the three extensometers in the revised 2027 land subsidence monitoring network. Figure 7-18 shows the revised RMS monitoring network, consisting of the most recent InSAR coverage from WY 2025 and three extensometer locations.
Table 7-12. Summary of Land Subsidence Extensometer Monitoring Network Well Construction Information DWR Extensometer
State Well Number
Total Well Depth
(feet bgs)
Top of Screen
Interval
(feet bgs)
Bottom of Screen
Interval
(feet bgs)
Measurement
Frequency
20N01E18L001M 940 767 894 Daily
19N01E35B002M 980 930 950 Daily
18N01E35L001M 899 816 836 Daily
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 136 Figure 7-18. Revised 2027 Land Subsidence RMS Monitoring Network Draft Butte Subbasin Periodic Evaluation, September 2026 Page 137
7.4.4 Sustainable Management Criteria Updates for Land Subsidence Even though the monitoring network has changed, the same methods used to define the subsidence SMC in the GSP are applicable to the revised monitoring networks. For example, InSAR and extensometer data will be compared to the MT of 0.5 foot of subsidence over a 5-year period instead of the GPS benchmarks in the original RMS network. However, the undesirable result definition is revised to better identify local conditions that could impact critical infrastructure. An undesirable result for subsidence is defined in the GSP as 25% of GPS monitoring benchmarks exceeding the MT of 0.5 foot over five years. The revised definition of undesirable results is when 25% of any PLSS Township (6 x 6 sections) exceeds the MT in two consecutive years within the same InSAR pixels (see discussion in Section 4.5.3).
7.5 SGMA Monitoring Network Module Reconciliation
The GSAs have updated SGMA Portal Monitoring Network Module (MNM) to accompany the changes identified in this monitoring network evaluation. Updates were also made to address DWR’s Determination Letter suggestions for monitoring network module improvements. The following revisions to the MNM were made:
• Remove the “Deep” principal aquifer designation – there are only Primary and Very Deep Principal Aquifers in the GSP.
• Verify that the RMS wells are all represented, there are 51 wells in the GSP RMS monitoring network and 55 wells in the MNM monitoring networks. • Update the sustainability indicators for well 17N03E05C003M which is listed as a groundwater level RMS well in the MNM, but it is actually only part of the ISW network • Add depth and screen intervals for wells with missing information in the MNM. The correct values were in the GSP and are provided in Table 7-2 of this evaluation for the following wells with missing information in the MNM:
o 17N01E10A001M
o 17N03E08K002M (no screen interval available)
o 18N02E25M001M
o 19N01E09Q001M
o 20N01W11N002M
o 21N01E08K002M
o 22N01E32E004M
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 138 • All GSP broad monitoring network wells that are not RMSs will be identified in the MNM. This includes wells 18N03E18F001M and 20N02E16P001M, which were identified as RMS in the GSP, but are moved to the GSP broad monitoring network through this evaluation. The GSP broad monitoring network is summarized in Appendix 7A.
• Remove ISW RMS well 18N01E05D002M from the RMS and broad monitoring network as it is no longer accessible.
• Include the three extensometer locations that are added to the subsidence RMS network and remove the 27 Sacramento Valley survey benchmarks that were removed from the subsidence RMS network.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 139 8 GSA AUTHORITIES AND ENFORCEMENT ACTIONS
During this evaluation period, the GSAs have made progress on and collaborated with or are aware of water management efforts led by the three Counties overlying the Subbasin, regarding the following five key items:
1. Funding approach for GSP implementation activities
2. County well ordinances and effects on the GSP implementation
3. County Drought Resilience Plans Update in compliance with SB 552
4. Domestic Well Mitigation and Demand Management Programs in Glenn and Colusa Counties
5. Relevant updates to General Plans since 2022
Some of these items were flagged as missing from the GSP in DWR’s review letter (as discussed in Section 2).
8.1 Funding
In the spring of 2024, the GSAs agreed to a Cost Share Agreement which constitutes the Second Amendment to the 2019 Cooperation Agreement (Appendix 8A). The County of Butte GSA serves as the fiscal administrator for the Butte Subbasin and also coordinates with DWR for TSS and Facilitation Support Services (FSS) on behalf of the 11 GSAs in the Subbasin. As set forth in the Cost Share Agreement, each GSA contributes to the administration and implementation of the GSP by funding an equal one-eleventh (1/11) share of the annually agreed-upon budget.
Table 8-1 below describes how each GSA funds its SGMA implementation obligations for its cost share contribution.
As described in Table 8-1, several GSAs fund SGMA implementation through their general funds. The Colusa Groundwater Authority’s (CGA) 2019 fee schedule has not changed since the GSP was submitted in January 2022. Two Butte Subbasin GSAs, Butte Water District and Reclamation District (RD) 2106, conducted Proposition 218 processes after submission of the 2022 GSP. The public engagement associated with those respective efforts is described below.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 140 Table 8-1. Butte Subbasin GSAs’ Sources of SGMA Implementation Funding Agency Funding Approach
(by GSA)
Additional Funding Sources
(Basin-wide) Further Information
County of Butte GSA ● Funding for County of Butte’s cost-share contribution is coming from County’s general
fund; no fees are being discussed at this time.
● DWR TSS implementation of five
wells for shallow well monitoring
system.
● DWR FSS Program for ongoing interagency coordination County of Butte GSA serves as the fiscal
administrator for the Butte Subbasin and
coordinates with DWR for TSS and FSS on behalf
of the 11 GSAs in the Subbasin
Butte Water District ● Conducted a successful Proposition 218 Process 2023-25.
○ Proposition 218 Process was a dual process.
Butte Water District looked at raising current
water rates as well as implementing a
Groundwater Management Fee.
○ Groundwater Management Fee is currently
set at $4.00/acre. The Board of Directors
could have gone as high as $12.78/acre.
See Table 8-2 (below), describing outreach and
engagement for Butte Water District’s Proposition
218 Process
Biggs-West Gridley
Water District
Agency General Funds
City of Biggs Agency General Funds
City of Gridley Agency General Funds
Colusa Groundwater
Authority
● For FY 2025-26, CGA is continuing to collect its
existing parcel charge, currently at $1.00/acre.
That fee has been in place since 2019 and can
be increased to as much as $1.50/acre.
● CGA is also reconsidering available funding
options and may initiate a new fee process in
2026-2027, but the specific type is still under
discussion.
● The CGA fee for 26-27 is $1.45 per acre.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 141 Agency Funding Approach
(by GSA)
Additional Funding Sources
(Basin-wide) Further Information
County of Glenn GSAButte ● County of Glenn GSA Butte Subbasin is currently funding Butte Subbasin
implementation through the General Fund but
there has been some interest in considering an
alternative funding approach.
Reclamation District 1004 Agency General Funds
Reclamation District 2106 ● Conducted a successful Proposition 218 process in 2024
● Enacted two fees that have been implemented
and are currently in effect:
○ Non-irrigable ($1.50/acre)
○ Irrigated ($4.85/per acre)
See Table 8-3 below describing outreach and
engagement RD 2106’s Proposition 218 Process
Richvale Irrigation District Agency General Funds
Western Canal Water
District
Agency General Funds
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 142
8.1.1 Butte Water District - Proposition 218 Outreach Approach Butte Water District's outreach approach was comprehensive and exceeded standard Proposition 218 compliance requirements. The District conducted an open, transparent, and inclusive process throughout. Numerous meetings and calls were held with landowners across the District, and notices were mailed to every parcel within the service area. Public meetings were convened to provide landowners with opportunities to voice concerns or submit formal objections. The District's efforts garnered additional public attention, with the Gridley Herald publishing two separate reports covering the process. For more information about the public meeting schedule, see Table 8-2.
Table 8-2. Public Meeting Schedule – Proposition 218 Process for Butte Water District Meeting Date Description
September 14,
2023
The Butte Water District Board of Directors reviewed a proposal submitted by Provost & Pritchard outlining the scope of the Proposition 218 process and associated Rate Study costs. The Board deferred action pending full Board attendance.
November 9,
2023 The Board of Directors approved the Rate Study proposal submitted by Provost & Pritchard.
March 14, 2024 Provost & Pritchard presented preliminary Rate Study findings to the Board of Directors. The Board requested revisions and modifications prior to final adoption.
September 18,
2024
Provost & Pritchard presented a revised Rate Study to the Board of Directors, incorporating previously requested edits and changes.
November 7,
2024
The Board of Directors approved the Rate Study and authorized commencement of the Proposition 218 process.
December 12,
2024
The Board of Directors approved the Cost-of-Service Report and authorized initiation of the formal Proposition 218 process.
January 21,
2025
Formal Proposition 218 notices were mailed to all 3,644 parcels within the District. The Cost-of-Service Study, Protest Form, and Objection Form were published on the Butte Water District website.
March 31, 2025
A public hearing was held at Butte Water District offices. Notices had been issued to all 3,644 parcels within the District. A total of 37 opposition votes were received, representing approximately 1% of parcels notified. A minimum of 1,823 opposition votes would have been required to reject the proposed rate changes. The Board of Directors adopted the authorizing resolutions and approved the new Groundwater Management Fee and Water Rate Schedules.
April 7, 2025 The Board of Directors established a Groundwater Management Fee of $4.00 per acre for fiscal year 2025, against a maximum allowable rate of $12.78 per acre.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 143
8.1.2 Reclamation District 2106 - Proposition 218 Outreach Approach RD 2106 also successfully utilized the Proposition 218 funding approach. Their outreach approach was thorough and designed to ensure full compliance with Proposition 218 requirements while maximizing transparency and public participation. The District conducted a structured, multi-phase process that included formal Board actions, statutorily required public notice, community engagement through a Town Hall meeting, and a formal public hearing.
Notices were mailed to all affected parcels within the required timeframe, and landowners were provided multiple opportunities to review the proposed rate changes and submit written protests or comments. For more information about the public meeting schedule, see Table 8-3.
Table 8-3. Public Meeting Schedule – Proposition 218 Process for RD 2106 Date / Event Description
May 15, 2023
1:00 PM
Special Board
Meeting
The Reclamation District 2106 Board of Directors convened a Special Board Meeting to accept the rate report, establish the Proposition 218 public hearing date, and authorize the printing and mailing of official notices of public hearing.
June 2, 2023
Notice Postmark
Date
Formal Proposition 218 notices were mailed to all affected parcels within the District, postmarked a minimum of 47 days in advance of the public hearing, in compliance with statutory requirements.
Notices included an announcement of the upcoming Town Hall meeting.
June 16, 2023
Newspaper
Advertisement
Pursuant to applicable statutory requirements, Reclamation District 2106 published notice of the upcoming public hearing in a newspaper of general circulation at least 30 days in advance of the scheduled hearing date.
June 13, 2023
1:00 PM
Town Hall Meeting
A Town Hall meeting was held at the local Fire Station to provide landowners and interested parties an opportunity to learn about the proposed rate changes, ask questions, and engage with District representatives prior to the formal public hearing.
July 18, 2023
1:00 PM
Proposition 218
Public Hearing
The formal Proposition 218 rate hearing was conducted before the Board of Directors. Following the close of the protest period and tabulation of written protests, the Board adopted a resolution authorizing the collection of fees on the County property tax roll.
Late July 2023
Tax Roll
Submission
The District submitted the approved fee schedule to the County for inclusion on the property tax roll, in accordance with Proposition 218 and applicable County deadlines.
8.2 County Well Ordinances and Permitting Processes
The three Counties (Butte, Colusa and Glenn), have updated their well ordinances since the GSP, as described below. The GSAs collaborated with the Counties on these updates.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 144
8.2.1 Butte County
The construction, repair or destruction of wells is subject to permitting by the Butte County Division of Environmental Health pursuant to Chapter 23B of the Butte County Code, Water Wells. The chapter provides minimum procedures for the proper construction of water wells and for the proper destruction of abandoned wells in order to ensure that water obtained from wells within the County of Butte will be suitable for the purposes for which used and that wells constructed or abandoned pursuant to this chapter will not cause pollution or impairment of the quality of the groundwater within the county. An additional purpose is to reduce potential well interference problems to existing wells and potential adverse impacts to the environment which could be caused by the construction of new wells or the repair or deepening of existing wells where a permit is required. Important provisions of the Section include:
• The construction, repair, reconstruction, deepening, abandonment and destruction of wells in Butte County must follow the standards in Bulletin 74‐81 and its supplement bulletin 74‐90, Water Well Standards, State of California. • After July 25, 1996, the pumping capacity of a new well cannot be greater than fifty (50) gallons per minute per acre to reasonably serve the overlying land, including contiguous parcels of land under the same ownership as the land upon which the well is located. • Wells can only be drilled by a person licensed to drill water wells pursuant to the provisions of Business and Professions Code section 7000 et seq. possessing a C‐57 water well contractor’s license required by section 13750.5 of the California Water Code. • Domestic well owners are required to ensure that a new well will operate properly assuming a repeat of the groundwater conditions experienced during the period 1987 through 1994 in the area in which the new well is located. • Well drillers reports must be filed with Butte County as well as with DWR. • Notification of well permit applications are required in specific instances to adjoining landowners and/or local agencies with an adopted groundwater management plan pursuant to part 2.75 of Division 6 of the California Water Code (commencing at section 10750). Landowners and/or local agencies are provided thirty (30) days to provide comments prior to permit issuance.
• Wells with a casing diameter greater than eight (8) inches are required to be drilled at specific distances away from existing wells.
• In addition to well sealing requirements specified within state well standards bulletin 74‐ 81 and bulletin 74‐90, the seal shall be extended five (5) feet into the first consolidated formation encountered below fifteen (15) feet to a maximum required sealing depth of fifty (50) feet.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 145 Butte County Public Health has begun a public process to revise sections of Chapter 23B, the Water and Well Ordinance. The County has initiated a Rural Water Safety Project, which includes a comprehensive review and update to the County’s existing ordinances and manuals governing water wells (Butte County Code Chapter 23B and Well Construction Manual) and onsite wastewater treatment (septic) systems (OWTS) (Butte County Code Chapter 19B and OWTS Manual). The ordinances and manuals will be reviewed and updated as needed to better align with current regulations, standards, policies, and priorities for community wellbeing.
8.2.2 Colusa County
8.2.2.1 Administration and Permitting
Permit applications for well construction and destruction are administered through Colusa County Environmental Health Services per Water Well Standards Bulletins 74-81 and 74-90. All applicable county agency conditions must be satisfied before a permit may be issued. Applicants must submit plot plans drawn to scale with all relevant site information.
8.2.2.2 Well Construction Standards
Siting and Drainage: Wells should be located upgradient from potential sources of pollution or contamination where feasible. Well casings must terminate above grade and above known flood levels. A concrete apron or base of at least four inches in thickness—sloped away from the well—is required around the casing, and surface drainage must be directed away from the well.
Annular Seal Minimum Depths: Minimum annular seal depths below ground surface vary by well type as follows: community and industrial wells require 50 feet; individual domestic, agricultural, air-conditioning, and all other well types require 20 feet. Seal material must additionally extend five feet into the nearest clay layer.
Casing Requirements: Well casing must be constructed to prevent wall collapse, convey water, and protect water quality.
Disinfection: All wells producing water for domestic use, including drinking water and food processing, must be disinfected following construction, repair, or pump work before being placed into service.
Minimum Separation Distances from Contamination Sources: Wells must maintain the following minimum horizontal distances from potential pollution sources: sewer lines (50 feet); watertight septic tanks (100 feet); subsurface sewage leaching fields or pits (100 feet); cesspools or seepage pits (150 feet); and animal or fowl enclosures (100 feet). Greater distances may be required at the time of permitting.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 146
8.2.2.3 Well Destruction Requirements
Purpose: Well destruction is required to protect and preserve groundwater supplies and to eliminate physical safety hazards.
Definition of Abandoned Wells: A well unused for one year is considered abandoned unless the owner demonstrates intent to continue use, in which case it is classified as inactive. To maintain inactive status, owners must ensure the well has no defects impairing water quality, is covered with a watertight tamper-resistant cap, is clearly marked, and has the surrounding area kept clear of brush and debris. Observation and test wells maintained by government agencies or research organizations are not considered abandoned provided they are properly capped, labeled, and locked when not in use.
Destruction Procedures: Abandoned wells must be completely filled to restore pre-construction subsurface conditions as nearly as possible. The process requires removal of obstructions, perforation of casing where necessary to ensure complete sealing, and filling from the bottom upward with approved materials. Approved sealing materials include neat cement, sand-cement grout, concrete, and bentonite clay. The upper 20 feet of all wells must be sealed with impervious material. Free fall of cement grout is limited to a maximum of 25 feet, and grout must be placed to prevent dilution or separation.
Additional Requirements in Urban Areas: In incorporated or densely developed unincorporated areas, the well casing must be excavated to five feet below ground surface and removed. The sealing material must form a cap over the excavation, which is then backfilled with native soil after the sealing material has set.
Notification: A minimum of 24 hours advance notice is required prior to installing an annular seal or destroying a well.
8.2.3 Glenn County
Since the submittal of the Butte Subbasin GSP in 2022, Glenn County amended its well ordinance. Adopted May 23, 2023, Ordinance 13239 amends Title 20, Chapter 20.08 of the Glenn County Code to regulate the placement, construction, replacement, repair, modification, and destruction of water wells within the county. The ordinance aims to protect groundwater from contamination and depletion, ensure water suitability for beneficial use, and safeguard public health, safety, and welfare. It is adopted pursuant to the Porter-Cologne Water Quality Control Act (Water Code Section 13000 et seq.).
8.2.3.1 Administration
9 Ordinance 1323. An Ordinance of the County of Glenn
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 147 The Glenn County Environmental Health Department serves as the Local Enforcement Agency (LEA), responsible for enforcing all provisions, issuing permits, conducting inspections, and issuing violation notices and stop-work orders.
8.2.3.2 Permit Requirements
A permit from the LEA is required before any person or entity may construct, replace, repair, modify, or destroy a well in Glenn County. Permit applications must include well location, contractor information, proposed depth and casing diameter, intended use, pump size, and a site plot plan. Permits expire after one year, with one possible extension. Beginning work without a permit may result in doubled permit fees.
8.2.3.3 Well Standards
The ordinance adopts California Department of Water Resources Bulletin standards (74-81 and 74-90) as the county's minimum construction standards, supplemented by local requirements including:
• Annular well seals extending at least five feet into the first low-permeability stratum and to a minimum of 50 feet
• Minimum setbacks of 50 feet from surface water bodies and 10 feet from property lines • Well casings extending at least 18 inches above grade and flood levels • Steel casings of no less than 10-gauge thickness
• Mandatory disinfection prior to placing wells into service • Water quality testing for total dissolved solids (TDS) or electrical conductivity (EC) prior to casing installation
8.2.3.4 Exempt Wells
Certain wells are exempt from consultant review requirements, including domestic and livestock wells producing less than 2 AFY, limited-purpose wells (fire suppression, cathodic protection, geothermal, soil borings, monitoring, and exploratory wells), and wells located outside the Corning, Colusa, or Butte Subbasins.
8.2.3.5 Consultant Review for Non-Exempt Wells
Non-exempt well permit applications require hydrogeologic review by a county-retained consultant. Applications are evaluated across five categories: groundwater level impacts, land subsidence, changes in groundwater storage, groundwater quality impacts, and impacts to public trust resources of navigable waters. Each category is assessed through a tiered analysis Draft Butte Subbasin Periodic Evaluation, September 2026 Page 148 framework, with progressively more comprehensive review required as potential impacts increase.
8.2.3.6 Minimum Depth Requirements
New, replacement, and repaired exempt wells must extend at least 50 feet below the minimum threshold established by the applicable Groundwater Sustainability Plan. This requirement applies only to wells within established high or medium priority groundwater basins.
8.2.3.7 Well Completion and Reporting
Upon completion, all persons who drill, abandon, or modify a well must file a Well Completion Report with the LEA within 60 days. Electric logging (E-Log) is required for wells larger than 8 inches in diameter.
8.2.3.8 Abandoned and Inactive Wells
A well unused for one year is considered abandoned and must be destroyed unless the owner demonstrates intent to reuse it and maintains required conditions. Inactive well owners must file a declaration of intent with the LEA. All abandoned wells must be destroyed in accordance with county standards to protect groundwater and eliminate physical hazards.
8.2.3.9 Enforcement and Penalties
Violations of the ordinance constitute a misdemeanor punishable by a fine of up to $5,000 per day and/or imprisonment of up to six months. The LEA may revoke or rescind permits for violations or misrepresentation. Decisions of the LEA may be appealed to the Board of Supervisors within 10 days of notice.
8.2.3.10 Review
The Board of Supervisors intends for this ordinance to be reviewed annually and modified as needed to reflect changes in hydrological conditions within Glenn County.
8.3 County Drought Resilience Plan Updates
Senate Bill (SB) 552 (2021) requires every California county to establish a standing Drought and Water Shortage Task Force and develop a County Drought Resilience Plan (County DRP) to protect state small water systems and domestic well users.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 149
8.3.1 Butte County
Butte County originally adopted a Drought Preparedness and Mitigation Plan10 in 2004.
Following the passage of SB 552 in 2021, the county initiated an update to comply with the new requirements.
As part of SB 552 compliance, the county is revising the Drought Preparedness and Mitigation Plan to include a drought and water shortage risk assessment, identify short-term and long-term response actions specifically addressing state small water systems and domestic wells, and incorporate other mitigation considerations. The County continues to use the existing Drought Task Force (established in 2004) for drought planning and implementation activities. The Drought Task Force is composed of various Butte County Department Directors and other local, state, and federal agency representatives.
As of the County's most recent reporting, the plan was adopted by the Board of Supervisors on XX.2026. Alongside the SB 552 plan revision, the Butte County Office of Emergency Management received funding through the State Water Resources Control Board for the Drought Resilience and Outreach Program (DROP) in 2024 designed to provide short- and long-term solutions, including water hauling and the repair or replacement of domestic wells impacted by drought to ensure property owners impacted by dry wells have access to water.
8.3.2 Colusa County
Colusa County has completed its SB 552 compliance process. Colusa County established a Drought Task Force beginning in 2015, as approved by the Board of Supervisors. The county received grant funds from the Department of Water Resources to help prepare the County's initial SB 552-compliant Drought Resilience Plan, with staff and consultant support working directly with the Task Force. On June 17, 2025, the Colusa County SB 552 Drought Resiliency Plan11 was approved by the County Board of Supervisors.
The Task Force continues to meet quarterly to monitor conditions, coordinate short-term response actions, and identify drought funding sources. On the groundwater side, in February 2025, the state Department of Water Resources approved the Revised Groundwater Sustainability Plan for the Colusa Subbasin, which includes enhanced programs to address subsidence, initiate a demand management program, and mitigate domestic wells impacted by declining water levels below GSP targets.
10 Butte County Emergency Drought Response & Resources 11 Colusa County SB 522 Drought Task Force and Resilience Planning Draft Butte Subbasin Periodic Evaluation, September 2026 Page 150
8.3.3 Glenn County
. Pursuant to a California DWR grant agreement and SB 552, Glenn County has established a standing drought task force to facilitate drought and water shortage preparedness for state small water systems, domestic wells, and homes. Glenn County had a drought response program, partnered with North Valley Community Foundation to implement the program, assisting with water tanks, water hauling, and drinking water supplies.
Glenn County’s Drought Resilience Plan12 has been adopted in May 2026, and includes a linkage to the GSP well mitigation program and SMC to protect domestic wells.
8.4 Domestic Well Mitigation Programs in Glenn and Colusa Counties A Pilot Domestic Well Mitigation Program was launched by January 1, 2026, which is a regional initiative to protect and support residents whose drinking water wells have been impacted by declining groundwater levels or land subsidence. The program is jointly led and funded by the Colusa Groundwater Authority13, Glenn Groundwater Authority14, and the Corning Sub-basin GSA, reflecting a shared commitment to protecting access to safe and reliable drinking water while responsibly managing groundwater resources.
8.5 General Plan Updates
The Butte Subbasin lies primarily within Butte County, and portions of it lie within Colusa and Glenn Counties. The County General Plans are applicable for all areas within the subbasins in these counties. Additionally, the Cities of Biggs and Gridley each have a General Plan that is applicable to lands within their GSA boundaries. Per GSP Regulations (Reg. §354.8f), this section provides relevant General Plan information related to Land Use Elements or other Topic Categories that may affect GSP implementation.
8.5.1 Butte County General Plan
The Butte County General Plan 2040 was adopted by the Butte County Board of Supervisors on March 28, 2023. The process was a targeted update, primarily focused on addressing the County’s ability to rebuild following the Camp and North Complex Fires and create a more resilient future. To achieve these goals, the General Plan Update included comprehensive revisions to the County’s Housing and Health and Safety Elements and introduced a new Environmental Justice Element. The Land Use, Circulation, and Water Resources Elements underwent significant revisions, primarily to address new requirements under State law. The 12 Glenn County SB552 Drought Resilience Plan March 2026 13 Colusa Groundwater Authority Pilot Domestic Well Mitigation Program 14 Glenn Groundwater Authority Pilot Domestic Well Mitigation Program Draft Butte Subbasin Periodic Evaluation, September 2026 Page 151 Economic Development, Agriculture, Conservation and Open Space, Public Facilities and Services, and Area Plans Elements sustained lesser revisions focused on maintaining internal consistency within the entire General Plan.
Water Resources Element has eight goals with associated policies and actions. The below summary illustrates where these goals are explicitly aligned with SGMA implementation. More information about the Butte County General Plan Update 2040 can be found at Butte County General Plan 2040 (Update): https://www.buttecounty.net/367/Butte-County-General-Plan2040.https://www.buttecounty.net/367/Butte-County-General-Plan-2040. The Water Element repeatedly and directly references SGMA and local GSPs as touchstones for county decisionmaking. Several provisions make GSP consistency a condition of county action:
• Policy W-P1.1 requires county planning and programs to be integrated with local GSPs as well as with Central Valley Regional Water Quality Control Board best management practices, establishing GSP consistency as a baseline for all relevant county planning efforts.
• Policy W-P2.2 commits the County to continuing participation in local GSAs, Groundwater Advisory Boards, and regional integrated water management efforts, including support for local GSPs.
• Policy W-P2.9 requires applicants for new major development projects to demonstrate adequate water supply, including an evaluation of potential cumulative impacts to surrounding groundwater users and the environment in accordance with local GSPs — directly tethering the development review process to GSP standards. • Policy W-P4.4 requires the County to consult applicable GSPs and local groundwater management agencies when making any decisions that could impact groundwater resources.
• Action W-A2.1 commits the County to participating in the implementation and update of GSPs to ensure long-term water resource sustainability.
8.5.1.1 Dedicated Groundwater Goals (Goals W-3 and W-4) Two goals are specifically devoted to groundwater management and GSA coordination, making them the most directly SGMA-relevant provisions in the element.
Goal W-3 focuses on protection and management of groundwater resources. Policy W-P3.1 establishes local regulatory authority over groundwater transfers and substitution programs to protect the sustainability of the County's economy, communities, and ecosystems, a locally controlled complement to SGMA's basin-level management framework. Policy W-P3.2 requires protection of both groundwater recharge and groundwater quality as considerations in new development review. Supporting actions commit the County to mapping groundwater recharge Draft Butte Subbasin Periodic Evaluation, September 2026 Page 152 areas countywide (W-A3.1), evaluating and filling gaps in existing standards to preserve groundwater resources and quality (W-A3.2), and improving scientific understanding of regional aquifer systems (W-A3.3).
Goal W-4 directly addresses coordination with the three GSAs operating within Butte County's subbasins. Policy W-P4.1 commits the County to working cooperatively with all three GSAs to support SGMA implementation. Policy W-P4.2 articulates the County's support for SGMA implementation across its full scope of groundwater levels, groundwater quality, and avoidance of land subsidence with a preference for locally managed, scientifically grounded, and compliance-oriented programs. Policy W-P4.3 emphasizes public information and involvement in groundwater sustainability planning. Action W-A4.1 calls for regional coordination with GSAs and other counties on data analysis and approaches to protect local beneficial uses and users.
8.5.1.2 Water Supply Adequacy and Groundwater Demand Management Several provisions in Goal W-2 have direct SGMA relevance by connecting land use approvals and water supply planning to groundwater sustainability:
• Policy W-P2.9 (noted above) is a key demand-management tool, requiring demonstrated water supply adequacy, including cumulative groundwater impacts consistent with GSPs before major development approvals.
• Action W-A2.3 directs the County to develop implementing criteria for W-P2.9, including project size thresholds and standards for evaluating impacts to surrounding groundwater users providing the operational framework for this GSP-linked review requirement.
• Policy W-P2.10 encourages the use of surface water to support agricultural economic development, which has an indirect SGMA benefit by reducing pressure on groundwater supplies.
• Policy W-P2.4 supports full utilization of the County's State Water Project allocation within Butte County when feasible, similarly oriented toward reducing groundwater dependence.
8.5.1.3 Groundwater Recharge Protection
Recharge protection, a core SGMA sustainability implementation approach, is addressed across multiple goals:
• Policy W-P3.2 directly requires protection of groundwater recharge in development decisions.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 153 • Policy W-P6.2 encourages permeable surfaces and rainwater catchment/retention systems to enhance groundwater recharge.
• Policy W-P6.5 calls for stormwater channels to be managed in ways that produce cobenefits, including supporting recharge. • Action W-A3.1 seeks funding for countywide mapping of water resources and groundwater recharge areas, a foundational data need for both GSP implementation and land use decision-making.
8.5.1.4 Groundwater Quality
Water quality protection is treated as integral to groundwater sustainability throughout the element:
• Policy W-P1.1 integrates GSP guidance into water quality planning. • Policy W-P1.2 commits the County to cooperating with state and local agencies to identify and eliminate point and non-point sources of pollution to both ground and surface waters, citing a broad range of pollution sources relevant to agricultural counties. • Policy W-P3.2 protects groundwater quality as a condition of development review. • Action W-A3.2 calls for evaluating gaps in existing standards and developing additional standards to protect groundwater quality.
8.5.1.5 Water Conservation as a Groundwater Sustainability Tool Goal W-5 addresses water conservation, which directly supports SGMA's objective of bringing basins into balance by reducing extraction demand:
• Policies W-P5.1 through W-P5.6 promote agricultural and urban water use efficiency, coordination with GSAs and conservation districts on consistent conservation measures, water recycling and reuse, greywater reuse, and water conservation requirements for new development.
• Policy W-P5.2 explicitly links conservation coordination to the Butte County GSAs, reinforcing the connection to SGMA implementation.
8.5.1.6 Climate Resilience and Drought (Goal W-8)
Goal W-8 addresses climate change resilience in ways relevant to long-term groundwater sustainability. Action W-A8.1 specifically calls for integrating climate change into water supply projections and management, an important element of forward-looking GSP implementation.
Policies and actions addressing drought preparedness (W-P8.2, W-A8.3, W-A8.4) are relevant to SGMA's requirement that GSPs account for long-term hydrologic variability.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 154
8.5.2 Colusa County General Plan
The Colusa County 2030 General Plan was adopted by the Colusa County Board of Supervisors in July 2012 and has not been updated since the Butte Subbasin GSP was submitted. The General Plan identifies the County’s vision for the future and provides a framework that guides decisions on growth, development, and conservation of open space and resources in a manner consistent with the quality of life desired by the County's residents and businesses. The following guiding principles were identified to guide the General Plan update and implementation:
• Maintain the County’s rural character and quality of life • Focus new development in and around existing communities • Ensure that growth is orderly
• Preserve and enhance the County’s agricultural heritage • Provide opportunities for the expansion of existing businesses and attraction of a diverse range of businesses that provide high‐quality jobs
• Promote a broader range of industries that support the County’s agricultural uses • Provide the young people in the County a future, in terms of employment and housing opportunities
• Ensure that adequate infrastructure is available to serve existing and new development The Colusa County 2030 General Plan includes sections on agriculture, circulation, community character, conservation, economic development, housing, land use, noise, open space, public services and facilities, safety, and implementation. The implementation of the Butte Subbasin GSP will provide for sustainable groundwater management and is not anticipated to affect water supply policies and assumptions in the Colusa County 2030 General Plan. Information on the Colusa County 2030 General Plan and related documents, including annual progress reports, can be found at: www.countyofcolusa.org/137/General‐Plan
8.5.3 Glenn County General Plan
The General Plan update process was completed in July 2023, following a multi-year process that involved extensive community outreach. On July 18, 2023, the Glenn County Board of Supervisors voted to adopt the General Plan Update and certify the Final Environmental Impact Report (EIR). The Glenn County General Plan applies to all of the unincorporated areas of the county outside of the cities of Willows and Orland, which have their own general plans. The General Plan represents the community’s view of its future and expresses the long‐term growth and development goals through addressing a range of important community topics like land use, housing, economic development, transportation, parks and recreation, public safety, and more.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 155 The Glenn County General Plan (2023) demonstrates broad and explicit alignment with SGMA's objectives. The Plan formally recognizes the Colusa Subbasin GSP and the roles of the CGA and GGA, commits to implementing the GSP, and integrates groundwater supply and quality considerations across its Land Use, Agriculture, Conservation and Open Space, Community Services and Facilities, and Safety Elements. Key implementation mechanisms include development review requirements tied to demonstrated water supply adequacy, well permit oversight, interagency coordination with GSA member agencies, recharge area protection, Low Impact Development and Best Management Practices standards, and monitoring of extraction and subsidence.
More Information on the General Plan update (2023) can be found at:
www.glenncounty.generalplan.org
8.5.4 City of Biggs and City of Gridley General Plans
The City of Biggs’ most recent General Plan update was completed in 2014. The General Plan identifies community values and objectives, sets land use policies, and establishes direction for future growth and development. More information on the General Plan is available on the City of Biggs website at www.biggs‐ca.gov.
The City of Gridley’s 2030 General Plan update was adopted in December 2009. The General Plan identifies long‐range objectives for physical development and conservation in the city and provides policies and programs to achieve those objectives. More information on the General Plan is available on the City of Gridley’s website at www.gridley.ca.us.
The implementation of the Butte Subbasin GSP will provide for sustainable groundwater management and is not anticipated to affect water supply policies and assumptions in the General Plans for the cities of Biggs and Gridley.
8.5.5 Effects of Land Use Plans Outside Subbasin
The Sutter County General Plan and zoning ordinance is the only land use plan adjacent to, but not included in, the Butte Subbasin. The Sutter County 2030 General Plan, adopted in 2011, is not anticipated to have any impact on the Butte GSP to maintain or achieve sustainable groundwater management. The GSAs in the Butte Subbasin will continue to monitor amendments to the Sutter County General Plan.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 156 9 OUTREACH, ENGAGEMENT, AND COORDINATION WITH OTHER
AGENCIES
This section demonstrates the GSAs’ continued public engagement with a summary of GSA board, advisory committee, and public meetings, and documentation of coordination efforts with neighboring GSAs, hydrologically connected basins, and tribal, state, and federal agencies.
9.1 Inter-agency Coordination Within the Subbasin
In 2019, the 11 GSAs operating within the Butte Subbasin entered into a Cooperation Agreement to comply with SGMA and manage the Subbasin sustainably through a coordinated approach to develop and implement a single, legally sufficient GSP for the entire Subbasin.
The 11 cooperating agencies are: Biggs West-Gridley Water District, Butte Water District, City of Biggs, City of Gridley, County of Butte, Colusa Groundwater Authority, County of Glenn, Reclamation District No. 1004, Reclamation District No. 2106, Richvale Irrigation District, and Western Canal Water District (Figure 9-1). Together, these agencies cover the entire Butte Subbasin, each retaining full legal authority and independent governance responsibility within their respective jurisdictions. Butte County staff serves as the GSP Plan Manager and is responsible for uploading annual reports, monitoring data, and Period Evaluations to the DWR SGMA Portal, and thus constitutes the primary point of contact with DWR for SGMA compliance.
A foundational principle of the cooperative arrangement is that membership does not diminish any agency's independent legal authority. Each GSA retains all powers granted under SGMA, including rights over water supplies, assets, facilities, well permitting, land use, and water management within its own jurisdiction. The 11 GSAs are separate public agencies—each governed by its own board—that have chosen to collaborate in developing and implementing a shared GSP while fully preserving their individual statutory powers and jurisdictional responsibilities.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 157 Figure 9-1. Groundwater Sustainability Agencies in the Butte Subbasin Draft Butte Subbasin Periodic Evaluation, September 2026 Page 158
9.1.1 Governance Structure
The cooperative governance structure centers on the BAB, established to coordinate GSP development, Periodic Evaluations, and GSP implementation across all 11 GSAs. GSA staff, referred to as GSA Managers, generally meet monthly to ensure coordination on GSP implementation across the Subbasin. They collaboratively prepare consensus-based recommendations on key planning and implementation actions and bring those to the BAB for discussion and concurrence. This serves as a central venue for public to be informed of subbasin conditions and activities. The BAB is comprised of representatives from each GSA Board (or their GSA Manager) and serves as a venue to discuss subbasin wide and GSP issues. Any recommendations that require GSA board action would then be referred to all GSAs for their individual consideration.
Composition and Appointments. The BAB consists of one Member appointed by each GSA’s governing body, who must themselves be a sitting member of that governing body. Each Agency also appoints one Alternate BAB member (who may be a GSA Manager), who may attend all BAB meetings and serve in the primary representative’s absence.
Terms and Removal. BAB members serve four-year terms and may be reappointed. Each Member and Alternate serves at the pleasure of their appointing body and may be removed at any time. Vacancies must be filled within 90 days.
Decision-Making Authority. The BAB strives for unanimous recommendations on coordinated topics, most importantly GSP development, Periodic Evaluations, and implementation and associated financial arrangements. When unanimity cannot be reached, the BAB documents the areas of disagreement to inform each GSA’s governing body. Ultimately, decision-making authority on all matters resides with each individual GSA governing body. The BAB's role is advisory; no decision of the BAB is binding on any GSA without action by that agency.
9.1.2 Subbasin Cost Sharing for SGMA Implementation
Equal Cost Sharing. As described in the cost sharing agreement (Amendment 2 to the Cooperation Agreement, Appendix 8A), each of the 11 GSAs contributes an equal share of the mutually agreed-upon annual budget.
Renegotiation Process. The equal-share methodology continues each fiscal year (July 1 – June 30) unless a GSA provides written notice to all other GSAs expressing intent to renegotiate prior to July 1. Upon timely delivery of such notice, all GSAs must meet and confer to negotiate a new or modified cost-sharing method to take effect at the start of the following fiscal year.
Budget Development and Oversight. GSA managers annually develop, monitor, and forecast the budget together, meeting as needed to consider adjustments necessary for SGMA compliance Draft Butte Subbasin Periodic Evaluation, September 2026 Page 159 and GSP implementation. Any budget increase requires unanimous approval of all GSAs representatives, with the increased amount apportioned under the same equal one-eleventh methodology. The County of Butte GSA-Butte Subbasin serves as Fund Administrator, receiving and administering all GSA contributions consistent with applicable law, providing quarterly budget updates to all GSAs. The Fund Administrator may be replaced by unanimous consent of the GSAs.
9.2 Public Outreach and Engagement
GSAs in the Subbasin collaborate to engage interested parties, the public, and beneficial users to provide updates on basin conditions during annual reporting, regularly share groundwater management information, solicit feedback on projects and management actions, and collect public comments during Periodic Evaluation drafting.
9.2.1 Public Participation Forums
Many forums are available for public participation in the Subbasin. The BAB meets periodically, with all meetings publicly noticed and conducted in accordance with the Ralph M. Brown Act.
These publicly noticed BAB meetings are an important forum for stakeholder engagement in GSP implementation as they provide opportunities for the public to engage with all of the GSAs in a one venue. During GSP development, the BAB met bi-monthly. Since 2022, the BAB has met semi-annually, and additional meetings were conducted in 2025 and 2026 during Periodic Evaluation development.
Equally significant, however, are the regular board meetings and special meetings of each of the 11 GSA governing bodies, which provide ongoing public forums for community input on GSPrelated matters within each agency's jurisdiction. There are additional collaborative and regional public forums which consistently address GSP implementation and provide opportunities for community engagement.
Taken together, the BAB, the 11 GSA governing boards and other collaborative public meetings provide a broad and geographically distributed set of public engagement opportunities across the Subbasin and the region. Regional forums are further discussed in the Interbasin Coordination section below. Butte Subbasin public engagement meetings are summarized in Table 9-1.Three GSAs in the Subbasin coordinate outreach efforts and Board meetings under a Joint Districts agreement, established in 1969:
• Biggs-West Gridley Water District GSA
• Butte Water District GSA
• Richvale Irrigation District GSA
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 160 Table 9-1. Butte Subbasin Public Engagement Meetings
Meeting Name GSAs Description Meeting Dates and Key Discussion Topics Additional Information Butte Subbasin
Advisory Board
(BAB)
All The BAB coordinates GSP development and
implementation across all 11 GSAs with
monthly GSA Manager meetings producing
consensus-based recommendations for BAB
review and individual GSA board action as
needed.
• 8/6/26: Draft PE overview and opportunity for public
comment
• 5/14/26: PE Update and Water Year (WY) 2025
Annual Report presentation and opportunity for
public comment
• 11/5/25: PE kickoff and approach
• 6/5/25: WY 2024 Annual Report
• 6/12/24: WY 2023 Annual Report
• 11/6/23: Cost Share Allocation Discussion
• 5/30/23: WY 2022 Annual Report
• 11/16/22: SGM Grant Round 2 Application approval
and direction to explore long-term GSP
implementation funding approach
https://www.buttebasingroundw
ater.org/meetings
Butte County
Water Commission
County of Butte GSA A 9-member advisory body appointed by the Butte County Board of Supervisors, meeting
monthly to review water resource issues,
develop policy recommendations, and monitor
state and federal water legislation —
supported by a seven-person Technical
Advisory Committee and providing a public
forum on water issues
The Butte County Water Commission meets bi-monthly
and includes the broad range of water resources topics
in Butte County. There is a standing item for GSP
implementation in all of the County’s groundwater
subbasins, including the Butte Subbasin.
https://www.buttecounty.net/12
21/Water-Commission
GSA Public
Meetings
All All 11 GSAs have regular board and council
meetings that are publicly noticed.
Agendas and meeting minutes for all GSA meetings
can be found at their respective websites.
• County of Butte GSA
• Butte Water District
• Biggs West Gridley Water
District
• City of Biggs
• City of Gridley
• Colusa Groundwater
Authority
• County of Glenn GSA
• RD 1004
• RD 2106
• Richvale Irrigation District
• Western Canal Water District
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 161 Meeting Name GSAs Description Meeting Dates and Key Discussion Topics Additional Information Joint Districts
Board Meetings
• Biggs-West Gridley
Water District
• Butte Water District
• Richvale Irrigation
District
The Joint Districts coordinate their efforts in
managing joint facilities and senior water
rights. In 1969, the Joint Districts entered into
a diversion agreement with the State allowing
for the diversion of up to 555,000 AF from the
Feather River at the Thermalito Afterbay
during the irrigation season following its
construction and the construction of Lake
Oroville as part of the State Water Project.
The joint Districts meet monthly. Meeting agendas
include a standing SGMA implementation item.
https://jointwaterdistricts.com/
Annual Landowner
Meetings
• Biggs-West Gridley
Water District
• Richvale Irrigation
District
• Western Canal
Water District
Annual landowner meetings in these districts
include district-wide updates, including GSP
implementation topics and grower education.
All landowners in these districts receive
invitations to these meetings via email or post.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 162
9.2.2 Communications, Public Notices, and Informational Materials Communications and Public Notices are conducted by each agency and through countywide interested parties lists for public engagement opportunities where GSP implementation is discussed. The Butte Subbasin GSAs maintain a collaborative website focused entirely on SGMA implementation efforts in the Subbasin: https://www.buttebasingroundwater.org. The website includes links to each of the GSAs, a tool to help stakeholders identify their appropriate GSA. All BAB meeting materials are posted on the website in advance of BAB meetings.
Additionally, informational and educational materials, brochures, and factsheets are also available through the website.
Since GSP submittal in 2022, the Butte County staff, as technical support to the Subbasin GSAs has prepared and broadly distributed annual report brochures15 as a companion to the full reports to make the information more accessible and engaging for the public. During the Periodic Evaluation process in 2025-2026, GSAs developed factsheets and brochures describing current implementation actions16 and to give an overview of the key components of the Periodic Evaluation. These materials are distributed by GSA managers at their publicly noticed board meetings and at BAB meetings and are available on the Subbasin website.
9.3 Coordination with Other Agencies
Butte Subbasin GSAs coordinate with other state and county agencies to perform monitoring and collect data across the Subbasin. In addition, the GSAs coordinate with adjacent GSAs in hydrologically connected subbasins to understand implementation activities and potential effects across basin boundaries, and to share data, as described in Section 10.1.
9.3.1 DWR Technical Support Services
In September 2021, staff from Western Canal Water District (WCWD) and County of Butte GSA began coordinating with the DWR Northern Region Office (NRO) to evaluate the TSS program as a potential fit for the shallow well installation PMA identified in the GSP. A kick-off meeting was held in October 2021 with all three entities, and in November 2021, DWR provided the TSS application and draft agreement templates required for each well. GSA staff completed the application and, in spring 2022, conducted GIS analysis to overlay shallow well installation polygons with parcel data to identify landowner and APN information. County of Glenn GSA staff assisted with ownership identification in Glenn County, and staff from the Colusa Groundwater Authority provided coverage for Colusa County. Since that time, WCWD, County of Butte GSA, and DWR staff have conducted numerous meetings and site visits with 15 Annual report brochures 16 Implementation action factsheets and brochures Draft Butte Subbasin Periodic Evaluation, September 2026 Page 163 landowners to identify suitable well locations and finalize project agreements with willing participants. More information on the TSS wells is included in Section 7.1.1.
9.3.2 Annual Groundwater Monitoring
Groundwater wells in the broad and RMS networks are monitored a minimum of four times per year, with many wells measured more frequently. Each March and October, DWR NRO staff conduct a round of measurements at nearly all network wells, with County of Butte GSA staff covering a subset of wells within Butte County. In recent years, County of Butte GSA has also undertaken follow-up visits to wells where DWR was unable to obtain a measurement or where a measurement was flagged as questionable, ensuring more complete and reliable network coverage and data collection.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 164 10 OTHER INFORMATION
This section captures supplemental information in three areas:
1. Description of GSA coordination with adjacent basins and an assessment whether plan implementation affects neighboring subbasin sustainability goals
2. Identification of significant technical and financial challenges to help inform future DWR assistance
3. Disclosure of active legal challenges or adjudications that may affect GSP implementation
10.1 Consideration of Adjacent Basins
The Butte Subbasin is actively engaged in a range of interbasin coordination activities that reflect both the hydrologic interconnectedness of the Northern Sacramento Valley's groundwater subbasins and the shared regional commitment to sustainable groundwater management under SGMA.
At the regional scale, the Northern Sacramento Valley Integrated Regional Water Management (NSV IRWM) program provides a standing venue for information sharing across the region. The Counties of Butte, Colusa, Glenn, Tehama, Shasta, and Sutter have entered into a Memorandum of Understanding to promote coordination, collaboration, and communication on shared water resources. Together, these six counties developed and are implementing the NSV IRWM Plan, a collaborative framework that engages multiple agencies, stakeholders, tribes, and the public to identify regional water-related needs and advance projects and programs that provide multiple benefits across the region. GSP implementation is a standing agenda item at each IRWM meeting, ensuring that groundwater sustainability remains integrated into the broader regional water management conversation.
At the corridor scale, the Butte Subbasin is leading and participating in two structured inter-basin coordination efforts. The North Sacramento River Corridor (NSRC) Inter-Basin Coordination (IBC) effort brings together GSAs from seven subbasins — Red Bluff, Antelope, Los Molinos, Corning, Vina, Colusa, and Butte — to advance coordinated groundwater management through the Periodic Evaluation process and into the next five-year implementation period. This effort builds on a foundational inter-basin coordination framework developed between 2020 and 2022, a completed technical joint GSP evaluation, and a collaboratively developed Framework Implementation Work Plan that will be reflected consistently across all participating subbasins' Periodic Evaluations. A separate Sutter Buttes Water Quality Working Group coordinates across the Butte, Colusa, Sutter, North Yuba, South Yuba, and Yolo Subbasins to address water quality concerns arising from the unique volcanic geology of Draft Butte Subbasin Periodic Evaluation, September 2026 Page 165 the Sutter Buttes area, with a focus on engaging DWR to improve basin characterization and regional groundwater quality monitoring. More in-depth information about each of these efforts is further described below.
In addition to these structured efforts, the Butte Subbasin GSA managers continue to engage in ongoing neighbor-to-neighbor inter-basin coordination discussions with adjacent subbasin staff, sharing relevant data and maintaining direct working relationships at shared boundaries. Taken together, these activities reflect a multi-layered approach to regional coordination, spanning bilateral boundary discussions, issue-specific working groups, corridor-wide facilitated collaboration, and valley-wide information sharing that positions the Butte Subbasin as an active and constructive partner in regional groundwater sustainability.
10.1.1 Inter-Basin Coordination in the North Sacramento River Corridor Overview
The NSRC IBC effort brings together GSAs from seven subbasins including: Red Bluff, Antelope, Los Molinos, Corning, Vina, Colusa, and Butte to advance collaborative groundwater management under SGMA (Figure 10-1). The Butte Subbasin served as the convener of this effort during the development of the GSP Periodic Evaluation in 2025-2026 with facilitation provided by the Consensus Building Institute (CBI) through the DWR FSS program.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 166 Figure 10-1. Northern Sacramento Valley Subbasins
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 167 Coordination Foundation: The 2020–2022 Inter-Basin Coordination Framework The most recent effort built directly on a predecessor project completed between 2020 and 2022, which produced the Northern Sacramento Valley Inter-Basin Coordination Report.17 That report established a foundational Interbasin Coordination Framework composed of five pillars: (1) information sharing; (2) joint analysis and evaluation; (3) coordination on mutually beneficial activities; (4) coordinated communication and outreach; and (5) an issue-resolution process. The framework was designed to guide GSAs through the 20-year implementation horizon of SGMA and was incorporated by reference into the Groundwater Sustainability Plans (GSPs) of each participating subbasin. Those GSPs, submitted between 2021 and 2024, each formally committed to pursuing inter-basin coordination consistent with the framework, reflecting the region's early recognition that groundwater management across these hydraulically connected subbasins requires and benefits from a shared, collaborative approach.
Technical Foundation: The Joint GSP Evaluation
A critical milestone which informed the recent facilitated process was the completion of a technical Joint GSP Evaluation for the Sacramento River Corridor subbasins, conducted by Montgomery & Associates (M&A) on behalf of the Vina and Wyandotte Creek GSAs with DWR grant support. This work advanced Pillar 2 of the framework—Joint Analysis and Evaluation—and was undertaken in anticipation of the GSP Periodic Evaluations due to DWR in January 2027.
The evaluation systematically compared key GSP elements across subbasins, with particular focus on HCMs, water budgets, SMCs, and monitoring networks. A central objective was 17 Inter-Basin Coordination in the Northern Sacramento Valley Draft Butte Subbasin Periodic Evaluation, September 2026 Page 168 establishing a common understanding of stream-aquifer interactions and cross-boundary groundwater flows, including the volume and direction of subsurface flows at each subbasin boundary. The Butte Basin Groundwater Model was updated as part of this work to better characterize these boundary conditions. Results from the Joint GSP Evaluation were documented in a Technical Memorandum (TM)18 that synthesized findings across the 2022 GSPs for Vina and Butte Subbasins and the 2024 resubmitted GSPs for the remaining subbasins, supplemented by the most recent annual reports.
The TM identified notable differences and uncertainties among subbasins related to hydrogeologic characterization, sustainable management criteria, and interconnected surface water management, which are also reflected in DWR's RCAs issued to each subbasin upon GSP approval. These RCAs, spanning topics such as groundwater level monitoring, subsidence thresholds, interconnected surface water, and water quality, provide a parallel driver and incentive for inter-basin coordination, as many of the data gaps and technical uncertainties identified by DWR are most effectively addressed through collaborative, cross-boundary analysis.
In general, the SMC established within the Butte Subbasin do not cause any impacts on neighboring subbasins. For example, the MTs for groundwater levels in Butte Subbasin are within the range of MTs of neighboring subbasins, except compared to Vina Subbasin, which are set much lower than in the Butte Subbasin. SMC for water quality are similar across neighboring Subbasins, all using salinity as their primary constituent of concern. Monitoring approaches could be further coordinated across subbasins.
The GSA Managers met in December 2024 to review and discuss the outcomes of the Joint GSP Evaluation, and a public webinar was held for the Sacramento River Corridor evaluation in July 2025. A public webinar was also held in August 2025 covering the results of an extensive modeling analysis of the DWR C2VSim model results of stream depletions.
The Inter-Basin Coordination Framework Five-Year Implementation Update Building on the 2020–2022 framework and the completed joint GSP evaluation, the participating GSAs have developed a Framework Five-Year Implementation Update (Framework Update, Appendix 10C) that will be incorporated into each subbasin's Periodic Evaluation. The Framework Update describes intended activities organized around the five framework pillars and covers both near-term deliverables for the Periodic Evaluation and longer-term coordination activities through the subsequent five-year implementation period (2027–2032). 18 Technical Memorandum, Joint GSP Evaluation for the North Sacramento River Corridor Draft Butte Subbasin Periodic Evaluation, September 2026 Page 169 Two public webinars were held during preparation of the Framework Update to inform the public on the overall effort, foster broad understanding of regional coordination activities, and receive input on work products. The first webinar, held on January 28, 2026, drew approximately 95 participants and provided an overview of the Interbasin Coordination Framework and the status of updates across all five pillars. Attendees engaged substantively with the technical content, raising questions about hydrological connectivity and the accuracy of groundwater models, the urgency of completing coordination work ahead of the January 2027 Periodic Evaluation deadline, and how cross-boundary issues such as interconnected surface water, ecological thresholds, and domestic well mitigation programs would be managed regionally.
Public input from the webinar was subsequently reviewed by GSA representatives and used to refine the draft Framework Update. A second public webinar was held in spring 2026 to gather additional public feedback.
The shared commitments GSAs have made in the Framework Update are described below.
Pillar 1: Information Sharing. GSAs will convene at least one regional coordination meeting annually — and more frequently as warranted — to keep each other informed of changing conditions such as surface water curtailments, land use changes, and policy developments relevant to groundwater management. Meetings will be supplemented by use of existing regional venues, including the Northern Sacramento Valley Integrated Regional Water Management (NSV IRWM) program and the Northern California Water Association (NCWA) Groundwater Task Force. Agendas and materials from formal meetings will be posted to the regional interbasin coordination website19 maintained by Butte County.
Pillar 2: Joint Analysis and Evaluation. This pillar has been substantially advanced through the Inter-Basin Coordination Analysis and Modeling Project completed in 2025 and funded with a DWR SGMA grant. The project compared GSP contents across the corridor, identified significant differences and uncertainties at subbasin boundaries, analyzed sustainable management criteria for cross-boundary impacts, and updated the Butte Basin Groundwater Model to compute inter-basin flows and stream-aquifer interactions. The results, documented in two Technical Memoranda and supported by publicly available webinar recordings and presentation materials, provide the technical foundation for coordinated action in the Periodic Evaluations and will be referenced in each subbasin's PE submission, and are available on the Butte County website.
Pillar 3: Coordination on Mutually Beneficial Activities. The Framework Update designates interconnected surface waters (ISW) as the priority regional activity for 2026–2032 (see Section
10.1.3.2). Each of the corridor subbasins received RCAs from DWR related to ISW, and the Framework Update calls for development and pursuit of funding for a regional approach to 19 https://www.buttecounty.net/2218/Inter-Basin-Coordination-in-the-Northern Draft Butte Subbasin Periodic Evaluation, September 2026 Page 170 coordinated monitoring and modeling that would allow subbasins to respond to those RCAs consistently and efficiently. NCWA is supporting this effort, and Proposition 4 has been identified as a potential funding source. Additional areas of ongoing interest include crossboundary flow analysis and regional modeling coordination, joint review of monitoring network gaps along shared boundaries, and collaborative pursuit of funding for regional supply projects such as groundwater recharge.
Pillar 4: Coordinated Communication and Outreach. Near-term activities include maintaining a current directory of GSA staff contacts, developing and periodically updating informational materials (Appendix 10A, 10B, 10C) for distribution through each subbasin's existing communication channels, and continuing to host public workshops. The regional interbasin coordination webpage hosted by Butte County will serve as the central public repository for work products. A public webinar held in January 2026 provided an initial opportunity for public engagement on the coordination effort.
Pillar 5: Issue Resolution. GSAs have confirmed their commitment to the issue-resolution process described in the original 2021 Inter-Basin Coordination Report, which provides an agreed-upon framework for identifying and resolving conflicts between agencies. An important near-term deliverable under this pillar is the development of shared language describing regional inter-basin coordination in the North Sacramento River Corridor for consistent use across all subbasins' Periodic Evaluation submissions, ensuring a coherent and unified regional narrative is presented to DWR. The issue-resolution process can be activated on an as-needed basis as coordination activities proceed.
10.1.2 Inter-Basin Coordination in the Sutter Buttes Area
10.1.2.1 Purpose and Background
A separate but related interbasin coordination effort is underway focused on water quality concerns surrounding the Sutter Buttes. While the surface expression of the Sutter Buttes lies within the Sutter Subbasin, the subsurface extent of the volcanic deposits and associated geologic structures reaches into adjacent subbasins and may influence groundwater quality across a broader region. Key concerns driving this effort include elevated arsenic concentrations in groundwater within the Sutter Buttes Rampart that exceed drinking water standards, reduced depths to the base of fresh groundwater resulting from the uplift of basement rocks, and the potential for faults associated with the Sutter Buttes to serve as conduits for the upward migration of saline or brackish connate water from deeper geologic formations.
The formation of a Sutter Buttes Water Quality Interbasin Working Group was formally proposed in the 2022 GSPs for the Butte and Colusa Subbasins, reflecting early recognition that Draft Butte Subbasin Periodic Evaluation, September 2026 Page 171 these geologically driven water quality issues extend beyond any single subbasin's jurisdiction and warrant a coordinated regional response.
10.1.2.2 Participating Agencies
The Working Group brings together GSA representatives from the Butte, Colusa, Sutter, North Yuba, and South Yuba Subbasins, with participation also extended to the Yolo Subbasin.
Participating agencies include the Butte County GSA, Richvale Irrigation District, Butte Water District, Biggs-West Gridley Water District, the Cities of Biggs and Gridley, Reclamation District 2106, County of Glenn GSA, the Glenn Groundwater Authority, the Colusa Groundwater Authority, and the Sutter County GSA, among others. Facilitation is provided by CBI and technical support by M&A.
10.1.2.3 Activities and Progress
The Working Group held its first meeting in November 2025, at which M&A provided a regional overview of water quality data, drawing on the State Water Board's Groundwater Ambient Monitoring and Assessment (GAMA) program and other relevant databases. Participants agreed that because salinity and arsenic in this area are naturally occurring and driven by the unique geology of the Sutter Buttes, the most effective path forward centers on engaging DWR to seek state-level support for improved basin characterization and groundwater monitoring. The group also acknowledged that water quality is primarily regulated through programs outside of SGMA, such as the Irrigated Lands Regulatory Program, and that GSAs' principal responsibility under SGMA is to understand and manage the potential impacts of degraded water quality on beneficial uses and users.
As an initial concrete step, M&A prepared comments to DWR on the draft Bulletin 118 recommending that the state improve basin characterization and water quality monitoring in the Sutter Buttes area. The Sutter Subbasin also committed to conducting geophysical surveys to better characterize the subsurface hydrogeology surrounding the Sutter Buttes and to sharing findings with the group as they become available.
A second Working Group meeting was convened in April 2026 as a direct engagement session with DWR, bringing together DWR's TSS Lead and Basin Characterization Program Lead to discuss the state's current tools and resources relevant to Sutter Buttes water quality, future resources under development, and potential avenues for technical assistance through other agencies such as GAMA.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 172
10.1.2.4 Anticipated Outcomes
The central goal of this effort is to work collaboratively with DWR on an ongoing basis to deepen the shared understanding of the Sutter Buttes' unique geological character and its influence on groundwater quality across subbasin boundaries, and to identify opportunities for enhanced regional monitoring supported by state resources. Participants are pursuing an ongoing dialogue with DWR around the potential for targeted technical support, including improved basin characterization and monitoring network development that would benefit multiple subbasins simultaneously.
Longer-term, the group may continue meeting to share monitoring data as it is collected, track progress on the geophysical surveys and any resulting hydrogeologic characterization and identify whether joint projects or management actions addressing water quality in the Sutter Buttes area are warranted.
10.1.3 Additional Inter-Basin Coordination Efforts
The Butte Subbasin, through its strategic position in the center of the north Sacramento Valley has many neighboring subbasins and shares boundaries with other subbasins. In addition, its major boundaries are comprised by the Sacramento River and the Feather River, which warrant coordination on items such as interconnected surface water evaluation and policy setting at Subbasin boundaries. Below are two more coordination activities that the Butte Subbasin GSAs have participated in.
10.1.3.1 Inter-Basin Coordination in the Feather River Corridor The Butte Subbasin GSAs also engaged in a Joint GSP Evaluation effort with the GSAs in the adjacent subbasins along the Feather River Corridor: Wyandotte Creek, Sutter, and North Yuba Subbasin (Figure 10-1). These GSAs met in December 2024 to review and discuss the outcomes of the Joint GSP Evaluation, which was documented in a TM20 prepared by M&A. A public webinar was held for the Feather River Corridor evaluation in August 2025. Given that the Subbasins in this corridor are largely sustainable, and SMC correlate well across boundaries, minimal additional coordination is needed at this time, and no further formal coordination has been undertaken.
10.1.3.2 Interconnected Surface Water Working Group
Butte Subbasin GSAs are coordinating with other neighboring GSAs in the Northern Sacramento Valley on best approaches to evaluate ISWs in streams at shared boundaries. GSA staff and their 20 Technical Memorandum, Joint GSP Evaluation for the Feather River Corridor Draft Butte Subbasin Periodic Evaluation, September 2026 Page 173 technical consultants have worked across subbasins to develop a Workplan for Regional Coordination and Collaboration to Address the Depletions of Interconnected Surface Water Sustainability Indicator in the Sacramento Valley (ISW Workplan). This ISW Workplan was shared with DWR staff and efforts are underway to develop an approach to implement this workplan over the next implementation period.
10.2 Challenges Not Previously Discussed
10.2.1 Assistance Needs for Plan Implementation
The Butte Subbasin has benefited greatly from two DWR support programs that have been essential to its ability to implement SGMA effectively. This support has been particularly critical given that the Subbasin did not receive a grant award under DWR's Round 2 SGMA Implementation Grant Program, leaving these state programs as a primary resource in addition to local cost share contributions for advancing SGMA implementation. Without these state investments, the cost of SGMA implementation would fall disproportionately on ratepayers in a Subbasin that is demonstrably meeting its groundwater sustainability objectives.
DWR's FSS program has been instrumental in enabling consistent, effective, and streamlined coordination across the Subbasin's 11 GSAs. In a medium priority basin with stable groundwater conditions, sustaining the institutional engagement and collaborative momentum needed for SGMA compliance across such a large and diverse group of agencies presents an ongoing organizational challenge. FSS-funded facilitation has directly addressed this challenge, fostering a cohesive working relationship among the GSAs and supporting the highly effective collaboration that has characterized the Subbasin's SGMA implementation to date. DWR's TSS program has likewise provided meaningful assistance, enabling the Subbasin to make tangible progress toward filling identified data gaps, most notably through the installation of a robust shallow groundwater monitoring network, described in Section 7.1.1.
Looking ahead, continued access to FSS and TSS, together with funding through Proposition 4 for regional technical efforts, will be essential to the Subbasin's ongoing success. The Northern Sacramento Valley is a hydrologically connected region where data gaps and sustainability challenges, such as the characterization of interconnected surface water, are most efficiently and effectively addressed through coordinated regional action. Sustained state investment in these programs will be critical to ensuring that the Butte Subbasin and its neighbors can continue to meet their SGMA obligations and protect this shared groundwater resource over the long term.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 174
10.3 Legal Challenges
The Butte Subbasin's GSP has been the subject of legal challenges brought by environmental advocacy organizations following DWR's approval of the Plan in 2023. The litigation centers on whether SGMA requires a GSP to give preferential priority to Public Trust Resources.
The environmental plaintiffs filed a reverse validation action contending that the Butte Subbasin GSP was legally insufficient because it did not establish groundwater management measures that prioritize the maintenance of groundwater levels specifically to protect Public Trust Resources.
The GSAs' position is that SGMA requires a balanced approach to groundwater management, one that considers Public Trust Resources alongside all other beneficial uses, but does not impose an absolute legal priority for environmental uses over agricultural or urban users, particularly in a basin that is sustainable and subject to natural cycles of depletion and recharge.
The trial court dismissed the environmental organizations' claims after granting plaintiffs three successive opportunities to amend their complaint. The court ruled on the doctrine of judicial abstention, concluding that it was not the proper role of the courts to intervene in and redesign a GSP that is subject to DWR's established review and oversight process under SGMA. The portion of the litigation directed at the GSAs was thus resolved in the GSAs' favor at the trial court level.
However, a related appeal remains pending. The environmental organizations have appealed the portion of the case directed at DWR, challenging DWR's decision to approve the Plan given its treatment of Public Trust Resources. The GSAs are participating in support of DWR's approval.
If the appellate court were to rule in favor of the environmental plaintiffs, it could establish a precedent requiring GSPs across the region to prioritize habitat and endangered species protection in ways that could constrain groundwater use during drought years, even in basins that are hydrologically sustainable over normal cycles of recharge and depletion. Similar litigation on related grounds is occurring in other groundwater basins in the surrounding region.
The outcome of this appeal has significant implications not only for the Butte Subbasin but for SGMA implementation more broadly, and the GSAs will continue to monitor and participate in proceedings as they advance.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 175 11 SUMMARY OF KEY TAKE-AWAYS AND CONTINUED
IMPLEMENTATION APPROACH
This Butte Subbasin’s First Periodic Evaluation assessment shows that the Butte Subbasin continues to be managed sustainably primarily through the use of reliable surface water supplies that allow the groundwater pumping volumes to stay within sustainable yields. As a result, no undesirable results have occurred during this evaluation cycle. Surface water supply is projected to continue to be reliable in normal to wet years and the aquifer is able to absorb temporary increases in pumping to make up for surface water limitations in dry years.
The GSAs addressed all three RCAs provided in DWR’s approval letter of the GSP.
Additional key take-aways for this First Periodic Evaluation include:
• Groundwater conditions remain stable within the Butte Subbasin • Sustainable Management Criteria have been confirmed
• Projects and Management Actions are on track
• Additional information solidifies understanding of Basin Setting • Conjunctive use of surface water and groundwater remains key to sustainability • Refined monitoring networks provide robust data acquisition • GSAs continue to collaboratively manage the Subbasin, engage stakeholders, and interact with GSAs in neighboring subbasins
The GSAs clarified SMC for groundwater levels, interconnected surface water and groundwater quality, and adopted a resolution to solidify this information (Appendix 11A). These revisions did not warrant a Plan Amendment since the general approach and intent of the GSP stayed the same and the Subbasin continues to be managed similarly. The updated SMC are summarized in Table 11-1.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 176 Table 11-1. Summary of Updated SMC
Sustainability
Indicator Evaluation Status MT Definition Undesirable Result Identification MO Definition Chronic Lowering
of Groundwater
Levels and
Reduction of
Groundwater
Storage
No indication of undesirable results
There were no groundwater level
measurements below the MT between
WY 2021 and 2025.
For each RMS well, the shallower of
either:
i. the shallowest 7th percentile of
nearby domestic wells, or
ii. the range of measured
groundwater levels, or 20 feet
(whichever is greater), below the
observed historic low.
If the resulting value is shallower than
the observed historic low, the MT is
set 10 feet deeper than the observed
historic low.
When 25% of each aquifer’s
RMS wells fall below their MTs
for 24 consecutive months (11
of 41 wells in the Primary
Aquifer and 3 of 10 in the Very
Deep Aquifer).
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well.
Depletion of
Interconnected
Surface Water
No indication of undesirable results
There are no groundwater level
measurements below the MT between
WY 2021 and 2025.
10 feet below the measured historical
low groundwater elevation at each
RMS well.
When 25% of the RMS wells (3
of 12) fall below their MTs for
24 consecutive months.
Static 5-year average (generally
2012-2017) of measured
groundwater level data for each
RMS well.
Degraded Water
Quality
No indication of undesirable results
One well routinely exceeds the MT due
to naturally occurring salinity. Otherwise,
there are no MT exceedances between
WY 2021 and 2025.
The higher of either 900 μS/cm or the
measured historical high of EC for
each RMS well.
When 25% of RMS wells
(2 of 8) exceed their MT for
2consecutive years.
Measured EC less than or equal
to 700 μS/cm at each RMS well.
Land Subsidence No indication of undesirable results
There were no subsidence monitoring
sites with a subsidence rate greater than
the MT between WY 2021 and 2025.
0.5 foot of subsidence over a 5-year
period within the same InSAR pixel.
When 25% of any PLSS
Township (6 x 6 sections)
exceeds the MT over two
consecutive years, within the
same InSAR pixels.
Up to 0.25 foot of subsidence
over a 5-year period, within the
same InSAR pixels. (0.05 foot
over 1 year; 1 foot over 20
years).
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 177 The GSAs are committed to continue their collaborative management of the Subbasin through stewardship of water resources with implementation of regular system maintenance and modernization.
Specific activities the GSAs plan to undertake during this next evaluation cycle:
1. Continue monitoring water levels, water quality, subsidence and ISW at all established monitoring sites
2. Coordinate installation of TSS-funded shallow monitoring wells to supplement the groundwater level and ISW networks
3. Continue exploring potential refinements to water quality monitoring network
4. Streamline annual report development and submittal
5. Participate in various inter-basin coordination meetings and activities
6. Engage with stakeholders on a regular basis and identify any impacted well owners if groundwater levels decline significantly in drought years or driven by other management actions.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 178 12 REFERENCES
Butte County Department of Water & Resource Conservation (Butte County). 2021. Butte Basin Groundwater Model, Model Documentation v 1.0. November.
Butte Subbasin Groundwater Sustainability Agencies (GSAs). 2021. Butte Subbasin Groundwater Sustainability Plan. Prepared by Davids Engineering. Available at:
https://sgma.water.ca.gov/portal/gsp/preview/98.
Butte Subbasin Groundwater Sustainability Agencies (GSAs), 2024, Butte Subbasin Annual Report WY 2023. Available at: https://sgma.water.ca.gov/portal/gspar/preview/292.
Butte Subbasin Groundwater Sustainability Agencies (GSAs), 2022, Butte Subbasin Annual Report WY 2021. Available at: https://sgma.water.ca.gov/portal/gspar/preview/92.
Butte Subbasin Groundwater Sustainability Agencies (GSAs), 2023, Butte Subbasin Annual Report WY 2022. Available at: https://sgma.water.ca.gov/portal/gspar/preview/244.
Butte Subbasin Groundwater Sustainability Agencies (GSAs), 2025, Butte Subbasin Annual Report WY 2024. Available at: https://sgma.water.ca.gov/portal/gspar/preview/436.
Butte Subbasin Groundwater Sustainability Agencies (GSAs), 2026, Butte Subbasin Annual Report WY 2025. Available at: https://sgma.water.ca.gov/portal/gspar/preview/492.
California Department of Water Resources (DWR), 2026. Best Management Practices of the Sustainable Management of Groundwater, Land Subsidence. January. https://water.ca.gov/- /media/DWR-Website/Web-Pages/Programs/Groundwater-Management/SustainableGroundwater-Management/Best-Management-Practices-and-GuidanceDocuments/Files/Land_Subsidence_BMP.pdf California Department of Water Resources (DWR), 2023a. Butte Subbasin Groundwater Sustainability Plan Determination Letter (Statement Of Findings Regarding The Approval Of The GSP). July. Available at: https://sgma.water.ca.gov/portal/gsp/assessments/98.
California Department of Water Resources (DWR), 2023b. Groundwater Sustainability Plan Implementation: A Guide to Annual Reports, Periodic Evaluations, & Plan Amendments.
October. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/GroundwaterManagement/Sustainable-Groundwater-Management/Best-Management-Practices-andGuidance-Documents/Files/GSP-Implementation-Guidance-Report.pdf State Water Resources Control Board. 2022. Groundwater Quality Considerations for High and Medium Priority Groundwater Basins. Letter. November 22.
Draft Butte Subbasin Periodic Evaluation, September 2026 Page 179 U.S. Geological Survey (USGS), 2008. Ground-water Quality Data in the Middle Sacramento Valley Study Unit, 2006 – Results from the California GAMA Program, by Schmitt, S.J., Fram, M.S., Milby-Dawson, B., J., and Belitz, K., USGS Data Series 385, 100 pgs.
Woodard & Curran. 2025. Technical Memorandum: Summary of Analysis of BBGM Version
1.0, Update to BBGM Version 1.2, and Time Series Extension.
Appendix 3A
Well Inventory of Supply Wells in the Butte Subbasin