Agenda packet — Portola City Council, June 10, 2026 (part 3 of 14)

Plumas County 4-147
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The County noted that while there has been some scattered record keeping describing the impacts of windstorms, there is little information to indicate that straight-line winds are little more than a nuisance.

For example, while winds can blow weeds that can create an additional expense for farmers, they often cause little long-term damage and there is little justification for allocating resources to combat them. Trees may be downed in the County. As detailed in the wildfire discussion, one of the most significant concerns in the County is the effect of high winds in fueling wildfires. Even before a wildfire starts, high winds can cause PG&E to put PSPS events into effect.

Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk
Assets at risk from high winds and tornadoes include people and populations; structures; critical facilities and infrastructure; and natural, historic, and cultural resources. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

Additionally, for each hazard in the FEMA NRI in Plumas County, the relevant portions of the hazard analysis done by FEMA is presented below. This includes the Hazard Risk Index Rating, Expected Annual Loss Values, Exposure Values, and the Historic Loss Ratio Rating. These are presented for the Plumas County Planning Area as a whole.

People and Populations
All populations are at some vulnerability to high winds and tornadoes. Certain vulnerable populations are at the greatest risk to the effects of high winds and tornadoes. These include the unsheltered, residents who live in mobile homes, residents who live in older buildings not built to withstand high winds and that may not have areas to shelter, such as below ground floors, and those working or caught outdoors during a wind or tornado event.

Structures
All structures in the Plumas County Planning Area have some measure of risk from high wind and tornadoes. Though most structures are built to withstand winds, high wind speeds can still cause damages in the Planning Area. Though rare, a tornado could cause damage to virtually all facilities in the tornado touchdown path. This would affect homes, businesses, commercial property, as well as government and critical facilities. During a wind or tornado event, power lines in the Planning Area can be damaged causing power outages and falling trees can cause damage to nearby structures by falling trees caused by wind. A tornado could also damage power lines. High winds are also a cause for PSPS events to be declared, causing power outages to occur. High winds and tornadoes could cause sparks from electric wires and other sources, which could lead to wildfire ignition. Once ignited, high winds can also cause wildfires to rapidly spread and become out of control. High wind conditions can cause wildfires to move into more urban areas destroying structures and whole subdivisions, as seen in recent wildfire events in California.

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Critical Facilities and Infrastructure
Though most critical facilities are built to withstand winds, high wind speeds can still cause damage in the Plumas County Planning Area. Though rare, a tornado could cause damage to virtually all facilities in the tornado touchdown path. Winds can cause PSPS events and lead to out-of-control wildfires, which can damage critical facilities and impact their abilities to perform their critical roles in the community.

Natural, Historic, and Cultural Resources
Natural resources, such as trees, are vulnerable to winds. Increased levee and streambank erosion can occur during periods of high water and high winds. High wave action during significant wind events can cause streambank erosion and impact marsh lands, wetlands, and habitat areas. Soil erosion can also occur during high winds. Most all natural, historic, or cultural resources are at risk to tornadoes. Tornadoes can tear apart habitats, causing fragmentation across ecosystems and can damage and destroy historic structures and cultural resources.

FEMA NRI Hazard Vulnerability Assessment
The NRI contains data and analysis for 18 separate natural hazards. The NRI hazard related to Plumas County’s High Winds and Tornadoes are: strong winds and tornadoes. As shown in Table 4-48, strong winds could cause a total of $15,164 in expected annual losses, with a historic loss rating of relatively moderate. Tornadoes could cause a total of $38,006 in expected annual losses with a historic loss ratio rating of very low. Strong winds and tornadoes have a relatively low rating for social vulnerability and a relatively high community resilience, with a risk index score of 5.4 out of 100 for strong winds and 8 out of 100 for tornadoes. According to the NRI, there have been 3 strong wind events between the years 1986- 2021 with an average of 0.1 events per year and 0 tornado events on record between the years 1950-2021, with an average of 0.1 events per year.

Table 4-48 Plumas County Planning Area – Strong Wind and Tornado Vulnerability Ratings from FEMA NRI
NRI Hazard Hazard Risk Index
Rating
Expected Annual
Loss Values
(total)**
Exposure Value
(total)***
Historic Loss Ratio
Rating
Strong Wind 5.4 $15,164 $237,469,962,102 Relatively moderate Tornado 8 $38,006 $237,469,962,102 Very low
Source: FEMA NRI Report
*Hazard Risk Index is calculated using data for only a single hazard type, and reflects a communities expected annual loss value, community risk factors, and the adjustment factor used to calculate the risk value. **Expected annual loss represents the average economic loss in dollars resulting from natural hazards each year. ***Exposure is a natural hazard consequence factor that is the representative value of buildings, populations, or agriculture potentially exposed to a natural hazard occurrence.

Additional information on the FEMA NRI can be found in Section 4.2 and Appendix H.

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Impacts from Severe Weather: High Wind and Tornado
Wind and tornadoes can cause damage to property and loss of life. While most tornado damage is caused by violent winds, the majority of injuries and deaths generally result from flying debris. Property damage can include damage to buildings, fallen trees, and power lines. Local industries may also be damaged or destroyed. Access roads and streets may be blocked by debris, delaying necessary emergency response.

Winds can push automobiles off roads, damage roofs and structures, cause power outages, and cause secondary damage due to flying debris and hazardous trees. High winds can also contribute to the spread and intensity of wildfires.

Tornadoes can cause damage to property and loss of life. While most tornado damage is caused by violent winds, the majority of injuries and deaths generally result from flying debris. Property damage can include damage to buildings, fallen trees and power lines, broken gas lines, broken sewer and water mains, and the outbreak of fires. Agricultural crops and industries may also be damaged or destroyed. Access roads and streets may be blocked by debris, delaying necessary emergency response.

Impacts to identified assets at risk to this hazard and the overall vulnerability of the County may be affected in the future by climate change (which was discussed in the hazard profile section above), changes in population patterns, and changes in land use and development. The influencing effects of these factors on this hazard are discussed further in the Future Conditions/Future Development discussion below.

Future Conditions/Future Development
This section provides a discussion of how future conditions will influence or affect the hazard over time and also discusses future development relative to each hazard.

Future Conditions
Future conditions may be affected by climate change, changes in population patterns (migration, density, or the makeup of socially vulnerable populations), and changes in land use and development. Findings on this for Plumas County include the following:
➢ Climate change is likely to exacerbate future wind conditions and associated impacts and vulnerability of the County. Climate change is not likely to affect future tornado conditions. ➢ Future population growth should be considered as having more or less people in a community affects the overall hazard vulnerability to the County. Population in Plumas County is expected to shrink over time. Changes in population and population patterns may or may not increase the impacts and vulnerability of the County to this hazard depending on the location and nature of growth (or decline) and continued planning for future hazard conditions.
➢ Land use planning should be proactive to address future hazard conditions. Building codes in the County ensure that new development is built to current building standards to withstand high winds, which should reduce the risk to future development in the County from high winds and tornadoes. With adherence to development standards, future losses to new development should be minimal. Changes in land use and increased development may amplify the impacts of high winds and tornadoes, as additional structures in the County increase the number of buildings at risk to high winds and tornadoes.

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Future Development
Future development considerations should be ongoing and proactive. Ensuring that comprehensive land use planning is conducted, regulatory requirements, such as building codes, are followed, and varying needs for different demographic groups that reside within the County are considered can help make for a more resilient community. Utilities at risk to high winds should be undergrounded as new facilities are improved or added. Whether high winds and tornadoes will occur, where, when, and of what intensity are all factors that evolve over the days and hours before they form and after they do. Improved weather forecasts coupled with new information technologies, including social media, have resulted in an increasingly large volume of risk information that is available to people when tornadoes and high winds threaten.

4.3.6. Agricultural Hazards: Severe Weather/ Pests/ Weeds Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent; past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

Hazard/Problem Description
In Plumas County, forest and agriculture lands are a fundamental component of the rural character, historic use, and way of life. Agriculture plays a significant role in the income and history of the County as well as in the current landscape. Agriculture provides not only local food production, agricultural lands make up open space and scenic vistas that are an intrinsic part of the Plumas County environment.

Agricultural losses occur on an annual basis and are usually associated with severe weather events, including heavy rains, floods, freeze, heat, and drought. The 2023 State of California Multi-Hazard Mitigation Plan attributes most of the agricultural disasters statewide to drought, freeze, and insect infestations. Other agricultural hazards include fires, crop and livestock disease, insects, and noxious weeds.

In addition to severe weather, pests and weeds, including invasive species, can affect the agricultural industry in the County. Invasive species are organisms that are introduced into an area beyond their natural range and become a pest in the new environment. This hazard also addresses the issues related to pests, weeds, and invasive species including those that pose a significant threat to the agricultural industry and are therefore a concern in the Plumas County Planning Area. This hazard does not address pests and plants that cause impacts to human health, as those issues are addressed in other planning mechanisms in the County.

According to the 2023 Plumas County Crop Report (the most recent available), many commodities are grown in Plumas County. The top three commodities for the County in 2023 were cattle; field crops; and timber products. According to the 2023 Crop Report, Plumas County’s 2023 estimated commodity production value is $38,262,907. Timber harvest dropped in 2023. This decrease represents the end of the

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Dixie Fire salvage and the resulting loss of harvestable timber in the County. A summation of crop production values, sourced from the Plumas County Agricultural Commissioner’s Annual Crop Reports, from 2020-2023 for Plumas County is shown in Table 4-49.

Table 4-49 Plumas County – Value of Agricultural Production 2020-2023 Crop 2020 2021 2022 2023
Apiary $26,305 $20,734 $54,000 $113,066
Fruit and Vegetable $285,000 $161,557 $311,359 $311,807 Field Crops $5,473,000 $4,234,300 $7,676,000 $3,493,359 Livestock Productions $10,595,000 $13,001,000 $25,572,000 $14,201,134 Livestock & Poultry
Products
$36,222 $43,419 $87,110 $29,653
Timber $16,666,000 $21,790,520 $24,067,294 $14,804,401
Rangeland $3,300,000 $3,300,000 $3,154,000 $3,291,828
Pastureland $1,896,000 $1,896,000 $1,902,560 $2,017,659 Organic Crops $55,358 $70,301 $82,937 $89,560
Total $38,332,885 $44,517,831 $62,907,260 $38,262,907
Source: Plumas County Agricultural Commissioner Reports. Retrieved 4/25/2025.

Natural Disasters and Severe Weather
According to the USDA, every year natural disasters, such as droughts, earthquakes, extreme heat and cold, floods, fires, earthquakes, hail, landslides, and tornadoes, challenge agricultural production. Because agriculture relies on the weather, climate, and water availability to thrive, it is easily impacted by natural events and disasters. Agricultural impacts from natural events and disasters most commonly include:
contamination of water bodies, loss of harvest or livestock, increased susceptibility to disease, and destruction of irrigation systems and other agricultural infrastructure. These impacts can have long lasting effects on agricultural production including crops, forest growth, and arable lands, which require time to mature.

Animal Pests
California is home to a large diversity of wildlife. Human-wildlife conflict can occur when humans and wild animals interact in an unwanted or unsafe way. In California, habitat loss and a changing climate have increased the frequency and type of wildlife incidents reported to the CDFW. It has become more common for people to observe, encounter, and interact with wildlife while recreating outdoors or living in a wildlandurban interface as wild animals search for food and water. (CDFW 2025). The HMPC noted that invasive species in animal groups like wild pigs are causing extensive damage to both residential areas and also agricultural areas. However, feral hog herds are not prevalent in Plumas County at this time.

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Insect Pests
Plumas County is threatened by a number of insects that, under the right circumstances, can cause severe economic and environmental harm to the agricultural industry. Insects of concern to plants and crops include the Spongy Moth, Japanese Beetle, and European Pine Shoot Moth. Grasshopper populations have fluctuated in the past, but more recently were an issue. Pest detection is a proactive program that seeks to identify exotic, invasive insects. These pests have wide host ranges and are difficult and costly to manage once established. Early detection is essential for quick and efficient eradication. Public participation is critical to the success of this program, since staff relies on the goodwill of property owners who allow traps to be placed on their properties. The Plumas County Department of Agriculture traps and monitors all these agricultural pests. Bark Beetles and Clear Wing Grasshoppers can also cause great destruction of crops within Plumas County
Weeds (both invasive and native)
Noxious weeds, defined as any plant that is or is liable to be troublesome, aggressive, intrusive, detrimental, or destructive to agriculture, silviculture, or important native species, and difficult to control or eradicate, are also of concern. Noxious weeds within the Planning Area include those listed on Table 4-50.

Table 4-50 Plumas County Weeds of Concern
Species of Concern
Barb Goatgrass Jointed Goatgrass Medusahead Yellow Starthistle Scotch thistle Musk Thistle Scotch Broom French Broom Tree of Heaven Perennial
pepperweed
Spotted
knapweed
Diffuse
Knapweed
Rush
Skeletonweed
Musk thistle Himalayan
blackberry
Leafy Spurge Dalmatian Toad
Flax
Puncture vine
Euphorbia Stinkwort Saltcedar Hounds Tongue Spanish Broom Dyer’s Woad Source: California Invasive Plant Council
The Plumas County Department of Agriculture uses mechanical and chemical control measures to contain all of these agricultural pests.

Location and Extent
Agricultural hazards occur throughout the County where lands are used for farming and grazing. The County has large swaths of agricultural lands in the non-federally owned areas of the County. These are shown in the Land Use Map for the County on Figure 4-20 above. Areas not as greatly affected by severe weather, insects, and pests are the City of Portola as well as the communities of Chester and Quincy in Plumas County, as well as the upper elevations of the County which all contain fewer agricultural acres.

However, while the communities may not be directly affected, they are indirectly affected economically when agricultural losses occur.

There is no scale that measures agricultural hazards. Agriculture in the County is at risk to many hazards:
insects, weeds, severe weather, as well as downturns in commodity prices. Each of these has a different duration and speed of onset. Some, such as freeze, can have a short onset and a short duration. Drought

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can have a long onset and long duration. Insects and weeds can have short or long onset, and short or long durations. All agricultural losses can have a significant impact on affected communities.

Wildlife and human interactions can take place anywhere in the County. The extent of these interactions ranges from peaceful viewing at a distance, but can also include deadly encounters for pets, livestock, and people.

Past Occurrences
Disaster Declaration History
The agricultural lands of Plumas County have historically been affected by weather related events such as extreme heat, freeze, heavy rain, and drought. The severe weather events can have devastating effects leading to losses in yield and affecting quality. The US Farm Services Agency provided information on disaster declarations from 2012 through 2024. These are shown in Table 4-51.

Table 4-51 Plumas County – USDA Disaster Declarations 2012 to 2024 Year Declared/Disaster Type Designation Number Primary or Contiguous Begin Date 2012
Drought-FAST TRACK S3248 Contiguous 5/31/2012
Drought-FAST TRACK S3268 Primary 7/12/2012
2013
Drought – FAST TRACK S3491 Contiguous 1/1/2013
2014
Drought – FAST TRACK S3637 Primary 1/14/2014
2015
Drought – FAST TRACK S3784 Primary 1/1/2015
2016
Drought – FAST TRACK S3952 Primary 1/1/2016
Excessive rain, high winds, cold
temperatures, and hail
S4170 Contiguous 3/1/2016
2017
– – – –
2018
Drought – FAST TRACK S4349 Contiguous 2/28/2018
2019
– – – –
2020
Drought – FAST TRACK S4675 Primary 4/14/2020
Drought – FAST TRACK S4691 Primary 4/21/2020

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Year Declared/Disaster Type Designation Number Primary or Contiguous Begin Date Drought – FAST TRACK S4697 Contiguous 4/21/2020
2021
Drought – FAST TRACK S4916 Primary 10/1/2020
2022
Drought – FAST TRACK S5146 Primary 4/8/2022
Freeze S5229 Contiguous 7/1/2022
Freeze S5332 Contiguous 4/12/2022
2023
Drought – FAST TRACK S5371 Primary 10/1/2022
Freeze S5556 Contiguous 2/16/2023
2024
– – – –
Source: US Farm Service Agency
NCDC Events
The NCDC does not track agriculture events.

Hazard Mitigation Planning Committee Events
These sections separate events since the 2019/2020 LHMPs and those before 2019.

Events since 2019/2020 LHMP
The County noted the following recent events:
2022 – The HMPC noted that agriculture events occur yearly, though with varying levels of damages to a variety of crops. The 2021 Plumas County Crop Report noted that 2021 saw multiple issues that affected the agricultural industry in the County. Drought has had large impacts in the County.

Due to the severity of the drought Plumas County was experiencing single digit relative humidity (RH) during the summer of 2021. This resulted in a lack of forage for livestock as well as a decrease in hay production. The lack of livestock feed had a ripple effect in that most of the ranchers had to reduce their herds to match the available forage.

On July 13, 2021, Plumas County dealt with another devastating blow, the Dixie Fire started to burn and continued to burn until October consuming half of the range land in as well as half of the timberland in Plumas County. It takes three years for the rangeland to recover and 70 years for timber to grow back. In 2021 and 2022 there was a spike in timber harvest receipts as there is a two-year window in which to harvest trees after a fire.

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The following statistics show how the County was affected:
➢ Timber production rose to $24,002,774 in 2022, due to the Dixie fire timber harvest, and then fell to $14, 804, 401 in 2023 and will continue to decline before stabilizing. ➢ Total hay tonnage decreased from 24,000 tons in 2022 to 14,000 tons in 2023 due to lack of surface water and grasshoppers.
➢ In 2021 some agricultural wells ran dry resulting in the loss of high dollar hay acreage that was turned into pasture for grazing.

Summer of 2023 – Grasshoppers are no longer experiencing winter kill due to the warming climate and are decimating pastureland and some hay fields. As the grasshopper population increases so does the blister beetle which feeds on grasshopper eggs. The blister beetle is highly toxic to livestock as well as causing blisters on people. Blister Beetle also affects the marketability of hay.

Events before 2019
Multiple events occurred before 2019. These are captured in other past occurrences in the rest of the hazard sections.

Likelihood of Future Occurrence
Highly Likely—As long as severe weather events, insects, pests, and weeds continue to be an ongoing concern to the Plumas County Planning Area, the potential for agricultural losses remains.

It is likely that climate change will increase the chance of future occurrence as well as future impacts. More information on climate change and agricultural hazards can be found in the next section. More information on future impacts can be found in the Future Conditions/Future Development section of the Vulnerability Assessment below.

Climate Change and Agricultural Hazards
According to the 2021 CAS (as well as the 2024 Draft CAS), addressing climate change in agriculture will encompass reducing vulnerability through adapting to the ongoing and predicted impacts of climate.

Agriculture in California is vulnerable to predicted impacts of climate change, including less reliable water supplies, increased temperatures, and increased pests.

The 2023 California State Hazard Mitigation Plan noted that California farmers contend with a wide range of crop-damaging invasive pests and pathogens. Continued climate change is likely to alter the abundance and types of many pests, lengthen pests’ breeding season, and increase pathogen growth rates. It also noted that change in climate can directly impact crop growth through new temperature patterns and northward shifts of pests and disease. Additionally, longer growing seasons may enable pest species to complete more reproductive cycles, which can increase severity of infestations.

Finally, the 2023 State Plan noted that temperature is not the only climatic influence on pests. For example, some insects are unable to cope in extreme drought, while others cannot survive in extremely wet conditions. Furthermore, while warming speeds up the life cycles of many insects, suggesting that pest

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problems could increase, some insects may grow more slowly as elevated carbon dioxide levels decrease the protein content of the leaves on which they feed.

Vulnerability Assessment
Vulnerability—High
According to historical hazard data, severe weather affecting agriculture and agricultural pests and weeds are an annual occurrence in the Plumas County Planning Area. If left unchecked, insects, pests, and weeds, including invasive species can threaten native species, biodiversity, ecosystem services, recreation, water resources, agricultural and forest production, cultural resources, economies and property values, public safety, and infrastructure.

The whole of the Planning Area has some measure of vulnerability to agricultural hazards. An assessment of a community’s vulnerability to this hazard begins with an understanding of local exposure to unincorporated Plumas County. This is included in the Local Concerns section below. After that, vulnerability is discussed in multiple sections that detail how this hazard can affect both the entire Plumas County Planning Area as well as the unincorporated portions of Plumas County. How the hazard affects the other participating jurisdictions and Districts can be found in their respective annexes to this LHMP Update. These sections below include assets at risk, impacts, and how future development can be affected by this hazard.

Local Concerns (unincorporated Plumas County)
The unincorporated County has certain specific concerns regarding this hazard. These concerns form a portion of the basis for the mitigation strategy and mitigation actions that seek to reduce vulnerabilities to this hazard.

USDA Concerns
According to the USDA, every year natural disasters, such as droughts, earthquakes, extreme heat and cold, floods, fires, earthquakes, hail, landslides, and tornadoes, challenge agricultural production. Because agriculture relies on the weather, climate, and water availability to thrive, it is easily impacted by natural events and disasters. Agricultural impacts from natural events and disasters most commonly include contamination of water bodies, loss of harvest or livestock, increased susceptibility to disease, and destruction of irrigation systems and other agricultural infrastructure. These impacts can have long lasting effects on agricultural production including crops, forest growth, and arable lands, which require time to mature. Specific impacts by hazard are listed below:
➢ Drought's most severe effects on agriculture include water quality and quantity issues. Other impacts include decreased crop yields, impact to feed and forage, altered plant populations and tree mortality. ➢ Earthquakes, though rare in Plumas County, can strike without warning and cause dramatic changes to the landscape of an area that can have devastating impacts on agricultural production and the environment. These impacts could include loss of harvest or livestock and destruction of irrigation systems and other agricultural infrastructure.

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➢ Extreme cold may result in loss of livestock, increased deicing, downed power lines, and increased use of generators. Deicing can impact agriculture by damaging local ecosystems and contaminating water bodies. Downed power lines cause people to run generators more often, which can release harmful air pollutants.
➢ Hot weather and extreme heat can worsen ozone levels and air quality as well as leading to drought conditions. Excessive heat and prolonged dry or drought conditions can impact agriculture by creating worker safety issues for farm field workers, severely damaging crops, and reducing availability of water and food supply for livestock.
➢ Wildfires can spread quickly and devastate thousands of acres of land, which may include agricultural lands. This devastation could lead to large losses in crops, forestry, livestock, and agricultural infrastructure.
➢ Flooding causes many impacts to agricultural production, including water contamination, damage to crops, loss of livestock, increased susceptibility of livestock to disease, flooded farm machinery, and environmental damage to and from agricultural chemicals. ➢ The County sees lots of alfalfa loss during heavy storm/rain and flood events. The alfalfa sits in these temporary “lakes” and rots. Additionally, ranchers can’t get the cattle in these areas when wet/flooded. ➢ Landslides and debris flow occur in all 50 states and commonly occur in connection with other major natural disasters such as earthquakes, volcanoes, wildfires, and floods. Some of the threats from landslides and debris flow include rapidly moving water and debris that can cause trauma; broken electrical, water, gas, and sewage lines; and disrupted roadways and railways. This can lead to agricultural impacts including contamination of water, change in vegetation, and harvest and livestock losses.

The Plumas County Agricultural Commissioner noted that of the above, the County is most concerned with drought, freeze, and insect infestations. Recent grasshopper infestation have occurred which caused damages to crops in the County.

Climate Change and Ag Hazards
A recent report from the Intergovernmental Panel on Climate Change (IPCC) shows that the climate crisis has already affected or resulted in the loss of ecosystems, and global warming will likely reach or exceed

1.5°C in the near-term. Agricultural experts warn that these changes are already threatening crop productivity worldwide. Extreme heat has a significant impact on crops, particularly affecting hay crops, rangeland, and livestock gains. Unusually hot temperatures cause smaller harvests and rising food prices, a phenomenon known as “heatflation”. This is also true with timber and other high value Plumas County crops.

Wildlife Interactions
Wildlife and human interactions can take place anywhere in the County. The extent of these interactions ranges from peaceful viewing at a distance, but can also include deadly encounters for pets, livestock, and people. Wildlife and human interaction can present the following hazards in Plumas County:

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Wildlife – Vehicle Collisions
Vehicle collisions with wildlife are an ongoing concern across Plumas County. Large animals such as deer, pronghorn, bears, mountain lions, and wolves cross rural roads, often at dawn and dusk. Vehicle drivers on narrow winding roads may encounter wildlife with little to no time to avoid an accident.

Wolf Attacks
Plumas County issued an emergency declaration on April 15, 2025, for the devastating economic toll wolves are having on ranchers in the county. Wolves have shown a lack of fear towards humans, attacking livestock near homes where both adults and children are present. Incidents in 2025 have shown that the wolves killed cattle without feeding on them, suggesting reasons beyond hunger. This behavior poses an imminent danger to local livestock producers and the families living and working on affected properties.

Between January and June 6, 2025, the CDFW reported 42 confirmed or probable wolf depredations.

Ranchers and homeowners are not allowed to shoot at or around the wolves because of their protected status under the California Endangered Species Act. Deterrent measures such as electric fencing, turbo fladry, and full-time riders in the herds have not proven to be feasible or cost-effective.

Bear Conflict
According to the Plumas County Sheriff’s Office bears are a weekly nuisance in the spring, summer, and fall, but they are usually not a public safety issue. Some bears have been known to break into homes, sheds and barns and cause much destruction in the process. Visitors and residents can mitigate the hazard by properly storing food and trash so as not to attract bears. Electric fencing is also a recommended mitigation measure. Bears also cause agricultural losses through livestock depredation and apiary destruction.

Mountain Lions
Mountain Lions are now frequently observed in rural communities. Most often they will prey on dogs and cats that are outside at night. Livestock depredation is also a known occurrence. Night penning can be a mitigation measure to prevent attacks. A Mountain Lion interacting directly with humans is considered a public safety concern.

Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk
Assets at risk from agricultural hazards include people and populations; structures; critical facilities and infrastructure; and natural, historic, and cultural resources. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

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People and Populations
Agricultural hazards (severe weather, pests and weeds, both native invasive species) have a minimal effect on people and populations in Plumas County, including vulnerable populations. However, damage to crops from invasive and nuisance species can cause significant increases in food prices and food insecurity among low-income communities.

Structures
Structures in Plumas County may be indirectly affected by agricultural hazards. The effects of severe weather on structures are included in the discussions on severe weather above. This section focuses then on native and invasive species, pests, and weeds. Some species can impact vegetation and can result in stream bank instability, erosion, and increased sedimentation, impacting ground stabilization and possibly causing foundation issues for nearby structures. If species cause trees and other vegetation to die, there is an increased risk of damage to roadways, powerlines, and buildings, and increased risk to wildfire. Some invasive plants have been shown to destabilize soil due to high densities and shallow root systems, negatively impacting nearby buildings and septic systems. Other nuisance plant species have been known to clog culverts and streams, increasing flooding risk.

Critical Facilities and Infrastructure
Critical facilities and infrastructure in Plumas County may be affected by agricultural hazards, though these effects are expected to be limited. The effects of severe weather on critical facilities are included in the discussions on severe weather above. This section focuses then on native and invasive species and insects, pests, and weeds. The 2023 State of California Hazard Mitigation Plan noted that water treatment plants could be impacted by invasive and nuisance species. Water that becomes polluted due to increased sedimentation and erosion will require additional treatment. If the system becomes clogged with these pollutants or with invasive aquatic species, the ability of water treatment plants to operate may become impaired. Additionally, soil that becomes unstable due to decaying vegetation can impact critical facilities that are built on or around these soils.

Natural, Historic, and Cultural Resources
Natural resources can be affected by agricultural hazards. The effects of severe weather on these resources are included in the discussions on severe weather above. This section focuses then on invasive species and pests. Negative impacts of pests and weeds to natural resources include the following:
➢ Loss of wildlife habitat and reduced wildlife numbers; ➢ Loss of native plant species;
➢ Reduced livestock grazing capacity;
➢ Increased soil erosion and topsoil loss;
➢ Diminished water quality and fish habitat.

Historic and cultural resources have a more limited risk from agricultural hazards.

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FEMA NRI Hazard Vulnerability Assessment
Agricultural hazards are not an NRI hazard; therefore, no data is presented.

Impacts from Agricultural Hazards
According to the California Invasive Plants Council. California annually suffers over $3 billion in agricultural losses due to these invasive species. The magnitude of this problem cannot be understated, as both the United States and California Departments of Agriculture are mandated to protect against the invasion of exotic pests and diseases.

The consequences of agricultural disasters to the Planning Area include ruined plant crops, dead livestock, ruined feed and agricultural equipment, monetary loss, job loss, and possible multi-year effects (i.e., trees might not produce if damaged, loss of markets, food shortages, increased prices, possible spread of disease to people, and loss or contamination of animal products). When these hazards cause a mass die-off of livestock, other issues occur that include the disposal of animals, depopulation of affected herds, decontamination, and resource problems. Those disasters related to severe weather may also require the evacuation and sheltering of animal populations. Overall, any type of severe agricultural disaster can have significant economic impacts on both the agricultural community and the entire Plumas County Planning Area.

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➢ Land use planning should be proactive to address future hazard conditions. Limited growth is occurring in the County, which should result in limited changes to land uses that would affect the agricultural industry. As the County continues to add new development, land uses may change resulting in a reduction in agricultural uses. However, for the next five years covered by this LHMP Update, changes in land use and development will likely not increase the impacts and associated vulnerabilities of the County to this hazard.

Future Development
Future development in the County is not likely to have an impact on agricultural hazards in Plumas County, except to the extent that agricultural lands are taken out of production as new development occurs reducing available land for agricultural uses, including those related to farming, timber production and grazing. In addition, the HMPC did note that with additional development in the County, there may be additional competition for water resources thus possibly impacting the agricultural industry.

4.3.7. Climate Change
Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent; past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

Hazard/Problem Description
Climate change is the distinct change in measures of weather patterns over a long period of time, ranging from decades to millions of years. More specifically, it may be a change in average weather conditions such as temperature, rainfall, snow, ocean and atmospheric circulation, or in the distribution of weather around the average. While the Earth’s climate has cycled over its 4.5-billion-year age, these natural cycles have taken place gradually over millennia, and the Holocene, the most recent epoch in which human civilization developed, has been characterized by a highly stable climate – until recently.

This LHMP Update is concerned with human-induced climate change that has been rapidly warming the Earth at rates unprecedented in the last 1,000 years. Since industrialization began in the 19th century, the burning of fossil fuels (coal, oil, and natural gas) at escalating quantities has released vast amounts of carbon dioxide and other greenhouse gases responsible for trapping heat in the atmosphere, increasing the average temperature of the Earth.

In Plumas County, the HMPC noted that each year it seems to get a bit warmer. California’s Adaptation Planning Guide (APG): Understanding Regional Characteristics has divided California into 11 different regions based on political boundaries, projected climate impacts, existing environmental setting, socioeconomic factors and regional designations. Plumas County falls within the North Sierra Region characterized as a sparsely settled mountainous region where the region’s economy is primarily tourismbased. The region is rich in natural resources, biodiversity, and is the source for the majority of water used

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by the state. Table 4-52 provides a summary of Cal-Adapt Climate Projections for the North Sierra Valley Region.

Table 4-52 North Sierra Region and Plumas County – Cal-Adapt Climate Projections Effect Ranges
Temperature
Change,
1990-2100
January increase in average temperatures: 2.5 °F to 4°F by 2050 and 6°F to 7°F by 2100. The largest changes are observed in the southern part of the region. July increase in average temperatures: 4 °F to 5°F by 2050 and 10°F by the end of the century, with the greatest change in the northern part of the region. (Modeled average temperatures; high emissions scenario) Precipitation Precipitation decline is projected throughout the region. The amount of decrease varies from 3 to 5 inches by 2050 and 6 inches to more than 10 inches by 2100, with the larger rainfall reductions projected for the southern portions of the region. (Community Climate System Model Version 3 (CCSM3) climate model; high carbon emissions scenario)
Heat wave Heat waves are defined as five consecutive days over 83 °F to 97°F depending on location. By 2050, the number of heat waves per year is expected to increase by two. A dramatic increase in annual heat waves is expected by 2100, eight to 10 more per year.

Snowpack Snowpack levels are projected to decline dramatically in many portions of the region. In southern portions of the region, a decline of nearly 15 inches in snowpack levels - a more than 60 percent drop - is projected by 2090. (CCSM3 climate model; high carbon emissions scenario) Wildfire Wildfire risk is projected to increase in a range of 1.1 to 10.5 times throughout the region, with the highest risks expected in the northern and southern parts of the region. (Geophysical Fluid Dynamics Laboratory climate model; high carbon emissions scenario) Source: Cal-Adapt 2017/2023.

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Figure 4-63 Plumas County – Climate Impact Regions
Source: Cal-Adapt 2017/2023
Location and Extent
Climate change is a global phenomenon. It is expected to affect the whole of the County. There is no scale to measure the extent of climate change. Climate change exacerbates other hazard, such as drought, extreme

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heat, flooding, wildfire, and others. The speed of onset of climate change is very slow. The duration of climate change is not yet known but is feared to be tens to hundreds of years.

Past Occurrences
Disaster Declaration History
Climate change has never been directly linked to any declared disasters, as shown in Table 4-4.

NCDC Events
The NCDC does not track climate change events.

Hazard Mitigation Planning Committee Events
While the HMPC noted that climate change is of concern, no specific climate change incidents could be recalled. HMPC members noted that the strength of storms does seem to be increasing, and the temperatures seem to be getting hotter. The HMPC also noted that snow levels seem to be higher (or larger) each year, and the recent atmospheric river winter rains were more intense. In addition, the Plumas County Air Quality District noted that within the last five years, Plumas County has experienced extreme droughts that have affected the health of the forest. The lack of rain lead to the Bark Beetle infestation, causing thousands upon thousands of trees to die. This has led to large areas of Plumas County to be declared high fire hazard zones as well. The combined effects of the drought, decline in the health of the forest, temperature changes, and wildfires in and around neighboring counties, much contributed to climate change, have also steered our way to poor air quality.

Likelihood of Future Occurrence
Highly Likely – Climate change is virtually certain to continue without immediate and effective global action. According to National Aerospace and Science Agency (NASA), 2024 was one of the hottest years on record. Without significant global action to reduce greenhouse gas emissions, the United Nations IPCC concludes in its Sixth Assessment Synthesis Report (2022) that average global temperatures are likely to exceed 1.5°C by the end of the 21st century, with consequences for people, assets, economies and ecosystems, including risks from heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise and storm surges.

Climate Scenarios
The United Nations IPCC developed several greenhouse gas (GHG) emissions scenarios based on differing sets of assumptions about future economic growth, population growth, fossil fuel use, and other factors.

The emissions scenarios range from “business-as-usual” (i.e., minimal change in the current emissions trends) to more progressive (i.e., international leaders implement aggressive emissions reductions policies).

Each of these scenarios leads to a corresponding GHG concentration, which is then used in climate models to examine how the climate may react to varying levels of GHGs. Climate researchers use many global climate models to assess the potential changes in climate due to increased GHGs.

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Key Uncertainties Associated with Climate Projections
➢ Climate projections and impacts, like other types of research about future conditions, are characterized by uncertainty. Climate projection uncertainties include but are not limited to:
✓ Levels of future greenhouse gas concentrations and other radiatively important gases and aerosols, ✓ Sensitivity of the climate system to greenhouse gas concentrations and other radiatively important gases and aerosols,
✓ Inherent climate variability, and
✓ Changes in local physical processes (such as afternoon sea breezes) that are not captured by global climate models.

Even though precise quantitative climate projections at the local scale are characterized by uncertainties, the information provided can help identify the potential risks associated with climate variability/climate change and support long term mitigation and adaptation planning.

Maps show projected change in average surface air temperature in the later part of this century (2071-2099) relative to the later part of the last century (1970-1999) under a scenario that assumes substantial reductions in heat trapping gases and a higher emissions scenario that assumes continued increases in global emissions.

These are shown in Figure 4-64.

Figure 4-64 Projected Temperature Change – Lower and Higher Emissions Scenario Source: National Climate Assessment
According to the California Natural Resource Agency (CNRA), climate change is already affecting California and is projected to continue to do so well into the foreseeable future. Current and projected changes include increased temperatures, sea level rise, a reduced winter snowpack altered precipitation patterns, and more frequent storm events. Over the long term, reducing GHGs can help make these changes less severe, but the changes cannot be avoided entirely. Unavoidable climate impacts can result in a variety

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of secondary consequences including detrimental impacts on human health and safety, economic continuity, ecosystem integrity and provision of basic services.

The CNRA’s 2021 CAS (as well as the 2024 Draft CAS) delineated how climate change may impact and exacerbate natural hazards in the future, including wildfires, extreme heat, floods, and drought:
➢ Climate change is expected to lead to increases in the frequency, intensity, and duration of extreme heat events and heat waves in Plumas County and the rest of California, which are likely to increase the risk of mortality and morbidity due to heat-related illness and exacerbation of existing chronic health conditions. Those most at risk and vulnerable to climate-related illness are the elderly, individuals with chronic conditions such as heart and lung disease, diabetes, and mental illnesses, infants, the socially or economically disadvantaged, and those who work outdoors. ➢ Higher temperatures will melt the Sierra snowpack earlier and drive the snowline higher, resulting in less snowpack to supply water to California users.
➢ Droughts are likely to become more frequent and persistent in the 21st century. ➢ Intense rainfall events, periodically ones with larger than historical runoff, will continue to affect California with more frequent and/or more extensive flooding. ➢ Storms and snowmelt may coincide and produce higher winter runoff from the landward side, while accelerating sea-level rise will produce higher storm surges during coastal storms. Together, these changes may increase the probability of floods and levee and dam failures, along with creating issues related to saltwater intrusion.
➢ Warmer weather, reduced snowpack, and earlier snowmelt can be expected to increase wildfire through fuel hazards and ignition risks. These changes can also increase plant moisture stress and insect populations, both of which affect forest health and reduce forest resilience to wildfires. An increase in wildfire intensity and extent will increase public safety risks, property damage, fire suppression and emergency response costs to government, watershed and water quality impacts, vegetation conversions and habitat fragmentation.

Vulnerability to Climate Change
Vulnerability—High
Climate change adaptation is a key priority of the State of California. The 2023 State of California MultiHazard Mitigation Plan stated that climate change is already affecting California. Sea levels have risen by as much as seven inches along the California coast over the last century, increasing erosion and pressure on the state’s infrastructure, water supplies, and natural resources. The State has also seen increased average temperatures, more extreme hot days, fewer cold nights, a lengthening of the growing season, shifts in the water cycle with less winter precipitation falling as snow, and earlier runoff of both snowmelt and rainwater in the year. In addition to changes in average temperatures, sea level, and precipitation patterns, the intensity of extreme weather events is also changing.

The whole of the County is at some measure of vulnerability to climate change. An assessment of a community’s vulnerability to this hazard begins with an understanding of local exposure to unincorporated Plumas County. This is included in the Local Concerns section below. After that, vulnerability is discussed in multiple sections that detail how this hazard can affect both the entire Plumas County Planning Area as well as the unincorporated portions of Plumas County. How the hazard affects the other participating

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jurisdictions can be found in their respective annexes to this LHMP Update. These sections below include assets at risk, impacts, and how future development can be affected by this hazard.

Local Concerns (unincorporated Plumas County)
The unincorporated County has specific concerns regarding this hazard. These concerns form a portion of the basis for the mitigation strategy and mitigation actions that seek to reduce risk and vulnerability to this hazard.

Anthropogenic and other GHGs will continue to cause more extreme climate change conditions unless steps are taken to minimize GHGs and carbon footprints. The County’s roadways, infrastructure in flood zones, disadvantaged communities, agriculture sector and sensitives species habitat are among the most vulnerable to climate change impacts.

Climate change threatens the food security and economic viability of agriculture in Plumas County.

Drought, flood, and severe weather are all poised to intensify as climate change continues to impact California. Agricultural practices need updating to become resilient and can be part of the solution to climate change in our region.

CAL FIRE noted that wildfires pose several significant public health concerns. Here are some of the chief local concerns:
➢ Respiratory Issues: Wildfire smoke contains fine PM2.5 that can penetrate deep into the lungs and even enter the bloodstream, exacerbating conditions like asthma, bronchitis, and chronic obstructive pulmonary disease (COPD).
➢ Cardiovascular Problems: Exposure to wildfire smoke can increase the risk of heart attacks, arrhythmias, and other cardiovascular issues.
➢ Vulnerable Populations: Certain groups, such as children, the unhoused, the elderly, pregnant women, and those with pre-existing health conditions, are more susceptible to the health impacts of wildfire smoke.
➢ Mental Health: The stress and trauma from experiencing wildfires and displacement can lead to mental health issues like anxiety, depression, and post-traumatic stress disorder (PTSD). ➢ Long-term Health Effects: Prolonged exposure to wildfire smoke can have lasting health impacts, including increased rates of respiratory infections and other chronic conditions. ➢ Emergency Room Visits and Hospitalizations: Increased air pollution from wildfires can lead to a rise in emergency room visits and hospitalizations for respiratory and cardiovascular issues which could overcome our small rural hospital’s ability to care for patients.

Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk
Assets at risk from climate change include people and populations; structures; critical infrastructure and vulnerable uses (with community lifelines); natural, historic, and cultural resources; and economic assets

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and community activities of value. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

People and Populations
Climate change affects people and populations within a community, especially those climate change issues related to increases in temperature over time. While all populations can be affected by temperature extremes, populations particularly vulnerable include the very old and very young, medically fragile people, people without means of shelter (and air conditioning) or transportation, people who are socially isolated and other socially vulnerable or underserved populations (as shown in Special Populations discussion in Section 4.2). Acclimatization to heat may help reduce risks from heat waves in the healthy general population but may not be sufficient to protect those with underlying issues and lack of resources.

Structures
Climate change, on its own, does not generally impact structures. However, structures in areas of increased wildfire, drought, extreme heat, or flood areas exacerbated by the effects of climate change would be at increased risk, as described throughout this LHMP. Due to the slow onset of climate change, many structures in the County are expected to undergo improvements and adapt over time to a new climate normal.

Critical Infrastructure and Vulnerable Uses
As with structures, critical and vulnerable facilities in areas of increased wildfire, flood areas, or in other hazard areas exacerbated by climate change, would be at increased risk. Climate change is expected to increase the general vulnerability of critical and vulnerable facilities to natural hazards. Due to the slow onset of climate change, critical and vulnerable facilities and community lifelines in the County are expected to undergo improvements and adapt over time to a new climate normal.

Natural, Historic, and Cultural Resources
The rivers, streams, and open space areas of the County supports rich biodiversity, including many specialstatus species. These are all at risk from the effects of climate change. In addition, if heat changes wildfire patterns, all areas (on land) of the County are at increased risk from fire – including natural, historic, and cultural resources. Furthermore, if climate change exacerbates the drought hazard, areas of wetlands in the County may dry up temporarily, which could damage habitat areas for waterfowl and other species that depend on these areas.

FEMA NRI Hazard Vulnerability Assessment
Climate change is not an NRI hazard; therefore, no data is presented.

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Impacts from Climate Change
The APG: Understanding Regional Characteristics identified the following impacts specific to the North Sierra region in which the Plumas County Planning Area is part of:
➢ Temperature increases
➢ Decreased precipitation
➢ Reduced snowpack
➢ Reduced tourism
➢ Ecosystem change
➢ Sensitive species stress
➢ Increased wildfire
Secondary impacts include changes in precipitation patterns, the global water cycle, melting glaciers and ice caps, and rising sea levels. According to the IPCC, climate change will “increase the likelihood of severe, pervasive and irreversible impacts for people and ecosystems” if unchecked.

Through changes to oceanic and atmospheric circulation cycles and increasing heat, climate change affects weather systems around the world. Climate change increases the likelihood and exacerbates the severity of extreme weather – more frequent or intense storms, floods, droughts, and heat waves. The 2023 State of California Hazard Mitigation Plan noted that there are direct and secondary impacts:
➢ Direct Impacts
✓ Increase in average temperature
✓ Changes in annual precipitation
➢ Secondary Impacts
✓ Increased frequency, intensity, and duration of extreme heat days and heat waves/events ✓ Increased flooding
✓ Increased wildfire
✓ Loss of snowpack and decreased water supplies
Consequences for human society include loss of life and injury, damaged infrastructure, long-term health effects, loss of agricultural crops, disrupted transport and freight, and more. Climate change is not a discrete event but a long-term hazard, the effects of which communities are already experiencing.

In addition to these sources, the 2023 State of California Hazard Mitigation Plan noted that according to California’s Fourth Climate Change Assessment, the state will experience the following climate impacts:
➢ Annual average daily high temperatures are expected to rise by 2.7 °F by 2040, 5.8°F by 2070, and

8.8°F by 2100 compared to observed and modeled historical conditions. These changes are statewide averages.
➢ Heat waves are projected to become longer, more intense, and more frequent. ➢ Warming temperatures are expected to increase soil moisture loss and lead to drier conditions. Summer dryness may become prolonged, with soil drying beginning earlier in the spring and lasting longer into the fall and winter.
➢ Droughts are likely to become more frequent and persistent through 2100.

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➢ The strength of the most intense precipitation and storm events affecting California is expected to increase.
➢ Snowpack levels are projected to decline significantly by 2100 due to reduced snowfall and faster snowmelt.
➢ Marine layer clouds are projected to decrease.
➢ Extreme wildfires (i.e., fires larger than 24,710 acres) would occur 50 percent more frequently. The maximum area burned statewide may increase 178 percent by the end of the century. ➢ Sea level rise is expected to continue to increase beach, cliff, and bluff erosion.

Future Conditions
Future conditions may be affected by climate change, changes in population patterns (migration, density, or the makeup of socially vulnerable populations), and changes in land use and development. Findings on this for Plumas County include the following:
➢ As climate change continues to accelerate over time, climate related impacts to the County will continue to increase.
➢ Climate change can influence population growth in the County over time. While the County is expected to decrease, the County could see population fluctuations as a result of climate impacts relative to those experienced in other regions, and these fluctuations are expected to impact demand for housing and other development. While there are currently no formal studies of specific migration patterns expected to impact the County region, climate-induced migration was recognized within the UNFCCC Conference of Parties Paris Agreement of 2015 and is expected to be the focus of future studies. So, while changes in population and population patterns are not expected to increase the impacts associated with this hazard, climate change may instead influence future population growth in the County. ➢ Land use planning should be proactive to address future hazard conditions. The increasing severity and frequency of storms and other weather events due to the changing climate should be taken into account while considering land use planning for the County. Ensuring that regulatory requirements, such as building codes, are updated and followed will help reduce future impacts to this hazard.

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4.3.8. Dam Failure
Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent; past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

Hazard/Problem Description
Dams are manmade structures built for a variety of uses including flood protection, power generation, agriculture, water supply, and recreation. When dams are constructed for flood protection, they are usually engineered to withstand a flood with a computed risk of occurrence. For example, a dam may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any one year. If prolonged periods of rainfall and flooding occur that exceed the design requirements, that structure may be overtopped or fail. Overtopping is the primary cause of earthen dam failure in the United States.

Dam failures can also result from any one or a combination of the following causes:
➢ Earthquake
➢ Inadequate spillway capacity resulting in excess overtopping flows ➢ Internal erosion caused by embankment or foundation leakage, or piping or rodent activity ➢ Improper design
➢ Improper maintenance
➢ Negligent operation
➢ Failure of upstream dams on the same waterway
In general, there are three types of dams: concrete arch or hydraulic fill, earth and rockfill, and concrete gravity. Each type of dam has different failure characteristics. A concrete arch or hydraulic fill dam can fail almost instantaneously; the flood wave builds up rapidly to a peak then gradually declines. An earthrockfill dam fails gradually due to erosion of the breach; a flood wave will build gradually to a peak and then decline until the reservoir is empty. A concrete gravity dam can fail instantaneously or gradually with a corresponding buildup and decline of the flood wave.

The CA DWR Division of Safety of Dams (DSOD) has jurisdiction over impoundments that meet certain capacity and height criteria. Embankments that are less than six feet high and impoundments that can store less than 15 acre-feet are non-jurisdictional. Additionally, dams that are less than 25 feet high can impound up to 50 acre-feet without being jurisdictional. FEMA and CA DWR DSOD assign hazard potential classifications to dams within the State that provides information on the potential impact should a dam fail.

The following two factors are considered when assigning hazard ratings: existing land use and land use controls (zoning) downstream of the dam. FEMA categorizes the downstream hazard potential into three categories in increasing severity: Low, Significant, and High. DSOD adds a fourth category of Extremely High. Dams are classified in four categories that identify the potential hazard to life and property:

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➢ Extremely High Hazard – Expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more.
➢ High Hazard – Expected to cause loss of at least one human life. ➢ Significant Hazard – No probable loss of human life but can cause economic loss, environmental damage, impacts to critical facilities, or other significant impacts. ➢ Low Hazard – No probable loss of human life and low economic and environmental losses. Losses are expected to be principally limited to the owner’s property.

Location and Extent
According to data provided by Plumas County, CA DWR DSOD, there are 22 dams in Plumas County that were constructed for flood control, storage, treatment impoundments, electrical generation, and recreational purposes. One dam outside the County (Indian Ole in Lassen County – a high hazard dam) could also affect areas inside the County. Of the 23 dams that could affect the County, 1 is rated as Extremely High Hazard, 16 are rated as High Hazard, 4 are rated as Significant Hazard, and 2 are rated as Low Hazard. Table 4-53 gives information on each of the dams in the County, and the dams outside the County that could affect areas in Plumas County.

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Figure 4-65 Plumas County Dam Inventory (Including Dams from Outside the County)

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Table 4-53 Plumas County – Dam Inventory (both Inside and Outside the County) Name River/Stream
Hazard
Classification Owner Dam Type
Capacity
(acre-ft)
Structural
Height (ft) Year Built
Inside Plumas County
Antelope Indian Creek High CA DWR Earth 22,566 113 1964 Bidwell
Lake**
Canyon Creek High Indian Valley
Community
Services
District
Earth and
Rock
5,200 35 1865
Bucks
Diversion
Brush Creek High PG&E Earth 5,843 99 1928
Bucks
Storage
Brush Creek High PG&E Rockfill 103,000 122 1928
Butt Valley Butt Creek High PG&E Earth 49,800 106 1924
Caribou
Afterbay
North Fork
Feather River
High PG&E Earth and
Rock
2,400 164 1959
Chester
Diversion
North Fork
Feather River
High Central
Valley Flood
Protection
Board
Earth 7 47 1975
Cresta North Fork
Feather River
High PG&E Gravity 4,400 103 1949
Eureka Eureka Creek Significant California
Dept. of
Parks and
Recreation
Earth 220 29 1866
Frenchman Last Chance
Creek
High CA DWR Earth 55,477 139 1961
Grizzly
Creek
Big Grizzly
Creek
High Private Gravity 140 39 1915
Grizzly
Forebay
Grizzly Creek High PG&E Arch 1,112 92 1928
Grizzly
Valley
Big Grizzly
Creek
High CA DWR Earth and
Rock
83,000 115 1966
Lake
Almanor
North Fork
Feather River
Extremely High PG&E Earth 1,308,000 130 1927
Little Grass
Valley
South Fork
Feather River
High South
Feather
Water and
Power
Agency
Rockfill 74,730 210 1961
Long Lake Gray Eagle
Creek
Significant Graeagle
Water
Company
Rockfill 1,478 12 1938
Lower Three
Lakes
Feather River Significant PG&E Rockfill 525 32 1928

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Name River/Stream
Hazard
Classification Owner Dam Type
Capacity
(acre-ft)
Structural
Height (ft) Year Built
Rock Creek North Fork
Feather River
High PG&E Gravity 4,660 120 1950
Silver Lake Silver Creek High Soper
Wheeler
Company
Earth and
Rock
650 21 1906
Slate Creek
Diversion
Slate Creek Low South
Feather
Water and
Power
Agency
Arch 643 72 1961
South Fork
Diversion
South Fork
Feather River
Low South
Feather
Water and
Power
Agency
Earth 88 70 1961
Taylor Lake Tributary of
Indian Creek
Significant Nature
Conservancy
Earth 380 14 1929
Dams Outside the County
Indian Ole
(Lassen
County)
Hamilton
Creek
High PG&E Flashboard
and Buttress
24,800 26 1924
Source: DSOD/Cal OES/NID Dam Status 1/25, National Performance of Dams Program *One Acre Foot=326,000 gallons
**this dam is locally known as Round Valley
Once a list of dams was established, dam inundation layers were obtained from DSOD in GIS. Only extremely high and high hazard dams were analyzed for this LHMP due to the potential for these dams to cause loss of life in the Plumas County Planning Area should a dam failure occur. Dams with an inundation area in the Plumas County Planning Area are shown on Figure 4-66 for extremely high hazard dams, and Figure 4-67 for high hazard dams. All extremely high and high hazard dams had available inundation mapping. Due to the size of Plumas County, inset maps derived from Figure 4-67 were created to zoom in on areas of dam inundation. These are shown from Figure 4-68 through Figure 4-71. Geographic flood extents as shown on the dam inundation maps are detailed by dam in Table 4-54 for both extremely high and high hazard dams. As shown, only the unincorporated County has areas located in the extremely high hazard dam inundation areas. The high hazard dam inundation zones are located in the unincorporated County, as well as in Portola. Information on the City of Portola can be found in its annex to this Plan.

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Figure 4-66 Plumas County Planning Area – Extremely High Hazard Dam Inundation Areas

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Figure 4-67 Plumas County Planning Area – High Hazard Dam Inundation Areas (Overview and Map Insets)

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Figure 4-68 Plumas County Planning Area – High Hazard Dam Inundation Areas (Map Inset 1)

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Figure 4-69 Plumas County Planning Area – High Hazard Dam Inundation Areas (Map Inset 2)

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Figure 4-70 Plumas County Planning Area – High Hazard Dam Inundation Areas (Map Inset 3)

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Figure 4-71 Plumas County Planning Area – High Hazard Dam Inundation Areas (Map Inset 4)

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Table 4-54 Plumas County Planning Area–Geographical Dam Inundation Extents Dam Inundation
Area
Total Acres % of Total
Acres
Improved
Acres
% of Total
Improved
Acres
Unimproved
Acres
% of Total
Unimproved
Acres
Extremely High Hazard Dams
Unincorporated County
Lake Almanor 2,355 0.14% 19 0.02% 2,337 0.15%
High Hazard Dams
City of Portola
Frenchman 78 2.2% 5 0.2% 73 5.8%
Grizzly Creek 61 1.7% 4 0.2% 57 4.5%
Grizzly Valley 234 6.7% 33 1.5% 202 16.1%
Unincorporated County
Antelope 9,462 0.6% 2,607 2.9% 6,855 0.4%
Bidwell Lake* 2,660 0.2% 509 0.6% 2,151 0.1%
Bucks Diversion 622 0.04% 2 0.002% 621 0.04%
Bucks Storage 833 0.05% 2 0.002% 831 0.05%
Butt Valley 1,648 0.10% 10 0.01% 1,638 0.1%
Caribou Afterbay 983 0.06% 7 0.01% 976 0.06%
Chester Diversion 2,781 0.2% 654 0.7% 2,127 0.1%
Cresta 44 0.003% 0 0.0% 44 0.003%
Frenchman 34,051 2.0% 16,764 18.5% 17,287 1.1%
Grizzly Creek 609 0.04% 81 0.1% 528 0.03%
Grizzly Forebay 192 0.01% 0 0.0% 192 0.01%
Grizzly Valley 16,282 1.0% 3,052 3.4% 13,230 0.8%
Indian Ole 25,743 1.5% 5 0.01% 25,738 1.6%
Little Grass Valley 523 0.03% 0 0.0% 523 0.03%
Rock Creek 571 0.03% 1 0.001% 571 0.04%
Silver Lake 971 0.06% 299 0.3% 672 0.04%
Source: DSOD/Cal OES/NID Dam Status 01/25
*this dam is locally known as Round Valley
There is no scale with which to measure dam failure, only a scale to measure dam failure vulnerability based on size of dam and proximity to development. Dam failure may range from a small breach to a total failure. While a dam may fill slowly with runoff from winter storms, a dam break can have a very quick speed of onset. The duration of dam failure is not long – only as long as it takes to empty the reservoir of water the dam held back.

For dam overtopping, the speed of onset is somewhat slower than that of a dam break, and the duration is longer (as evidenced in the 2017 Oroville Dam event). The Plumas County Planning Area would be affected for as long as the flood waters from the dam failure takes to drain downstream.

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Past Occurrences
Disaster Declaration History
There have been no disaster declarations related to dam failure in Plumas County, as shown in Table 4-4.

NCDC Events
There have been no NCDC dam failure events in Plumas County.

National Performance of Dams Program Events
The National Performance of Dams Program at Stanford University tracks dam failures. A search of the National Performance of Dams Program database showed no past dam failure events in Plumas County.

Hazard Mitigation Planning Committee Events
No events of dam failure were noted to have occurred in Plumas County.

Likelihood of Future Occurrence
Unlikely—No dam failure events have occurred in the County. Further, based on historical data and input from the HMPC, it is unlikely that major dam failure event will occur in Plumas County. There is concern that many of the State’s older dams, including those in Plumas County, could start experiencing problems associated with aging infrastructure and deferred maintenance issues.

It is likely that climate change will increase the chance of future occurrence as well as future impacts. More information on climate change and dam failure can be found in the next section. More information on future impacts can be found in the Future Conditions/Future Development section of the Vulnerability Assessment below.

Climate Change and Dam Failure
The 2023 California State Hazard Mitigation Plan noted that modeling described in California’s Fourth Climate Change Assessment projects less frequent but more extreme daily precipitation. Year-to-year precipitation will become more volatile, and the number of dry years will increase by mid-century. As the climate continues to warm, atmospheric rivers will carry more moisture, and extreme precipitation may increase. Climate model projections show a tendency for the northern part of the State to become wetter.

Increases in both precipitation and heat causing snow melt in areas upstream of dams could increase the potential for dam failure and uncontrolled releases in Plumas County.

Vulnerability Assessment
Vulnerability—Extremely High
Dam failure flooding can occur as the result of a partial or complete collapse of an impoundment. Dam failures often result from prolonged rainfall and flooding. An earthquake event can also contribute to a

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Plumas County 4-184
Local Hazard Mitigation Plan Update
January 2026
dam failure. The primary danger associated with dam failure is the high velocity flooding of those properties downstream of the dam. A dam failure can range from a small, uncontrolled release to a catastrophic failure. Vulnerability to dam failures is generally confined to the areas subject to inundation downstream of the facility. Secondary losses would include loss of the multi-use functions of the facility and associated revenues that accompany those functions. Dam failure flooding would vary by community depending on which dam fails and the nature and extent of the dam failure and associated flooding.

The whole of the Planning Area has some measure of vulnerability to dam failure. An assessment of a community’s vulnerability to this hazard begins with an understanding of local exposure to unincorporated Plumas County. This is included in the Local Concerns section below. After that, vulnerability is discussed in multiple sections that detail how this hazard can affect both the entire Plumas County Planning Area as well as the unincorporated portions of Plumas County. How the hazard affects the other participating jurisdictions can be found in their respective annexes to this LHMP Update. These sections below include assets at risk, impacts, and how future development can be affected by this hazard.

The County noted that there are many dams that can affect the County, but none are directly owned by any County entity. The County noted that it does outreach and education to dam owners, assists with yearly inspections, and plans. The greatest concerns are to the people and property downstream of the dams, as discussed in the sections below.

Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk from Dam Failure
Assets at risk from dam failure include people and populations; structures; critical facilities and infrastructure; and natural, historic, and cultural resources. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

Methodology (Location, Parcels, and Structures)
In order to analyze people and populations, structures, and critical facilities at risk in dam inundation areas, GIS was used to quantify those values. The 2024 Plumas County Parcel/Assessor Data was provided by Plumas County and was used as the basis for the inventory of assessed values for both improved and unimproved parcels within the Plumas County Planning Area. The dam inundation areas, obtained from DSOD, were then overlaid on the parcel layer. For the purposes of this analysis, if the dam inundation layer

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January 2026
intersected any portion of a parcel polygon, the entire parcel was considered to be in the dam inundation area. The parcels were segregated and analyzed in this fashion for the entire Planning Area. This analysis was performed on extremely high and high hazard dam inundation areas only. No analyses for significant, low or unknown hazard classification dams analysis were performed.

This dam inundation analysis can be misleading since there are many large polygons throughout the County.

All extremely high and high classification dams in the County have mapped dam inundation areas. Those dams, their hazard classification, mapping status, and whether they inundate areas in Plumas County are shown on Table 4-55.

Table 4-55 Plumas County Planning Area – Dam Inundation Mapping Status and Availability Dam
Number
Source
County
Cal OES Dam Inventory DSOD Dam Inundation Data Mapped Plumas County
Affected
(analyzed)
Dam
Inventory
Previous Plan
Classification
DSOD
Data
Available
DSOD
Classification
1 Plumas
County
Lake
Almanor
Extremely High Lake
Almanor
Extremely High Yes Yes
2 Plumas
County
Antelope High Antelope High Yes Yes
3 Plumas
County
Bidwell
Lake
High Bidwell
Lake
High Yes Yes
4 Plumas
County
Bucks
Diversion
High Bucks
Diversion
High Yes Yes
5 Plumas
County
Bucks
Storage
Extremely High Bucks
Storage
High Yes Yes
6 Plumas
County
Butt Valley High Butt Valley High Yes Yes
7 Plumas
County
Caribou
Afterbay
High Caribou
Afterbay
High Yes Yes
8 Plumas
County
Chester
Diversion
High Chester
Diversion
High Yes Yes
9 Plumas
County
Cresta High Cresta High Yes Yes
10 Plumas
County
Frenchman High Frenchman High Yes Yes
11 Plumas
County
Grizzly
Creek
High Grizzly
Creek
High Yes Yes
12 Plumas
County
Grizzly
Forebay
High Grizzly
Forebay
High Yes Yes
13 Plumas
County
Grizzly
Valley
High Grizzly
Valley
High Yes Yes
14 Lassen
County
Indian Ole High Indian Ole High Yes Yes

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Plumas County 4-186
Local Hazard Mitigation Plan Update
January 2026
Dam
Number
Source
County
Cal OES Dam Inventory DSOD Dam Inundation Data Mapped Plumas County
Affected
(analyzed)
Dam
Inventory
Previous Plan
Classification
DSOD
Data
Available
DSOD
Classification
15 Plumas
County
Little Grass
Valley
High Little Grass
Valley
High Yes Yes
16 Plumas
County
Rock Creek High Rock Creek High Yes Yes
17 Plumas
County
Silver Lake High Silver Lake High Yes Yes
18 Plumas
County
Eureka Significant Eureka Significant Yes No
19 Plumas
County
Long Lake Significant Long Lake No DSOD
Inundation/Class
Available
No No
20 Plumas
County
Lower
Three Lakes
Significant Lower
Three Lakes
Significant Yes No
21 Plumas
County
Taylor Lake Significant Taylor Lake No DSOD
Inundation/Class
Available
No No
22 Plumas
County
Slate Creek
Diversion
Low Slate Creek
Diversion
No DSOD
Inundation/Class
Available
No No
23 Plumas
County
South Fork
Diversion
Low South Fork
Diversion
No DSOD
Inundation/Class
Available
No No
Source: Cal OES/DSOD
People and Populations
All people and populations located in dam inundation areas are vulnerable to dam failure. Certain vulnerable populations may be at increased risk to dam failure, especially during a large event with minimal advance notice. These vulnerable populations may include: the unhoused, those with limited mobility, and those that lack the resources to leave the area.

Plumas County residents that live in these dam inundation areas are often the most vulnerable. Not only are the residents at risk, but their homes and contents are all at risk, compounding the impacts associated with significant hazard events. To further evaluate the impact to the Plumas County Planning Area’s residential populations located in these hazard areas, a separate analysis was performed to determine residential populations in the dam inundation areas. The DSOD dam inundation areas were overlayed on the parcel layer and linked to Assessor Data. Those residential parcel polygons that intersect the dam inundation areas were counted and multiplied by the Census Bureau average household factors for each jurisdiction. This is shown in Table 4-56.

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Plumas County 4-187
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January 2026
Table 4-56 Plumas County Planning Area – Residential Population in Extremely High and High Hazard Dam Inundation Area by Jurisdiction
Dam Inundation Area
City of Portola Unincorporated Plumas County
Improved Residential
Parcels
Population Improved
Residential Parcels
Population
Extremely High Hazard Dams
Lake Almanor 0 0 8 19
High Hazard Dams
Antelope 0 0 170 406
Bidwell Lake 0 0 45 108
Bucks Diversion 0 0 0 0
Bucks Storage 0 0 0 0
Butt Valley 0 0 3 7
Caribou Afterbay 0 0 2 5
Chester Diversion 0 0 1,023 2,445
Cresta 0 0 0 0
Eureka 0 0 0 0
Frenchman 13 30 108 258
Grizzly Creek 12 27 29 69
Grizzly Forebay 0 0 0 0
Grizzly Valley 82 188 396 946
Indian Ole (Lassen County) 0 0 143 342
Little Grass Valley 0 0 0 0
Rock Creek 0 0 0 0
Silver Lake 0 0 64 153
Source: Cal OES, DSOD, 2024 Plumas County Parcel/Assessor Data, 2023 US Census American Community Survey Average Household Size – City of Portola (2.29); Unincorporated Plumas County (2.39).

Structures
Many structures in the Plumas County Planning Area have some measure of risk to dam failure. Dam failure flooding can affect the built environment of many locations in the Plumas County Planning Area.

Structures in dam inundation areas are at risk and depending on flood depths, can range from slight damage to total inundation. Analysis by extremely high hazard dam and high hazard dam follows.

Extremely High Hazard Dam Analysis
Parcel counts, land, and improved values (i.e., those with a structure improvement on the parcel), personal property, estimated content replacement values, and total values that fall within each extremely high hazard dam inundation areas in the Planning Area are presented below by dam. For the extremely high hazard dam inundation areas, only the unincorporated County is affected. Breakdowns of parcel counts by property

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use in each inundation are for the incorporated jurisdictions in the County are shown in their respective annexes to this LHMP Update. Analysis is presented in the following tables. ➢ Table 4-57 shows the summary counts and values of parcels in all extremely high hazard dam inundation areas in the Plumas County Planning Area. It should be noted that only the unincorporated County is affected by extremely high hazard dam inundations. ➢ Table 4-58 shows the count and value of parcels in the Lake Almanor Dam inundation area detailed by property use for the unincorporated County.

Table 4-57 Plumas County Planning Area – Summary Counts and Values of Parcels and Structures in Extremely High Hazard Dam Inundation Area by Dam Dam
Inundation
Area
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total
Value
Lake Almanor 47 12 $1,493,588 $2,266,764 $62,488 $2,049,844 $5,872,684 Source: Cal OES/DSOD, 2024 Plumas County Parcel/Assessor Data Table 4-58 Unincorporated Plumas County – Count and Value of Parcels and Structures in Extremely High Hazard Dam Inundation Area (Lake Almanor) by Property Use
Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Lake Almanor
Agricultural 2 $55,771 $0 $0 $0 $55,771
Commercial 4 3 $109,485 $1,736,350 $56,350 $1,736,350 $3,638,535 Federal Lands 9 0 $0 $0 $0 $0 $0
Government 2 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 1 0 $3,716 $0 $0 $0 $3,716
Recreational 1 1 $122,330 $96,574 $638 $96,574 $316,116 Residential 22 8 $1,202,286 $433,840 $5,500 $216,920 $1,858,546 ROW/Utilities 6 0 $0 $0 $0 $0 $0
Unincorporated
Plumas County
Total
47 12 $1,493,588 $2,266,764 $62,488 $2,049,844 $5,872,684 Source: Cal OES/DSOD, 2024 Plumas County Parcel/Assessor Data High Hazard Dam Analysis
Parcel counts, land, and improved values (i.e., those with a structure improvement on the parcel), personal property values, estimated content replacement values, and total values that fall within each high hazard

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Plumas County 4-189
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January 2026
dam inundation areas in the Planning Area are presented below by dam. For the high hazard dam inundation areas, both the City of Portola (Frenchman, Grizzley Creek, Grizzly Valley) as well as the unincorporated County is affected. The results are presented in tables below by:
➢ Table 4-59 shows the shows the summary counts and values of parcels in all high hazard dam inundation areas. This table includes summaries for both the unincorporated County, as well as the City of Portola.
➢ Table 4-60 shows the count and value of parcels in each high hazard dam inundation area detailed by property use for the unincorporated County.

More information on dam inundations affecting the City of Portola is shown in their annex to this LHMP Update.

Table 4-59 Plumas County Planning Area – Summary Counts and Values of Parcels and Structures in High Hazard Dam Inundation Area by Dam
Dam
Inundation
Area
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Antelope 445 203 $32,907,880 $41,089,602 $2,946,487 $24,732,037 $101,676,006 Bidwell Lake 276 63 $16,141,238 $12,518,086 $773,289 $9,227,265 $38,659,878 Bucks
Diversion
7 1 $64,771 $44,198 $0 $44,198 $153,167
Bucks Storage 10 1 $64,771 $44,198 $0 $44,198 $153,167
Butt Valley 32 6 $588,715 $1,940,477 $56,350 $1,838,414 $4,423,956 Caribou
Afterbay
23 5 $211,447 $1,874,126 $56,350 $1,805,238 $3,947,161
Chester
Diversion
1,496 1,167 $73,477,181 $200,553,635 $25,417,370 $121,650,730 $421,098,916 Cresta 1 0 $0 $0 $0 $0 $0
Frenchman* 487 187 $47,205,467 $43,065,368 $2,913,406 $30,157,820 $123,342,061 Grizzly Creek* 188 59 $11,183,050 $14,622,880 $65,215 $10,508,935 $36,380,080 Grizzly
Forebay
6 0 $59,280 $0 $0 $0 $59,280
Grizzly Valley* 1,207 667 $78,302,639 $136,953,183 $914,730 $90,820,304 $306,990,856 Indian Ole 193 143 $70,765,788 $74,205,539 $3,608 $37,102,770 $182,077,705 Little Grass
Valley
7 0 $86,653 $0 $0 $0 $86,653
Rock Creek 8 1 $64,771 $44,198 $0 $44,198 $153,167
Silver Lake 122 64 $8,144,051 $10,846,489 $49,550 $5,859,745 $24,899,835 Source: Cal OES/DSOD, 2024 Plumas County Parcel/Assessor Data * Affect the City of Portola and the unincorporated County

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Plumas County 4-190
Local Hazard Mitigation Plan Update
January 2026
Table 4-60 Unincorporated Plumas County– Count and Value of Parcels and Structures in High Hazard Dam Inundation Area by Dam and Property Use Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Antelope
Agricultural 76 21 $10,400,951 $3,369,109 $89,560 $3,369,109 $17,228,729 Commercial 10 8 $1,142,392 $3,834,150 $193,933 $3,834,150 $9,004,625 Federal Lands 16 0 $0 $0 $0 $0 $0
Government 17 0 $0 $0 $0 $0 $0
Industrial 7 2 $483,334 $446,826 $0 $670,239 $1,600,399 Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 3 2 $82,464 $277,561 $40,139 $277,561 $677,725 Residential 280 170 $20,798,739 $33,161,956 $2,622,855 $16,580,978 $73,164,528 ROW/Utilities 35 0 $0 $0 $0 $0 $0
Total 445 203 $32,907,880 $41,089,602 $2,946,487 $24,732,037 $101,676,006 Bidwell Lake
Agricultural 22 6 $4,673,292 $1,382,310 $20,260 $1,382,310 $7,458,172 Commercial 24 8 $1,658,161 $4,161,251 $662,673 $4,161,251 $10,643,336 Federal Lands 2 0 $0 $0 $0 $0 $0
Government 11 0 $0 $0 $0 $0 $0
Industrial 8 2 $384,681 $57,661 $0 $86,492 $528,834
Institutional 1 0 $27,886 $0 $0 $0 $27,886
Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 4 2 $99,018 $277,561 $40,139 $277,561 $694,279 Residential 175 45 $9,298,200 $6,639,303 $50,217 $3,319,652 $19,307,372 ROW/Utilities 28 0 $0 $0 $0 $0 $0
Total 276 63 $16,141,238 $12,518,086 $773,289 $9,227,265 $38,659,878 Bucks Diversion
Agricultural 2 0 $55,771 $0 $0 $0 $55,771
Commercial 1 1 $9,000 $44,198 $0 $44,198 $97,396
Federal Lands 2 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0

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Plumas County 4-191
Local Hazard Mitigation Plan Update
January 2026
Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Residential 1 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 7 1 $64,771 $44,198 $0 $44,198 $153,167
Bucks Storage
Agricultural 2 0 $55,771 $0 $0 $0 $55,771
Commercial 1 1 $9,000 $44,198 $0 $44,198 $97,396
Federal Lands 2 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 1 0 $0 $0 $0 $0 $0
Residential 3 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 10 1 $64,771 $44,198 $0 $44,198 $153,167
Butt Valley
Agricultural 2 0 $55,771 $0 $0 $0 $55,771
Commercial 4 3 $109,485 $1,736,350 $56,350 $1,736,350 $3,638,535 Federal Lands 9 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 10 3 $423,459 $204,127 $0 $102,064 $729,650 ROW/Utilities 6 0 $0 $0 $0 $0 $0
Total 32 6 $588,715 $1,940,477 $56,350 $1,838,414 $4,423,956 Caribou Afterbay
Agricultural 2 0 $55,771 $0 $0 $0 $55,771
Commercial 4 3 $109,485 $1,736,350 $56,350 $1,736,350 $3,638,535 Federal Lands 6 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0

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Plumas County 4-192
Local Hazard Mitigation Plan Update
January 2026
Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Recreational 0 0 $0 $0 $0 $0 $0
Residential 5 2 $46,191 $137,776 $0 $68,888 $252,855
ROW/Utilities 5 0 $0 $0 $0 $0 $0
Total 23 5 $211,447 $1,874,126 $56,350 $1,805,238 $3,947,161 Chester Diversion
Agricultural 13 0 $297,165 $0 $0 $0 $297,165
Commercial 163 121 $17,051,266 $35,160,368 $24,614,834 $35,160,368 $111,986,836 Federal Lands 3 0 $0 $0 $0 $0 $0
Government 33 0 $0 $0 $0 $0 $0
Industrial 19 13 $1,454,167 $2,954,682 $671 $4,432,023 $8,841,543 Institutional 16 9 $307,206 $1,412,790 $11,680 $1,412,790 $3,144,466 Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 1 1 $53,060 $265,302 $0 $265,302 $583,664
Residential 1,187 1,023 $54,314,317 $160,760,493 $790,185 $80,380,247 $296,245,242 ROW/Utilities 60 0 $0 $0 $0 $0 $0
Total 1,496 1,167 $73,477,181 $200,553,635 $25,417,370 $121,650,730 $421,098,916 Cresta
Agricultural 0 0 $0 $0 $0 $0 $0
Commercial 0 0 $0 $0 $0 $0 $0
Federal Lands 0 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 0 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 1 0 $0 $0 $0 $0 $0
Frenchman
Agricultural 90 36 $17,901,349 $7,132,959 $2,470,258 $7,132,959 $34,637,525 Commercial 23 16 $2,097,396 $5,229,719 $46,465 $5,229,719 $12,603,299 Federal Lands 16 0 $0 $0 $0 $0 $0
Government 32 0 $0 $0 $0 $0 $0
Industrial 1 0 $83,626 $0 $0 $0 $83,626
Institutional 1 0 $0 $0 $0 $0 $0

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Plumas County 4-193
Local Hazard Mitigation Plan Update
January 2026
Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 7 5 $798,618 $1,727,948 $0 $1,727,948 $4,254,514 Residential 203 108 $23,913,007 $24,844,657 $380,018 $12,422,329 $61,560,011 ROW/Utilities 44 0 $0 $0 $0 $0 $0
Total 418 165 $44,793,996 $38,935,283 $2,896,741 $26,512,955 $113,138,975 Grizzly Creek
Agricultural 2 0 $147,112 $0 $0 $0 $147,112
Commercial 15 9 $1,310,886 $2,433,410 $5,500 $2,433,410 $6,183,206 Federal Lands 4 0 $0 $0 $0 $0 $0
Government 13 0 $0 $0 $0 $0 $0
Industrial 1 0 $83,626 $0 $0 $0 $83,626
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 2 2 $138,946 $1,305,609 $0 $1,305,609 $2,750,164 Residential 68 29 $7,447,841 $7,371,019 $43,050 $3,685,510 $18,547,420 ROW/Utilities 19 0 $0 $0 $0 $0 $0
Total 125 40 $9,128,411 $11,110,038 $48,550 $7,424,529 $27,711,528 Grizzly Forebay
Agricultural 2 0 $59,280 $0 $0 $0 $59,280
Commercial 0 0 $0 $0 $0 $0 $0
Federal Lands 3 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 0 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 6 0 $59,280 $0 $0 $0 $59,280
Grizzly Valley
Agricultural 43 11 $6,190,243 $1,837,037 $103,680 $1,837,037 $9,967,997 Commercial 64 45 $6,309,222 $15,256,180 $113,595 $15,256,180 $36,935,177 Federal Lands 16 0 $0 $0 $0 $0 $0
Government 53 0 $0 $0 $0 $0 $0
Industrial 24 17 $2,243,403 $2,399,030 $190 $3,598,545 $8,241,168

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Plumas County 4-194
Local Hazard Mitigation Plan Update
January 2026
Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Institutional 1 1 $9,399 $175,873 $0 $175,873 $361,145
Miscellaneous 1 0 $0 $0 $0 $0 $0
Recreational 53 34 $2,404,275 $5,183,330 $36,430 $5,183,330 $12,807,365 Residential 595 396 $52,209,444 $85,627,475 $253,480 $42,813,738 $180,904,137 ROW/Utilities 69 0 $0 $0 $0 $0 $0
Total 919 504 $69,365,986 $110,478,925 $507,375 $68,864,703 $249,216,989 Indian Ole
Agricultural 5 0 $201,864 $0 $0 $0 $201,864
Commercial 4 0 $371,757 $0 $0 $0 $371,757
Federal Lands 5 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 3 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 159 143 $70,192,167 $74,205,539 $3,608 $37,102,770 $181,504,084 ROW/Utilities 16 0 $0 $0 $0 $0 $0
Total 193 143 $70,765,788 $74,205,539 $3,608 $37,102,770 $182,077,705 Little Grass Valley
Agricultural 2 0 $82,644 $0 $0 $0 $82,644
Commercial 0 0 $0 $0 $0 $0 $0
Federal Lands 4 0 $0 $0 $0 $0 $0
Government 0 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 1 0 $4,009 $0 $0 $0 $4,009
Recreational 0 0 $0 $0 $0 $0 $0
Residential 0 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 7 0 $86,653 $0 $0 $0 $86,653
Rock Creek
Agricultural 2 0 $55,771 $0 $0 $0 $55,771
Commercial 1 1 $9,000 $44,198 $0 $44,198 $97,396
Federal Lands 3 0 $0 $0 $0 $0 $0
Government 1 0 $0 $0 $0 $0 $0

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Dam
Inundation
Area/ Property
Use
Total
Parcel
Count
Improved
Parcel
Count
Total Land
Value
Improved
Structure
Value
Personal
Property
Value
Estimated
Contents
Value
Total Value
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 1 0 $0 $0 $0 $0 $0
ROW/Utilities 0 0 $0 $0 $0 $0 $0
Total 8 1 $64,771 $44,198 $0 $44,198 $153,167
Silver Lake
Agricultural 10 4 $1,336,802 $856,234 $49,550 $856,234 $3,098,820 Commercial 3 1 $186,866 $16,767 $0 $16,767 $220,400
Federal Lands 8 0 $0 $0 $0 $0 $0
Government 4 0 $0 $0 $0 $0 $0
Industrial 0 0 $0 $0 $0 $0 $0
Institutional 0 0 $0 $0 $0 $0 $0
Miscellaneous 0 0 $0 $0 $0 $0 $0
Recreational 0 0 $0 $0 $0 $0 $0
Residential 90 59 $6,620,383 $9,973,488 $0 $4,986,744 $21,580,615 ROW/Utilities 7 0 $0 $0 $0 $0 $0
Total 122 64 $8,144,051 $10,846,489 $49,550 $5,859,745 $24,899,835 Source: Cal OES/DSOD, 2024 Plumas County Parcel/Assessor Data Critical Facilities and Infrastructure
Dam failure presents a threat to critical facilities and infrastructure. For example, transportation routes, including bridges and highways, are vulnerable to dam inundation and have the potential to be unpassable and even destroyed, creating isolation and supply chain issues. Those critical facilities that are most vulnerable are those that are already in poor condition and would not be able to withstand a large water surge. The following GIS analysis identifies critical facilities and infrastructure at risk to dam inundation flooding, separated out by extremely high hazard and high hazard dams.

Extremely High Hazard Dam Inundation Analysis
Dam failure presents a threat to critical facilities and infrastructure. The following analysis discusses critical facilities and infrastructure. A separate analysis was performed on the critical facility inventory in the Plumas County Planning Area to determine critical facilities in extremely high hazard dam inundation areas. Using GIS, the DSOD extremely high hazard dam inundation areas were overlayed on the critical facility GIS layer. Figure 4-72 shows critical facilities, as well as the extremely high hazard dam inundation areas. Table 4-61 details critical facilities by extremely high hazard dam inundation areas by facility type and count for the Planning Area. Details of critical facility category, type, name and address by extremely

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high hazard dam inundation areas are listed in Appendix G. As shown, 47 critical facilities fall in the extremely high hazard dam inundation area, all of which are located in the unincorporated County.

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Figure 4-72 Plumas County Planning Area – Critical Facilities in Extremely High Hazard Dam Inundation Areas (Overview Map with Inset)

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Table 4-61 Plumas County Planning Area – Critical Facilities in Extremely High Hazard Dam Inundation Zones by Jurisdiction and Critical Facility Category Jurisdiction / Extremely High Hazard
Dam Inundation Area
Critical Facility Category Facility Count
Portola
Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 16
At Risk Populations Facilities 8
Hazardous Materials Facilities 0
Outside of Extremely High Hazard Dam
Inundation Area Total
24 Portola Total 24
Unincorporated Plumas County
Extremely High Hazard Dam Inundation Area
Essential Services Facilities 47
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Extremely High Hazard Dam Inundation
Area Total
47 Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 782
At Risk Populations Facilities 33
Hazardous Materials Facilities 4
Outside of Extremely High Hazard Dam
Inundation Area Total
819
Unincorporated Plumas County Total 866
Unincorporated Butte County
Extremely High Hazard Dam Inundation Area
Essential Services Facilities 2
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Extremely High Hazard Dam Inundation
Area Total
2 Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 2
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of Extremely High Hazard Dam
Inundation Area Total
2 Unincorporated Butte County Total 4
Unincorporated Lassen County
Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 34
At Risk Populations Facilities 0
Hazardous Materials Facilities 0

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Jurisdiction / Extremely High Hazard
Dam Inundation Area
Critical Facility Category Facility Count
Outside of Extremely High Hazard Dam
Inundation Area Total
34 Unincorporated Lassen County Total 34
Unincorporated Sierra County
Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 4
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of Extremely High Hazard Dam
Inundation Area Total
4 Unincorporated Sierra County Total 4
Unincorporated Tehama County
Outside of Extremely High Hazard Dam
Inundation Area
Essential Services Facilities 2
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of Extremely High Hazard Dam
Inundation Area Total
2 Unincorporated Tehama County Total 2
Grand Total 934
Source: Cal OES/DSOD; Plumas County GIS
High Hazard Dam Inundation Analysis
Dam failure presents a threat to critical facilities and infrastructure. The following analysis discusses critical facilities and infrastructure. A separate analysis was performed on the critical facility inventory in the Plumas County Planning Area to determine critical facilities in high hazard dam inundation areas. Using GIS, the DSOD high hazard dam inundation areas were overlayed on the critical facility GIS layer. Figure 4-73 shows critical facilities, as well as the high hazard dam inundation areas. Figure 4-74 through Figure 4-77 show the zoomed inset maps from Figure 4-73. Table 4-62 details critical facilities by high hazard dam inundation areas by facility type and count for the Planning Area. Details of critical facility category, type, name and address by high hazard dam inundation areas are listed in Appendix G. As shown, 8 critical facilities that fall in the high hazard dam inundation area are located in the City of Portola, 186 in the unincorporated County, and 3 in the surrounding counties.

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Figure 4-73 Plumas County Planning Area – Critical Facilities in Extremely High Hazard Dam Inundation Areas (Overview Map)

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Figure 4-74 Plumas County Planning Area – Critical Facilities in High Hazard Dam Inundation Areas (Map Inset 1)

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Figure 4-75 Plumas County Planning Area – Critical Facilities in High Hazard Dam Inundation Areas (Map Inset 2)

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Figure 4-76 Plumas County Planning Area – Critical Facilities in High Hazard Dam Inundation Areas (Map Inset 3)

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Figure 4-77 Plumas County Planning Area – Critical Facilities in High Hazard Dam Inundation Areas (Map Inset 4)

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Table 4-62 Plumas County Planning Area – Critical Facilities in Extremely High Hazard Dam Inundation Zones by Jurisdiction and Critical Facility Category Jurisdiction / High Hazard Dam
Inundation Area
Critical Facility Category Facility Count
Portola
High Hazard Dam Inundation Area
Essential Services Facilities 7
At Risk Populations Facilities 1
Hazardous Materials Facilities 0
High Hazard Dam Inundation Area Total 8
Outside of High Hazard Dam Inundation Area
Essential Services Facilities 9
At Risk Populations Facilities 7
Hazardous Materials Facilities 0
Outside of High Hazard Dam Inundation
Area Total
16 Portola Total 24
Unincorporated Plumas County
High Hazard Dam Inundation Area
Essential Services Facilities 172
At Risk Populations Facilities 13
Hazardous Materials Facilities 1
High Hazard Dam Inundation Area Total 186
Outside of High Hazard Dam Inundation Area
Essential Services Facilities 657
At Risk Populations Facilities 20
Hazardous Materials Facilities 3
Outside of High Hazard Dam Inundation
Area Total
680
Unincorporated Plumas County Total 866
Unincorporated Butte County
High Hazard Dam Inundation Area
Essential Services Facilities 3
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
High Hazard Dam Inundation Area Total 3
Outside of High Hazard Dam Inundation Area
Essential Services Facilities 1
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of High Hazard Dam Inundation
Area Total
1 Unincorporated Butte County Total 4
Unincorporated Lassen County
Outside of High Hazard Dam Inundation Area Essential Services Facilities 34

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Jurisdiction / High Hazard Dam
Inundation Area
Critical Facility Category Facility Count
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of High Hazard Dam Inundation
Area Total
34 Unincorporated Lassen County Total 34
Unincorporated Sierra County
Outside of High Hazard Dam Inundation Area
Essential Services Facilities 4
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of High Hazard Dam Inundation
Area Total
4 Unincorporated Sierra County Total 4
Unincorporated Tehama County
Outside of High Hazard Dam Inundation Area
Essential Services Facilities 2
At Risk Populations Facilities 0
Hazardous Materials Facilities 0
Outside of High Hazard Dam Inundation
Area Total
2 Unincorporated Tehama County Total 2
Grand Total 934
Source: Cal OES/DSOD, Plumas County GIS
Natural, Historic, and Cultural Resources
A major dam failure event and associated flooding could have a devastating impact on the Plumas County Planning Area. Large flood events can affect all natural, historic, and cultural resources that lie in the dam inundation areas. There are a number of ways floodwaters associated with a dam failure event can impact natural resources and the environment. Wildlife habitats can be destroyed by floodwaters. Contaminated floodwater can pollute rivers and habitats. Silt and sediment can destroy natural areas. Riverbanks and natural levées can be eliminated as rivers reach bankfull capacity. Rivers can be widened, and deposition can increase downstream. Trees can be uprooted by high-velocity water flow. Plants that survive the initial flood may die due to being inundated with water. Historic structures and cultural resources may also be significantly affected and even destroyed. Generally, the impacts are associated with damage to these structures within the inundated areas, but other cultural resources such as those associated with Native Americans and old tribal areas can also be disturbed, damaged, and lost during extreme flood events.

FEMA NRI Hazard Vulnerability Assessment
Dam failure is not an NRI hazard; therefore, no data is presented.

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Impacts from Dam Failure
Impacts to the Plumas County Planning Area from dam failure flooding could be extensive and widespread and include loss of life and injury, flooding and damage to property and structures, damage to critical facilities and infrastructure, loss of natural resources, and all other flood related impacts. Additionally, mass evacuations and associated economic losses can also be significant.

Other impacts associated with dam failure include landslides, bank erosion, and destruction of habitat. Dam failures can cause downstream flooding and can transport large volumes of sediment and debris and contaminants from the floodwaters. Other environmental impacts can include contamination from septic system failures and releases of contaminants from hazardous materials facilities, contamination of potable water supplies; changes in configurations of streams; loss of wildlife habitats; and degradation of wetlands.

A large dam failure event could have significant and catastrophic impacts.

Future Conditions
Future conditions may be affected by climate change, changes in population patterns (migration, density, or the makeup of socially vulnerable populations), and changes in land use and development. Findings on this for Plumas County include the following:
➢ Climate change is likely to exacerbate future rain and storm conditions and associated impacts and vulnerability of the County to dam failure.
➢ Population in Plumas County is expected to slightly decrease. Vulnerable population groups could face disproportionate effects from a dam failure and should be planned for. Changes in population and population patterns may or may not increase the impacts and vulnerability of the County to this hazard depending on the location and nature of growth and continued planning for future hazard conditions. ➢ Land use planning should be proactive to address future hazard conditions. Locating new development, structures, and critical facilities and infrastructure within or near areas of dam failure risk may put additional development at risk. However, County building codes are in effect to partially reduce this risk and should be updated as necessary to continue to address future dam failure conditions. Thus, depending on the location of new development and adherence to protective building codes, changes in land use and development may or may not increase the impacts and associated vulnerabilities of the County to this hazard.

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GIS Analysis
Plumas County provided 13 future development areas which were used as the basis for the inventory of future development for the unincorporated County. These were mapped in GIS. Only high hazard dam inundation areas affect future development in the County. As such, no extremely high hazard or significant hazard dam mapping or analysis is presented. Figure 4-78 shows the locations of the future development areas overlayed on the high hazard dam inundation areas. 2 dams intersect the future development areas.

Future development affected by the Chester Diversion Dam is detailed in Table 4-63, while future development affected by Frenchman Dam is detailed in Table 4-64.

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Figure 4-78 Unincorporated Plumas County – Future Development Areas in High Hazard Dam Inundation Areas

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Table 4-63 Unincorporated Plumas County – Future Development Areas in Chester Diversion Dam Inundation Area by Parcels and Acres
Dam Hazard
Inundation
Areas /
Future
Development
Status and
Type
Site / Map
ID and
Future
Development
Name
Total
Parcel
Count
Improved
Parcel
Count
Unimproved
Parcel
Count
Total
Acres
Total
Improved
Acres
Total
Unimproved
Acres
Chester Diversion
Approved
Commercial G - Seneca
Hospital
1 0 1 11.80 0.00 11.80
Chester Diversion Total 1 0 1 11.80 0.00 11.80
Source: Cal OES, DSOD, Plumas County GIS
Table 4-64 Unincorporated Plumas County – Future Development Areas in Frenchman Dam Inundation Area by Parcels and Acres
Dam Hazard
Inundation
Areas /
Future
Development
Status and
Type
Site / Map
ID and
Future
Development
Name
Total
Parcel
Count
Improved
Parcel
Count
Unimproved
Parcel Count
Total
Acres
Total
Improved
Acres
Total
Unimproved
Acres
Frenchman
Approved
Residential J - Meadow
Edge Mobile
Home Park
1 1 0 42.74 42.74 0.00
Frenchman Total 1 1 0 42.74 42.74 0.00
Source: Cal OES, DSOD, Plumas County GIS
Future Development
Future development considerations should be ongoing and proactive. Ensuring that comprehensive land use planning is conducted, regulatory requirements, such as building codes, are followed, and varying needs for different demographic groups with intersecting identities that reside within the County are considered can help to make for a more resilient community. Although new growth and development corridors would fall in the area flooded by a dam failure, given the limited potential of total dam failure and the large area that a dam failure would affect, development in the dam inundation area will continue to occur.

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4.3.9. Drought and Water Shortage (w/Tree Mortality)
Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent; past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

Hazard/Problem Description
Drought
Drought is a gradual phenomenon. Although droughts are sometimes characterized as emergencies, they differ from typical emergency events. Most natural disasters, such as floods or forest fires, occur relatively rapidly and afford little time for preparing for disaster response. Droughts occur slowly, over a multi-year period, and it is often not obvious or easy to quantify when a drought begins and ends.

Drought is a complex issue involving (see Figure 4-79) many factors—it occurs when a normal amount of precipitation and snow is not available to satisfy an area’s usual water-consuming activities. Drought can often be defined regionally based on its effects:
➢ Meteorological drought is usually defined by a period of below average water supply. ➢ Agricultural drought occurs when there is an inadequate water supply to meet the needs of the state’s crops and other agricultural operations such as livestock. ➢ Hydrological drought is defined as deficiencies in surface and subsurface water supplies. It is generally measured as streamflow, snowpack, and as lake, reservoir, and groundwater levels. ➢ Socioeconomic drought occurs when a drought impacts health, well-being, and quality of life, or when a drought starts to have an adverse economic impact on a region.

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Figure 4-79 Causes and Impacts of Drought
Source: National Drought Mitigation Center (NDMC)
Location and Extent of Drought
Drought is a regional phenomenon that affects the entire Plumas County Planning Area. Drought in the United States is monitored by the National Integrated Drought Information System (NIDIS). A major component of this portal is the U.S. Drought Monitor. The Drought Monitor concept was developed jointly by the NOAA’s Climate Prediction Center, the NDMC, and the USDA’s Joint Agricultural Weather Facility in the late 1990s as a process that synthesizes multiple indices, outlooks and local impacts, into an assessment that best represents current drought conditions. The final outcome of each Drought Monitor is a consensus of federal, state, and academic scientists who are intimately familiar with the conditions in their respective regions. A snapshot of the current (and 2024) drought conditions in California and the Plumas County Planning Area can be found in Figure 4-80. Snapshots from 2014 to 2023 are shown in Figure 4-81 and Figure 4-82. As seen in these figures and related data, the Planning Area has been in and out of drought over the past ten years since the last drought started in 2014.

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Figure 4-80 Plumas County Planning Area – Current (2025) and 2024 Drought Status Source: US Drought Monitor

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Figure 4-81 Previous Drought Status in Plumas County (2018-2023) Source: US Drought Monitor

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Figure 4-82 Previous Drought Status in Plumas County (2014-2017) Source: US Drought Monitor
Cal DWR says the following about drought:

One dry year does not normally constitute a drought in California. California’s extensive system of water supply infrastructure—its reservoirs, groundwater basins, and inter-regional conveyance facilities—mitigates the effect of shortterm dry periods for most water users. Defining when a drought begins is a function of drought impacts to water users. Hydrologic conditions constituting a drought for water users in one location may not constitute a drought for water users elsewhere, or for water users having a different water supply. Individual water suppliers may use criteria such as rainfall/runoff, amount of water in storage, or expected supply from a water wholesaler to define their water supply conditions.

The drought issue in California is further compounded by water rights. Water is a commodity possessed under a variety of legal doctrines. The prioritization of water rights between domestic uses, farming and federally protected fish habitats in California further contributes to this issue.

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As shown on the previous figures, drought is tracked by the US Drought Monitor. The Drought Monitor includes a scale to measure drought intensity:
➢ None
➢ D0 (Abnormally Dry)
➢ D1 (Moderate Drought)
➢ D2 (Severe Drought)
➢ D3 (Extreme Drought)
➢ D4 (Exceptional Drought)
Drought has a slow onset and long duration. Drought is not initially recognized as a problem because it normally originates in what is considered good weather, which typically includes a dry late spring and summer in Mediterranean climates, such as in California. This is particularly true in Northern California where drought impacts are delayed for most of the population by the wealth of stored surface and ground water. The drought complications normally appear more than a year after a drought begins. The most direct and likely most difficult drought impact to quantify is to local economies, especially agricultural economies. Drought has the potential to affect the entire Plumas County Planning Area.

Water Shortage
Northern Sacramento Valley counties, including Plumas County, generally have sufficient groundwater and surface water supplies to mitigate even the severest droughts of the past century. Many other areas of the State, however, also place demands on these water resources during severe drought. Water supply in the County is discussed in the below sections.
2035 Plumas County General Plan Water Resources Element The 2035 Plumas County General Plan Water Resources Element noted that the amount of precipitation received throughout the watershed varies but greatly contributes to the significant amount of water available in the County and throughout the region. The Sierra Crest, centrally located within Plumas County, acts as a barrier to storm systems between the western and eastern portions of the County. The western side of the Sierra Nevada Mountains receives over 90 inches of precipitation annually while the area east of the Sierra Crest receives only 11 inches. Snowpack levels in the County’s higher elevation areas serve as natural water reservoirs for surface water that becomes available as the snow melts and drains into the regional waterway system.

Plumas County Flood Control and Water Conservation District The HMPC noted that the Plumas County Flood Control and Water Conservation District (PCFCWCD) is one of the 29 State Water Contractors within the State of California. The PCFCWCD has annual entitlements to 2,700-acre feet of water from the State Water project with 3 water customers. Water customers include City of Portola, Grizzly Ranch Golf Club and Grizzly Lake CSD. The project water is transported from Lake Davis by Grizzly Valley Pipeline to the Lake Davis Water Treatment plant and transported to City of Portola through the pipeline. Grizzly Ranch Golf Club has an intake pump on Grizzly Creek and pumps the water for irrigating the golf course. City of Portola currently purchases 946 acre-feet, Grizzly Golf Club 250 Acre-feet and Grizzly Lake CSD purchases 57 acre-feet. Grizzly Lake currently

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does not use the entitlement but could in the future access the water by building a pipeline from the Lake Davis treatment plant. Grizzly Lake CSD currently pumps groundwater, and Portola utilizes a natural spring seasonally. Over 1,400 acre-feet annual entitlement is currently available for sale but not utilized due to lack of customers.

Upper Feather River Watershed
The Upper Feather River watershed covers a majority of the County (98%), which is about 72% of the watershed. The tributaries of the Upper Feather River watershed drain over 2 million acres of land in the Sierra Nevada Mountains, flowing southwest into Lake Oroville in neighboring Butte County. The Upper Feather River watershed is divided into four main branches with respective watersheds: the West Branch, the North Fork, the Middle Fork and the South Fork of the Feather River. The North Fork Feather River drainage area is the largest drainage area in the watershed covering approximately 1.4 million acres and contributing a yearly average flow of over 2.3 million acre-feet of water to Lake Oroville. The South Fork Feather River drainage is the smallest of the four drainage areas and contributes an average of over 189,000 acre-feet to Lake Oroville each year. The Upper Feather River watershed serves as an important supply of surface water resources. Water has been a valuable export from Plumas County since the State Water Project (SWP) located its main storage facility fed by the Feather River at Lake Oroville. This watershed supplies 3.2 million acre-feet per year for downstream urban, industrial and agricultural use as part of the State Water Project and delivers water to 29 agencies. The State Water Project also operates three reservoirs in Plumas County; Antelope Lake, Frenchman Lake and Lake Davis, which flow into Lake Oroville.

The main stems of the Upper Feather River watershed in addition to many of the tributaries exhibit some level of degradation, primarily due to human activities. The east side of the County experiences much more erosion than the west side, which greatly affects surface water quality. Timber harvesting, water diversion, irrigation practices, road and railroad construction, grazing and mining have all contributed to in-stream water quality issues, such as increased sediment transport, that impact aquatic life and riparian vegetation.

Plumas County Groundwater Basins
Plumas County contains fourteen groundwater basins, which are primarily located in the valleys on the east side of the Sierra Crest. These groundwater basins are also shown Figure 4-83. Sierra Valley is the largest groundwater basin, covering 125,250 acres, and underlies the Middle Fork of the Feather River. The smallest groundwater basin identified in the figure is Yellow Creek Valley Groundwater Basin covering 2,310 acres. Some of the County’s groundwater basins have been depleted as a result of high extraction rates and slow recharge. For example, the Sierra Valley groundwater basin has experienced significant declines due to human activity and agricultural practices.

Groundwater quality is currently monitored in nine of the County’s groundwater basins. Groundwater quality in the County varies by basin. Water quality in the Sierra Valley basin is primarily affected by geothermal activity which causes the groundwater to contain high concentrations of boron, fluoride, iron and sodium. Some wells within the Sierra Valley Sub-Basin also exhibit high levels of arsenic and manganese. Localized groundwater quality has been influenced in some areas by the use of septic systems.

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Figure 4-83 Plumas County – Hydrologic Features
Source: Plumas County General Plan Water Resources Element

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Plumas County Drought Plan (SB552)
This Plan is in process. The most recent draft available for inclusion into this LHMP Update was the November 2025 Draft. The Plumas County Drought Resilience Plan documents how Plumas County, its Drought and Water Shortage Task Force members, and other entities with water supply and drought management responsibilities will address water supply vulnerabilities for three types of systems in the County: domestic wells, as defined in California Health and Safety Code (HSC) Section 116275(n) and Section 10609.51(d); state small water systems (SSWS), as defined in HSC Section 116275(n) and Section 10609.51(m); and local small water systems (LSWS), as defined by Plumas County Code Sec. 6-9.04. – Definitions. The County Drought Resilience Plan was prepared pursuant to SB 552: Drought Planning for Small Water Suppliers, SSWS, and Domestic Well Communities (Hertzberg; see Section 1.2 for additional detail). This County DRP was developed by the County with funding and technical support provided by the CA DWR Drought Resilience Planning Assistance Program.

This plan fulfills county requirements for preparation of a plan that includes potential drought and water shortage risk and proposed interim and long-term solutions for domestic wells and SSWS within the County’s jurisdiction (CWC Section 10609.70). While measures to protect small water suppliers and nontransient noncommunity water systems are not within the scope of this document, this plan considers integration opportunities consistent with the intent of SB 552.

Location and Extent of Water Shortage
Since water shortage generally happens on a regional scale, the entirety of the Plumas County Planning Area is at risk. There is no established scientific scale to measure water shortage. The speed of onset of water shortage tends to be lengthy. The duration of water shortage can vary, depending on the severity of the drought that accompanies it and the status of the water sources going into the drought. The extent of the drought considers several factors, including the nature, source, and reliability of water. While drought conditions persist, the Planning Area has generally sufficient water supply from water supplies, which lessens the extent of water shortage impacts in the Planning Area.

Past Occurrences
Disaster Declaration History
There have been 1 state and 1 federal disaster declarations for Plumas County. These are shown on Table 4-65. More information on USDA disaster declarations from drought (13 events) can be found in Table 4-51 in the Past Occurrences in Section 4.3.6.

Table 4-65 Plumas County – Federal and State Drought Disaster Declarations 1950-2025 Disaster Type State Declarations Federal Declarations
Count Years Count Years
Drought 1 2014 1 1997
Source: Cal OES, FEMA

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NCDC Events
There have been 2 NCDC drought events in Plumas County. This is shown on Table 4-66.

Table 4-66 NCDC Drought Events for Plumas County 1993-7/31/2024* Event Type Number
of Events
Deaths Deaths
(indirect)
Injuries Injuries
(indirect)
Property
Damage
Crop
Damage
Drought 2 0 0 0 0 $50,000 $0
Source: NCDC
*Note: Losses reflect totals for all impacted areas, some of which fell outside of Plumas County CA DWR Events (with Hazard Mitigation Planning Committee Input) This section is not separated by events since the 2019/2020 LHMPs since much of it covers multiple year droughts.

Historically, California has experienced multiple severe droughts. According to the DWR, droughts exceeding three years are relatively rare in Northern California, the source of much of the State’s developed water supply. Figure 4-84 depicts California’s Multi-Year Historical Dry Periods, 1850-2000. The 1929- 34 drought established the criteria commonly used in designing storage capacity and yield of large northern California reservoirs. Figure 4-85 compares the 1929-34 drought to the 1976-77, 1987-92, 2007-09, and 2012-2016 droughts.

Figure 4-84 California’s Multi-Year Historical Dry Periods, 1850-2000 Source: Cal DWR
Notes: Dry periods prior to 1900 estimated from limited data; covers dry periods of statewide or major regional extent

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Figure 4-85 Average Annual Runoff and Precipitation for Historic Droughts Source: CA DWR – 2012-2016 California Drought: Historical Perspective (https://ca.water.usgs.gov/california-drought/californiadrought-comparisons.html) Figure 4-86 depicts runoff for the State from 1905 to 2021. This gives a historical context for the 2014- 2021 drought to compare against past droughts.

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Figure 4-86 Annual California Runoff –1905 to 2021
Source: Cal DWR
The 2018 and 2023 California State Hazard Mitigation Plan discussed the major droughts from 1900 to 2022. The discussion below appends to the tables and figures above.

The 1975-1977 Drought
From November 1975 through November 1977, California experienced one of its most severe droughts.

Although people in many areas of the state are accustomed to very little precipitation during the growing season (April to October), they expect it in the winter. In 1976 and 1977, the winters brought only one-half and one-third of normal precipitation, respectively. Most surface storage reservoirs were substantially drained in 1976, leading to widespread water shortages when 1977 turned out to be even drier.

The 1987-1992 Drought
From 1987 to 1992, California again experienced a serious drought due to low precipitation and run-off levels. The hardest-hit region was the Central Coast, roughly from San Jose to Ventura. In 1988, 45 California counties experienced water shortages that adversely affected about 30 percent of the state’s population, much of the dry-farmed agriculture, and over 40 percent of the irrigated agriculture. Fish and wildlife resources suffered, recreational use of lakes and rivers decreased, forestry losses and fires increased, and hydroelectric power production decreased. In February 1991, DWR and Cal OES surveyed drought conditions in all 58 California counties and found five main problems: extremely dry rangeland,

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irrigated agriculture with severe surface water shortages and falling groundwater levels, widespread rural areas where individual and community supplies were going dry, urban area water rationing at 25 to 50 percent of normal usage, and environmental impacts.

Storage in major reservoirs had dropped to 54 percent of average, the lowest since 1977. The shortages led to stringent water rationing and severe cutbacks in agricultural production, including threats to survival of permanent crops such as trees and vines. Fish and wildlife resources were in critical shape as well. Not since the 1928-1934 drought had there been such a prolonged dry period. In response to those conditions, the Governor established the Drought Action Team. This team almost immediately created an emergency drought water bank to develop a supply for four critical needs: municipal and industrial uses, agricultural uses, protection of fish and wildlife, and carryover storage for 1992. The large-scale transfer program, which involved over 800,000 acre-feet of water, was implemented in less than 100 days with the help and commitment of the entire water community and established important links between state agencies, local water interests, and local governments for future programs.

The 2007-2009 Drought
Water years 2007-2009 were collectively the 15th driest three-year period for DWR’s eight-station precipitation index, which is a rough indicator of potential water supply availability to the State Water Project (SWP) and Central Valley Project (CVP). Water year 2007 was the driest single year of that drought and fell within the top 20 percent of dry years based on computed statewide runoff. In June 2008, a state emergency proclamation was issued due to water shortage in selected Central Valley counties. In February 2009, for the first time in its history, the State of California proclaimed a statewide drought. The state placed unprecedented restrictions on CVP and SWP diversions from the Delta to protect listed fish species, a regulatory circumstance that exacerbated the impacts of the drought for water users.

The greatest impacts of the 2007–2009 drought were observed in the CVP service area on the west side of the San Joaquin Valley, where hydrologic conditions combined with reduced CVP exports resulted in substantially reduced water supplies (50 percent supplies in 2007, 40 percent in 2008, and 10 percent in 2009) for CVP south-of Delta agricultural contractors. Small communities on the west side highly dependent on agricultural employment were especially affected by land fallowing due to lack of irrigation supplies, as well as by factors associated with current economic recession. The coupling of the drought and economic recession necessitated emergency response actions related to social services, such as food banks and unemployment assistance.

The 2012-Current Drought (which is ongoing after a pause in 2017 and 2023) The statewide drought of 2012-2017, which is still ongoing, will be remembered as one of the most severe and costliest droughts of record in California. The drought that spanned water years 2012 through 2017 included the driest four-year statewide precipitation on record (2012-2015) and the smallest SierraCascades snowpack on record (2015, with 5 percent of average). It was marked by extraordinary heat:
2014, 2015, and 2016 were California’s first, second, and third warmest years in terms of statewide average temperatures. By the time the drought was declared officially over in April 2017, the state had expended $6.6 billion in drought response and mitigation programs and had been declared a federal disaster area.

This coincides with many of the drought declarations from the USDA (discussed in more detail in Section

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4.3.6 above). The HMPC reported that fire danger in 2013 and 2014 reached record levels as the drought produced dry, highly flammable fuel loads. The HMPC also noted that multiple tree mortality declarations were issued in 2020, 2021, 2023, and 2024 from the elevated risk to tree mortality from drought.

Water Shortage
Figure 4-87 illustrates several indicators commonly used to evaluate water conditions in California. The percent of average values are determined by measurements made in each of the ten major hydrologic regions. The chart describes water conditions in California between 2009 and 2021. The chart illustrates the cyclical nature of weather patterns in California.

Figure 4-87 Water Supply Conditions, 2009 to 2021
Source: 2023 State of California Hazard Mitigation Plan Beginning in 2012, snowpack levels in California dropped dramatically. 2015 estimates place snowpack as 5 percent of normal levels. Snowpack measurements have been kept in California since 1950 and nothing in the historic record comes close to 2015’s severely depleted level. The previous record for the lowest snowpack level in California, 25 percent of normal, was set both in 1976-77 and 2013-2014. In “normal” years, the snowpack supplies about 30 percent of California’s water needs, according to the California Department of Water Resources. Snowpack levels began to increase in 2016, and in 2017 snowpack increased to the largest in 22 years, according to the State Department of Water Resources. In late 2017 and early 2018, drought conditions began to return to California but have been dampened by periods of above average rainfall in the first part of 2019. Snowpack stayed low until the winters of 2021/2022 and 2022/2023 when atmospheric rivers caused heavy rains and snows in the area. Snowpack has been relatively normal since then.

The Sierra Valley Groundwater District provided graphs of District water levels from 1980 to 2016. These can be seen on Figure 4-88, Figure 4-89, and Figure 4-90.

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Figure 4-88 Plumas County – Loyalton Water Levels 1980-2016 Source: Sierra Valley Groundwater District. Retrieved 5/2/2025.

Figure 4-89 Plumas County – Vinton Water Levels 1980-2016 Source: Sierra Valley Groundwater District. Retrieved 5/2/2025.

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Figure 4-90 Plumas County – Chilcoot Water Levels 1980-2016 Source: Sierra Valley Groundwater District. Retrieved 5/2/2025.

Likelihood of Future Occurrence
Drought
Likely—Historical drought data for the Plumas County Planning Area indicate there have been 5 significant droughts in the last 87 years. This equates to a drought every 16.4 years on average or a 6.1 percent chance of a drought in any given year. Based on this data and given the multi-year length of droughts and future climate change affects, the Plumas County Planning Area determined that future drought occurrence continues to be likely.

Water Shortage
Occasional — Recent historical data for water shortage indicates that Plumas County may at some time be at risk to both short and prolonged periods of water shortage. Based on this it is possible that water shortages will affect the County in the future during extreme drought conditions. However, to date, Plumas County has continued to have relatively consistent water supply.

It is likely that climate change will increase the chance of future occurrence as well as future impacts. More information on climate change and drought and water shortage can be found in the next section. More information on future impacts can be found in the Future Conditions/Future Development section of the Vulnerability Assessment below.

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Climate Change and Drought and Water Shortage
Climate scientists studying California find that drought conditions are likely to become more frequent and persistent over the 21st century due to climate change. The experiences of California during recent years underscore the need to examine more closely the state’s water storage, distribution, management, conservation, and use policies. The 2021 CAS (as well as the Draft 2024 CAS) stresses the need for public policy development addressing long term climate change impacts on water supplies. The CAS notes that climate change is likely to significantly diminish California’s future water supply, stating that: California must change its water management and uses because climate change will likely create greater competition for limited water supplies needed by the environment, agriculture, and cities.

A 2018 report from the Public Policy Institute of California noted that thousands of Californians – mostly in rural, small, disadvantaged communities – already face acute water scarcity, contaminated groundwater, or complete water loss. Climate change would make these effects worse.

Cal-Adapt has modeled future risk of drought. Recent research suggests that extended drought occurrence (“mega-drought”) could become more pervasive in future decades. This tool explores data for two 20-year drought scenarios (using the quad that contains the City of Portola in Figure 4-91) derived from LOCA downscaled meteorological and hydrological simulations – one for the earlier part of the 21st century, and one for the latter part:
➢ The upper chart represents a mid-century dry spell from 2023-2042 identified from the HadGEM2-ES RCP 8.5 simulation. The extended drought scenario is based on the average annual precipitation over 30 years. This average value equates to 66.4% of historical median annual precipitation averaged over the North Coast and Sierra California Climate Tracker regions. ➢ The lower chart represents a late century dry spell from 2051–2070 identified from the HadGEM2-ES RCP 8.5 simulation. The extended drought scenario is based on the average annual precipitation over 20 years. This average value equates to 74.4% of historical median annual precipitation averaged over the North Coast and Sierra California Climate Tracker regions.

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Figure 4-91 Plumas County Planning Area – Future Extended Drought Scenarios Source: Cal Adapt – Extended Drought Scenarios (retrieved 2/26/2025)

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Vulnerability Assessment
Vulnerability—High
Based on historical information, the occurrence of drought in California, including the Plumas County Planning Area, is cyclical, driven by weather patterns. Drought has occurred in the past and will occur in the future. Periods of actual drought with adverse impacts can vary in duration, and the period between droughts can be extended. Although an area may be under an extended dry period, determining when it becomes a drought is based on impacts to individual water users. The vulnerability to drought may vary and include reduction in water supply, turf losses, impacts to natural resources, and an increase in dry fuels and tree dieback.

Tree Mortality and Drought
One of the specific impacts of drought in the Plumas County Planning Area is the increased risk to trees from beetle kill and other insects, pathogens and parasites, and other tree mortality and die back issues.

Drought weakens trees and makes them more susceptible to insect infestation and other pathogens. Insects, such as bark beetles and others, frequently attack trees weakened by drought, disease, injuries, or other factors that may stress the tree. These insects and other pathogens can contribute to the decline and eventual death of trees throughout the Planning Area, especially in those areas on the west where the elevations increase.

The tree mortality and dieback problem are a high priority because of the issue of hazardous trees and increased wildfire hazard in the wildland areas and the potential for worsening conditions and expansion into other forested sites in future years. Fuel loading and fire potential are greater in stands as compared to healthy stands due to the higher proportion of dead and dying material. Removal of dead and dying trees near structures, powerlines, and roadways is also a high priority because of the immediate falling hazard.

University of California, Berkeley’s Department of Environmental Science, Policy, and Management (ESPM) was commissioned in 2021 to investigate the symptoms and possible causes of the dieback.

Preliminary results show that the dieback is driven mostly by environmental stressors, including but not limited to drought, predisposing trees to disease caused by opportunistic pathogens. The report includes best management practices to limit the spread of pathogens. It is important to continue investigating the causes of the tree mortality and dieback to help Planning Area jurisdictional staff track changes over time, recognize where other areas may potentially be susceptible to tree mortality, and apply appropriate best management practices.

Under current drought conditions, competition for resources within forested areas will likely continue to intensify with greater stress increasing tree susceptibility to insect damage and disease and ultimately increase mortality.

In all vegetation types, drought extends the length of the fire season because of its effect on live and dead fuel moisture, with critical low fuel moisture levels occurring early in spring or summer and extending at least through the first significant rains that typically come in late fall.

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The whole of the Planning Area has some measure of vulnerability to drought and water shortage. An assessment of a community’s vulnerability to this hazard begins with an understanding of local exposure to unincorporated Plumas County. This is included in the Local Concerns section below. After that, vulnerability is discussed in multiple sections that detail how this hazard can affect both the entire Plumas County Planning Area as well as the unincorporated portions of Plumas County. How the hazard affects the other participating jurisdictions can be found in their respective annexes to this LHMP Update. These sections below include assets at risk, impacts, and how future development can be affected by this hazard.

The County noted that PSPS events, or severe storm events that knock out power, can cause water to stop being pumped by water companies or by individual wells in the County. PSPS events are usually not longlasting, but severe weather can knock the power out for five to seven days. Water shortage can cause tree mortality issues in the County.

Plumas County is developing a County Drought Resilience Plan to help prepare state small water systems and domestic wells for future droughts and water shortages. Being prepared helps reduce hardships and improves our community's water supply resiliency. That plan noted that there are several areas in the County with more advanced water shortage vulnerability. This can be seen on Figure 4-92.

This includes the areas shown on Table 4-67.

Table 4-67 Plumas County Drought Task Force Risk Assessment Findings Source: Plumas County Drought Task Force

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Figure 4-92 Plumas County Planning Area – Regions with Water Shortage Vulnerability Source: Plumas County Drought Task Force

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Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk
Assets at risk from drought and water shortage include people and populations; structures; critical facilities and infrastructure; and natural, historic, and cultural resources. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

People and Populations
The people and populations of the Plumas County Planning Area are not directly affected by general drought conditions; although, their turfed areas, trees, and other water-depending resources can all be affected. In extreme drought conditions, however, residents and other populations within the Planning Area may be vulnerable to drought and water shortage issues. Wells can go dry. Drought and water shortage can lead to an increase in wildfires threatening Planning Area residents. Water shortages can have an effect on all populations in the Planning Area but often have a greater effect on the unhoused and other vulnerable populations that may be unable to access and afford clean drinking water during shortages. During periods of drought as the costs of water usage may increase, those who are economically disadvantaged may be unable to afford the increased costs of potable water.

Structures
Structures have a limited vulnerability to drought and water shortage. It is the secondary hazard of drought (wildfire) which causes risks to structures. Drought can also stress trees, causing die off. These trees may fall on structures adjacent to them.

Critical Facilities and Infrastructure
Most critical facilities and infrastructure have a limited vulnerability to drought and water shortage. Should drought conditions be severe enough to cause water shortage reliability issues, some facilities and infrastructure may be affected. Water and wastewater systems may be impacted during times of reduced water supply and need to employ contingencies to remain functional and fully operational. Other waterdependent systems may also be adversely affected. Further, the secondary hazard of drought (increased tree mortality and potential for spread of urban fires and wildfire) can pose a significant risk to critical facilities and infrastructure. Drought can also stress trees, causing die off. Dead and weakened trees may fall on critical infrastructure adjacent to them and impact power lines and other utilities, often causing power outages. Fallen trees can also temporarily block roads and access which can impact first responders in an emergency.

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Natural, Historic, and Cultural Resources
Drought and water shortage can have a significant impact on natural resources. Water levels in reservoirs and lakes may be reduced, and a loss of wetlands and marsh areas may occur. Severe drought conditions can contribute to an increase in erosion of soils and lead to poor soil quality. Further, all of the trees in the Planning Area are at risk to drought impacts and a reduction in water supply. These trees provide a wealth of social and environmental benefits to Planning Area residents and visitors, from shade and beauty to air quality, carbon reduction and stormwater management. Increased tree mortality during extended droughts can be substantial and widespread. This potential increase in tree mortality in the Planning Area can also add to the available fuel which can exacerbate the spread of wildfires. Drought can devastate crops and dry out pastures, dry out forests and critical habitat areas, and reduce food and water available for wildlife and livestock. Additionally, drought conditions can also cause soil to compact and not absorb water well, potentially making an area more susceptible to flooding. It is unlikely that drought and water shortage would have a significant impact on historic and cultural resources in the Planning Area unless they are impacted by a drought precipitated wildfire event.

FEMA NRI Hazard Vulnerability Assessment
The HMPC looked at the FEMA NRI for additional specific data on Plumas County’s vulnerability to drought. The NRI contains data and analysis for 18 separate natural hazards, including drought. As shown in Table 4-68, drought could cause a total of $371,450 in expected annual losses, with a historic loss rating of relatively moderate. Drought has a relatively low rating for social vulnerability and a relatively high community resilience, with a risk index score of 87.4 out of 100. According to the NRI, there have been 1,400 drought events between the years 2000-2021 with an average of 61 events per year.

Table 4-68 Plumas County Planning Area – Drought Vulnerability Ratings from FEMA NRI NRI Hazard Hazard Risk Index
Rating
Expected Annual
Loss Values
(total)**
Exposure Value
(total)***
Historic Loss Ratio
Rating
Drought 87.4 $371,450 $3,622,270 Relatively moderate
Source: FEMA NRI Report
*Hazard Risk Index is calculated using data for only a single hazard type, and reflects a communities expected annual loss value, community risk factors, and the adjustment factor used to calculate the risk value. **Expected annual loss represents the average economic loss in dollars resulting from natural hazards each year. ***Exposure is a natural hazard consequence factor that is the representative value of buildings, populations, or agriculture potentially exposed to a natural hazard occurrence.

Additional information on the FEMA NRI can be found in Section 4.2 and Appendix H.

Impacts from Drought and Water Shortage
The vulnerability to drought is Countywide, but impacts may vary and include reduction in water supply and an increase in dry fuels. The potential for a reduction in water supply during drought conditions generally leads to both mandated and voluntary conservations measures during extended droughts. During these times, the costs of water can also increase. Also of concern, the increased dry fuels and fuel loads associated with drought conditions can result in an increased fire danger. In areas of extremely dry fuels,

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the intensity and speed of fires can be significant. Water supply and flows for fire suppression can also be an issue during extended droughts. Drought that occurs during periods of extreme heat and high winds can be a contributing factor to PSPS events to be declared in the Planning Area. More information on PSPS can be found above in Section 4.3.

Other qualitative impacts associated with drought in the Plumas County Planning Area are those related to water intensive activities such as municipal usage, commerce, tourism, and recreation use. With more precipitation likely falling as rain instead of snow and warmer temperatures causing decreased snowfall to melt faster and earlier, water supply is likely to become more unreliable. In addition, drought and water shortage is predicted to become more common. This means less water available for use over the long run, and additional challenges for water supply reliability, especially during periods of extended drought.

Drought Impact Reporter
Drought impacts are wide-reaching and may be economic, environmental, and/or societal. Tracking drought impacts can be difficult. The Drought Impact Reporter from the National Drought Mitigation Center (NDMC) is a useful reference tool that compiles reported drought impacts nationwide. Table 4-69 show drought impacts for County from 1850 to May 2025. The data represented is skewed, with the majority of these impacts from records within the past ten years.

Table 4-69 Plumas County Drought Impacts 1850-5/1/2025
Category Plumas County Number of Impacts
Agriculture 28
Business and Industry 11
Energy 8
Fire 16
Plants & Wildlife 30
Relief, Response, and Restrictions 69
Society and Public Health 28
Tourism and Recreation 13
Water Supply and Quality 67
Total 270
Source: National Drought Mitigation Center. Retrieved 5/1/2025.

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Future Conditions
Future conditions may be affected by climate change, changes in population patterns (migration, density, or the makeup of socially vulnerable populations), and changes in land use and development. Findings on this for Plumas County include the following:
➢ Climate change is likely to exacerbate future drought conditions and associated impacts and vulnerability of the County to drought and water shortage. ➢ Future population growth should be considered as having more or less people in a community and can affect the overall hazard vulnerability to the County. Population growth in Plumas County is expected to decrease. Plumas County has access to large quantities of water through its groundwater as well as surface water. However, any future population growth in the County will add additional pressure to water companies during periods of drought and water shortage. Water companies will need to continue to plan for and add infrastructure capacity for population growth. As the population grows, the nature and makeup of populations will shift and change along with it. Vulnerable and underserved populations, such as those with low incomes and the unhoused that might not always have access to affordable clean water, will need to be considered as future development continues, since they may experience a disproportionate impact from drought and water shortage. Potential population growth will be a challenge not only with regard to Plumas County’s water access for agricultural production, but state- and nation-wide with regard to food production. Should the County see a growth in population, it will increase the vulnerability and impacts to the County from this hazard. ➢ Land use planning should be proactive to address future hazard conditions. If the County begins to grow, more cropland may be taken out of production to provide housing to accommodate for population growth. As Plumas County’s agricultural lands are reduced, it seems likely that there would be less of a competing demand for water. However, more development will also require an increase in water supply and associated infrastructure. Changes in land use and development may or may not increase the impacts and associated vulnerabilities of the County to this hazard depending on where and how this future growth occurs.

4.3.10. Earthquake
Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent; past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

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Hazard/Problem Description
An earthquake is caused by a sudden slip on a fault. Stresses in the earth’s outer layer push the sides of the fault together. Stress builds up, and the rocks slip suddenly, releasing energy in waves that travel through the earth’s crust and cause the shaking that is felt during an earthquake. Earthquakes can cause structural damage, injury, and loss of life, as well as damage to infrastructure networks, such as water, power, gas, communication, and transportation. Earthquakes may also cause collateral emergencies including dam and levee failures, seiches, hazmat incidents, fires, avalanches, and landslides. The degree of damage depends on many interrelated factors. Among these are: the magnitude, focal depth, distance from the causative fault, source mechanism, duration of shaking, high rock accelerations, type of surface deposits or bedrock, degree of consolidation of surface deposits, presence of high groundwater, topography, and the design, type, and quality of building construction. This section briefly discusses issues related to types of seismic hazards.

Ground Shaking
Ground shaking is motion that occurs as a result of energy released during faulting. The damage or collapse of buildings and other structures caused by ground shaking is among the most serious seismic hazard.

Damage to structures from this vibration, or ground shaking, is caused by the transmission of earthquake vibrations from the ground to the structure. The intensity of shaking and its potential impact on buildings is determined by the physical characteristics of the underlying soil and rock, building materials and workmanship, earthquake magnitude and location of epicenter, and the character and duration of ground motion.

Actual ground breakage generally affects only those buildings directly over or nearby the fault. Ground shaking generally has a much greater impact over a greater geographical area than ground breakage. The amount of breakage and shaking is a function of earthquake magnitude, type of bedrock, depth and type of soil, general topography, and groundwater. As with most communities in Northern California near active faults, much of Plumas County could be susceptible to moderate to more violent ground shaking.

Seismic Structural Safety
Older buildings constructed before building codes were established, and even newer buildings constructed before earthquake-resistance provisions were included in the codes, are the most likely to be damaged during an earthquake. Buildings one or two stories high of wood-frame construction are considered to be one of the most structurally resistant to earthquake damage. Older masonry buildings without seismic reinforcement (unreinforced masonry) and soft story buildings are the most susceptible to the type of structural failure that causes injury or death.

The susceptibility of a structure to damage from ground shaking is also related to the underlying foundation material. A foundation of rock or very firm material can intensify short-period motions which affect lowrise buildings more than tall, flexible ones. A deep layer of water-logged soft alluvium can cushion lowrise buildings, but it can also accentuate the motion in tall buildings. The amplified motion resulting from softer alluvial soils can also severely damage older masonry buildings.

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Other potentially dangerous conditions include but are not limited to: building architectural features that are not firmly anchored, such as parapets and cornices; roadways, including column and pile bents and abutments for bridges and overcrossings; and above-ground storage tanks and their mounting devices. Such features could be damaged or destroyed during strong or sustained ground shaking.

Liquefaction Potential
Liquefaction is a process whereby soil is temporarily transformed to a fluid formed during intense and prolonged ground shaking. Areas most prone to liquefaction are those that are water saturated (e.g., where the water table is less than 30 feet below the surface) and consist of relatively uniform sands that are loose to medium density. In addition to necessary soil conditions, the ground acceleration and duration of the earthquake must be of sufficient energy to induce liquefaction.

Liquefaction during major earthquakes can cause severe damage to structures on level ground as a result of settling, titling, or floating. Such damage occurred in San Francisco on bay-filled areas during the 1989 Loma Prieta earthquake, even though the epicenter was several miles away. If liquefaction occurs in or under a sloping soil mass, the entire mass may flow toward a lower elevation. Also, of particular concern in terms of developed and newly developing areas are fill areas that have been poorly compacted.

No known liquefaction areas exist in Plumas County.

Landslide/Debris Flows
Landslides can occur as a result of horizontal seismic inertia induced in the slopes by the ground shaking.

The most common earthquake-induced landslides include shallow, disrupted landslides such as rock falls, rockslides, and soil slides. Debris flows are created when surface soil on steep slopes becomes totally saturated with water. Once the soil liquefies, it loses the ability to hold together and can flow downhill at very high speeds, taking vegetation and/or structures with it. Slide risks increase after an earthquake during a wet winter. This is discussed in the vulnerability section below, and as a stand-alone hazard in Section

4.3.13. Settlement
Settlement can occur in poorly consolidated soils during ground shaking. During settlement, the soil materials are physically rearranged by the shaking to result in a less stable alignment of the individual minerals. Settlement of sufficient magnitude to cause significant structural damage is normally associated with rapidly deposited alluvial soils or improperly founded or poorly compacted fill. These areas are known to undergo extensive settling with the addition of irrigation water, but evidence due to ground shaking is not available.

Fire Following Earthquake
Fire ignitions and spread following a major earthquake can further compound the potentially catastrophic impacts of a large earthquake event. Fire following earthquake is a major hazard in earthquake-prone areas which are densely developed and have high inventories of wood frame buildings. The capability of fire departments to combat fires following an earthquake is impacted by the regional nature of an earthquake,

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damage to transportation and water supply systems, and the potential failure of telecommunications. A local fire department may suffer impacts to its personnel, buildings, equipment, and communications systems. Moreover, fewer resources are immediately available as mutual aid – due to the commitment of nearby fire departments responding to the earthquake within their own jurisdictions. Transportation system damage may adversely impact the mobilization of off-duty personnel and fire and rescue mutual aid resources from unaffected areas. Maneuvering fire crews within the affected areas of the earthquake may be impeded by debris or damage to roads and bridges. Additionally, the municipal water supply system which ordinarily supports firefighting operations is likely to be impacted.

Fire following earthquake is an issue that could impact any California community that experiences an earthquake – both urban and rural. According to the FM Global Insurance company, “Fires after earthquakes commonly initiate from electrical or fuel related sources because their use is widespread. In the US, natural gas is a factor 15 to 50 percent of the time, and electrical ignitions account for 40 percent or more of post-earthquake wildfires. Ignitable liquid spills, chemical reactions, and the contact of combustibles with heat sources have also initiated or contributed to many fires after earthquakes.” Location and Extent
California is seismically active because it sits on the boundary between two of the earth’s tectonic plates.

Most of the state ‐ everything east of the San Andreas Fault ‐ is on the North American Plate. The cities of Monterey, Santa Barbara, Los Angeles, and San Diego are on the Pacific Plate, which is constantly moving northwest past the North American Plate. The relative rate of movement is about two inches per year. The San Andreas Fault is considered the boundary between the two plates, although some of the motion is taken up on faults as far away as central Utah.

Faults
A fault is defined as “a fracture or fracture zone in the earth’s crust along which there has been displacement of the sides relative to one another.” For the purpose of planning there are two types of faults, active and inactive. Active faults have experienced displacement in historic time, suggesting that future displacement may be expected. Inactive faults show no evidence of movement in recent geologic time, suggesting that these faults are dormant. This does not mean, however, that faults having no evidence of surface displacement within the last 11,000 years are necessarily inactive. For example, the 1975 Oroville earthquake, the 1983 Coalinga earthquake, and the 1987 Whittier Narrows earthquake occurred on faults not previously recognized as active. Potentially active faults are those that have shown displacement within the last 1.6 million years (Quaternary). An inactive fault shows no evidence of movement in historic (last 200 years) or geologic time, suggesting that these faults are dormant.

Two types of fault movement represent possible hazards to structures in the immediate vicinity of the fault:
fault creep and sudden fault displacement. Fault creep, a slow movement of one side of a fault relative to the other, can cause cracking and buckling of sidewalks and foundations even without perceptible ground shaking. Sudden fault displacement occurs during an earthquake event and may result in the collapse of buildings or other structures that are found along the fault zone when fault displacement exceeds an inch or two. The only protection against damage caused directly by fault displacement is to prohibit construction in the fault zone.

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The 2035 Plumas County General Plan Public Health and Safety Element noted that the risk of seismic hazards to residents of Plumas County is based on the approximate location of earthquake faults within and outside of the County. Several potentially active faults pass through Plumas County. The Almanor Fault, Butt Creek Fault Zone, and the Mohawk Valley Fault traverse the County. The Indian Valley Fault is also considered an active fault located within the County. Additionally, the Honey Lake and Fort Sage Faults are two active faults located east of the County that may pose a risk to the County Planning Area.

Figure 4-93 shows fault locations in and near Plumas County. The HMPC noted that an area of concern for the County is the Sierra Valley.

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Figure 4-93 Faults in and near Plumas County

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The amount of energy released during an earthquake is usually expressed as a magnitude and is measured directly from the earthquake as recorded on seismographs. An earthquake’s magnitude is expressed in whole numbers and decimals (e.g., 6.8). Seismologists have developed several magnitude scales. One of the first was the Richter Scale, developed in 1932 by the late Dr. Charles F. Richter of the California Institute of Technology. The Richter Magnitude Scale is used to quantify the magnitude or strength of the seismic energy released by an earthquake. Another measure of earthquake severity is intensity. Intensity is an expression of the amount of shaking at any given location on the ground surface (see Table 4-70).

Seismic shaking is typically the greatest cause of losses to structures during earthquakes.

Table 4-70 Modified Mercalli Intensity (MMI) Scale
MMI Felt Intensity
I Not felt except by a very few people under special conditions. Detected mostly by instruments.

II Felt by a few people, especially those on upper floors of buildings. Suspended objects may swing.

III Felt noticeably indoors. Standing automobiles may rock slightly.

IV Felt by many people indoors; by a few outdoors. At night, some people are awakened. Dishes, windows, and doors rattle.

V Felt by nearly everyone. Many people are awakened. Some dishes and windows are broken. Unstable objects are overturned.

VI Felt by everyone. Many people become frightened and run outdoors. Some heavy furniture is moved. Some plaster falls.

VII Most people are alarmed and run outside. Damage is negligible in buildings of good construction, considerable in buildings of poor construction.

VIII Damage is slight in specially designed structures, considerable in ordinary buildings, and great in poorly built structures. Heavy furniture is overturned.

IX Damage is considerable in specially designed buildings. Buildings shift from their foundations and partly collapse. Underground pipes are broken.

X Some well-built wooden structures are destroyed. Most masonry structures are destroyed. The ground is badly cracked. Considerable landslides occur on steep slopes.

XI Few, if any, masonry structures remain standing. Rails are bent. Broad fissures appear in the ground.

XII Virtually total destruction. Waves are seen on the ground surface. Objects are thrown in the air.

Source: Multi-Hazard Identification and Risk Assessment, FEMA 1997 Other Hazards
Earthquakes can also cause landslides and dam failures. Earthquakes may cause landslides (discussed in Section 4.3.13), particularly during the wet season, in areas of high water or saturated soils. Finally, earthquakes can cause dams to fail (see Section 4.3.7 Dam Failure). Earthquakes can also cause levees to fail.

Past Occurrences
Disaster Declaration History
There have been no disaster declarations in the County related to earthquakes, as shown on Table 4-4.

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NCDC Events
Earthquake events are not tracked by the NCDC database.

United States Geologic Survey (USGS) Events
The USGS National Earthquake Information Center database contains data on earthquakes in the Plumas County area. Table 4-71 shows the approximate distances earthquakes can be felt away from the epicenter.

According to the table, a magnitude 5.0 earthquake could be felt up to 90 miles away. The USGS database was searched for magnitude 5.0 or greater on the Richter Scale within 90 miles of the City of Portola in Plumas County. These 43 results are detailed in Table 4-72.

Table 4-71 Approximate Relationships between Earthquake Magnitude and Intensity Richter Scale Magnitude Maximum Expected Intensity (MM)* Distance Felt (miles)

2.0 - 2.9 I – II 0

3.0 - 3.9 II – III 10

4.0 - 4.9 IV – V 50

5.0 - 5.9 VI – VII 90

6.0 - 6.9 VII – VIII 135

7.0 - 7.9 IX – X 240

8.0 - 8.9 XI – XII 365
*Modified Mercalli Intensity Scale.

Source: USGS Earthquake Intensity Zonation and Quaternary Deposits, Miscellaneous Field Studies Map 9093, 1977.

Table 4-72 Magnitude 5.0 Earthquakes or greater within 90 Miles of Plumas County* Date Richter Magnitude Location
5/12/2023 5.16 4 km NW of Canyon dam, CA
5/11/2023 5.48 Lake Almanor Earthquake
5/24/2013 5.69 10 km WNW of Greenville, California
4/26/2008 5.1 1 km NW of Mogul, Nevada
8/10/2001 5.2 Northern California
11/26/1998 5.1 7 km NW of Redding, CA
11/28/1980 5.1 Northern California
2/22/1979 5.3 Northern California
11/27/1976 5 Northern California
8/2/1975 5.2 Northern California
8/2/1975 5.1 Northern California
8/1/1975 5.7 0 km WSW of Palermo, California
4/29/1968 5 Northern California
9/12/1966 5.91 Northern California
4/1/1959 5.6 Northern California

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Date Richter Magnitude Location
9/26/1953 5.3 Nevada
3/22/1953 5 Northern California
5/9/1952 5.1 Nevada
12/14/1950 5.6 Northern California
3/20/1950 5.5 Lassen Peak area, California
12/29/1948 6 Northern California
7/7/1946 5 Lassen Peak area, California
3/30/1943 5.3 Northern California
12/17/1942 5.1 Northern California
12/3/1942 5.5 Nevada
2/8/1940 5.7 Northern California
4/24/1914 6.4 Nevada
2/18/1914 6 Nevada
6/23/1909 5.7 Northern California
3/3/1909 5 Northern California
6/20/1889 5.9 Northern California
4/29/1888 5.9 Northern California
6/3/1887 6.3 Nevada
1/31/1885 5.7 Northern California
1/7/1881 5.6 Near Red Bluff, California
7/10/1877 5.5 Lake Tahoe area, California-Nevada border 1/24/1875 6.2 South of Janesville, California
12/27/1869 6.2 Near Carson City, Nevada
12/27/1869 6.4 Northwest of Virginia City, Nevada
5/30/1868 6 Near Virginia City, Nevada
3/15/1860 6.5 East of Reno, Nevada
9/3/1857 6 California-Nevada Border east of Truckee
1/25/1855 5.5 Sierra County, California
Source: USGS, National Earthquake Information Center
*Search dates 1950 – March 31, 2025
Figure 4-94 shows major historical earthquakes in California from 1769 to 2017. Since 2017, California has experienced four additional 6.0 or greater earthquakes:
➢ 2022 - 6.4 Ferndal
➢ 2021 - 6.0 Antelope Valley
➢ 2019 - 6.4 and 7.1 Ridgecrest
None of these earthquakes (over 6.0) since 2017 have affected the Plumas County Planning Area.

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Figure 4-94 Historic Earthquakes in California 1769 to 2017 Source: 2023 State of California Multi-Hazard Mitigation Plan

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Hazard Mitigation Planning Committee Events
These section separates events since the 2019/2020 LHMPs and those before 2019.

Events since 2019/2020 LHMP
In November of 2019, a small cluster of earthquakes ranging from 2.5 to 3.3 on the Richter Scale occurred near Taylorsville. No damages occurred, but the earthquakes were felt.

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Figure 4-95 Taylorsville Earthquakes
Source: Plumas County

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May 2023 – a 5.5m earthquake was noted at 15:19 and a 5.2m quake on May 12th at 02:18. Aftershocks were ongoing through 5/13/2023. Seneca Healthcare noted that it conducted a complete facility assessment with no issues noted. Seneca experienced no damage but did prepare for facility evacuation in the event of further quakes causing damage to the facility and roadways. Plumas Pines Resort had the biggest damages.

April 4-5, 2024 – Senecal Healthcare noted earthquakes near Belden in Plumas County – a total of seven earthquakes the largest being 4.4m between the hours of 1834 on 4/4/24 thru 4/5/25 @0346. Assessment showed no issues at Seneca Healthcare due to Earthquakes.

Events before 2019
The HMPC noted that during past regional earthquake events, Plumas County has experienced groundshaking. In addition, in some instances, nearby earthquake events have caused cracking in the plaster on walls and in windowsills.

The HMPC member from Viera Ranch noted that on August 1, 1975, the Oroville earthquake occurred in nearby Butte County. There was strong shaking noted in Quincy. Other events noted by this member include:
➢ December 24, 1992 – an earthquake of 5.6 magnitude hit, causing ground shaking in Quincy. ➢ January 16, 1993 – a small earthquake was felt in Quincy. ➢ August 13, 1994 – a small earthquake of 3.0 magnitude was felt in Quincy. ➢ June 8, 1995 – an earthquake of 4.1 was felt in the Cromberg area. ➢ February 1, 1996 – a small earthquake was felt in Quincy. ➢ November 5, 1997 – a 4.8 earthquake was felt in the Spring Garden Area. ➢ June 5, 2001 – a small earthquake was felt in Quincy. ➢ June 10, 2001 – a 5.5 earthquake was felt. It was centered in Portola. The County Courthouse was checked for cracks.
➢ January 7, 2003 – a 2.8 earthquake was felt in Quincy. 2013 – As shown on Table 4-72, a series of earthquakes occurred near Lake Almanor on May 24, 2013.

The series of earthquakes included a 5.7 magnitude earthquake near Canyon Dam, near the southern end of Lake Almanor. See Figure 4-96 for location of the May 24th earthquake series. Injuries were reported and damage to infrastructure and homes were sustained. Lake Almanor Mutual Water Company sustained a water main rupture which resulted in water supply loss, and 600 PG&E customers on the Lake Almanor peninsula lost power.

A USGS Report on this event noted that Felt intensity among the communities around Lake Almanor appeared to vary significantly. Lake Almanor West, Lake Almanor Country Club, and Hamilton Branch experienced MMI ≥7, whereas other communities around the lake experienced MMI ≤6; the maximum observed intensity was MMI 8, in Lake Almanor West. Damage in the high intensity areas consisted of broken and collapsed chimneys, ruptured pipes, and some damage to foundations and to structural elements within houses. Although this shaking damage is not usually expected for an Mw 5.7 earthquake, the intensities at Lake Almanor Country Club correlate with the peak ground acceleration (38 percent g) and peak ground velocity (30 centimeters per second) recorded by the California Strong Motion Instrumentation Program accelerometer located at the nearby Lake Almanor Fire Station. The intensity distribution for the

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three hardest hit areas (LAW, LACC, and HB) appears to increase as the azimuth from epicenter to the intensity sites approaches the fault strike. The small communities of Almanor and Prattville on the southwestern shore of Lake Almanor experienced somewhat lower intensities. The town of Canyon Dam experienced a lower intensity as well, despite its location up-dip of the earthquake rupture.

As a result of the 5.7 event, Plumas County OES instituted an emergency proclamation. This provides businesses and homeowners official documentation in potential damage claim activity. Over one million dollars in damages were reported and over 50 homes in the Lake Almanor basin were impacted. Broken or toppled chimneys were the most common report, however broken water lines caused flooding and water damage. At least one residential structure was shifted off its foundation as a result of ground shaking.

Figure 4-97 depicts damage to a home in the Lake Almanor area.

Figure 4-96 Plumas County – May 2013 Canyon Dam Earthquakes Source: USGS

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Figure 4-97 Plumas County – Home Damaged by Canyon Dam Earthquake Source: Plumas County
Figure 4-98 shows interior damage done to a house in the Lake Almanor Country Club area. Figure 4-99 shows a water tank. The earthquake caused a coupling to break, spilling 50,000 gallons of water before the LACC Mutual Water Company shut off the leak. Figure 4-100 shows the exterior of a house where a chimney collapsed.

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Figure 4-98 Damage at Home in Lake Almanor Country Club Area Source: Chapman, K., Gold, M.B., Boatwright, J., Sipe, J., Quitoriano, V., Dreger, D., and Hardebeck, J., 2016, Faulting, damage, and intensity in the Canyon dam earthquake of May 23, 2013: U.S. Geological Survey Open-File Report 2016-1145, 49 p., http://dx.doi.org/10.3133/ofr20161145.

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Figure 4-99 Water Tank in Lake Almanor Country Club Area Source: Chapman, K., Gold, M.B., Boatwright, J., Sipe, J., Quitoriano, V., Dreger, D., and Hardebeck, J., 2016, Faulting, damage, and intensity in the Canyon dam earthquake of May 23, 2013: U.S. Geological Survey Open-File Report 2016-1145, 49 p., http://dx.doi.org/10.3133/ofr20161145.

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Figure 4-100 Chimney Collapse at a Home in Lake Almanor Country Club Area Source: Chapman, K., Gold, M.B., Boatwright, J., Sipe, J., Quitoriano, V., Dreger, D., and Hardebeck, J., 2016, Faulting, damage, and intensity in the Canyondam earthquake of May 23, 2013: U.S. Geological Survey Open-File Report 2016-1145, 49 p., http://dx.doi.org/10.3133/ofr20161145.

Likelihood of Future Occurrence
Unlikely (major earthquake); Likely (minor earthquake)— A few sizeable earthquakes have been recorded within the County. The possibility of an earthquake is an ever-present phenomenon in California

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and Plumas County. The combination of plate tectonics and associated California coastal mountain range building geology essentially guarantees earthquake as a result of the periodic release of tectonic stresses.

It is likely that climate change will not affect the chance of future occurrence as well as future impacts.

More information on climate change and earthquake can be found below the Mapping of Future Occurrences section. More information on future impacts can be found in the Future Conditions/Future Development section of the Vulnerability Assessment below.

Mapping of Future Occurrences
Mapping of future occurrences is presented in the following sections:
➢ CGS Analysis
➢ UCERF Analysis
California Geological Survey (CGS) Analysis
Maps indicating the maximum expectable intensity of ground shaking for the County are available through several sources. Figure 4-101, prepared in 2016 by the California Division of Mines and Geology, shows the expected relative intensity of ground shaking and damage in California from anticipated future earthquakes. The shaking potential is calculated as the level of ground motion that has a 2% chance of being exceeded in 50 years, which is the same as the level of ground-shaking with about a 2,500-year average repeat time. Although the greatest hazard is in areas of highest intensity as shown on the map, no region is immune from potential earthquake damage.

Figure 4-101 Maximum Expectable Earthquake Intensity – 2% Chance in 50 Years Source: California Division of Mines and Geology (2016)

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UCERF Analysis
In 2014, the USGS and the CGS released the time‐dependent version of the Uniform California Earthquake Rupture Forecast (UCERF III) model. The UCERF III results have helped to reduce the uncertainty in estimated 30‐year probabilities of strong ground motions in California. The UCERF map is shown in Figure 4-102 and indicates that Plumas County has a low to moderate risk of earthquake occurrence.

Figure 4-102 Probability of Earthquake Magnitudes Occurring in 30 Year Time Frame Source: United States Geological Survey Open File Report 2015‐3009 Climate Change and Earthquake
Climate change is unlikely to increase earthquake frequency or strength.

Vulnerability Assessment
Vulnerability—High
Earthquake vulnerability is primarily based on population and the built environment. Urban areas in high seismic hazard zones are the most vulnerable, while more rural areas, such as Plumas County are less

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vulnerable. The primary impacts of concern are life safety and property damage. Several faults are in and near the Plumas County Planning Area, and seismic hazard mapping indicates that the Planning Area has low to moderate seismic hazard potential. Depending on the location and magnitude of an earthquake event, some degree of structural damage due to stronger seismic shaking could be expected.

An assessment of a community’s vulnerability to this hazard begins with an understanding of local exposure to unincorporated Plumas County. This is included in the Local Concerns section below. After that, vulnerability is discussed in multiple sections that detail how this hazard can affect both the entire Plumas County Planning Area as well as the unincorporated portions of Plumas County. How the hazard affects the other participating jurisdictions can be found in their respective annexes to this LHMP Update. These sections below include assets at risk, impacts, and how future development can be affected by this hazard.

Seismic events can have particularly negative effects on older buildings constructed of Unreinforced Masonry (URM), including materials such as brick, concrete and stone. The Uniform Building Code (UBC) identifies four seismic zones in the United States. The zones are numbered one through four, with Zone 4 representing the highest level of seismic hazard. The UBC establishes more stringent construction standards for areas within Zones 3 and 4. All of California lies within either Zone 3 or Zone 4. Plumas County is within the less hazardous Zone 3. The County Building Department does not track or keep an inventory of URM buildings. The first building permit was issued in 1959, and at that time they were issued by the Assessor’s office. The Building Department did not come into existence until 1968 and at that time the 1967 edition of the Uniform Building Code was in force. Under the 67 UBC Chapter 24, all habitable structures constructed of masonry were required to be reinforced. It would be safe to assume that prior to the existence of the Building Department in 1968, any structure built of masonry may not be reinforced.

The HMPC noted that an area of concern for the County is the Sierra Valley.

Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

Assets at Risk
Assets at risk from earthquake include people and populations; structures; critical facilities and infrastructure and community lifelines; natural, historic, and cultural resources; and economic assets and community activities of value. These are discussed in the following sections for the Plumas County Planning Area and unincorporated Plumas County.

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People and Populations
All people and populations are at risk from earthquake shaking and other subhazards related to an earthquake event. Those at heightened risk include:
➢ The unsheltered
➢ Infants and children under age five and their caregivers ➢ Elderly (65 and older)
➢ Individuals with disabilities
➢ Individuals dependent on medical equipment
➢ Individuals with impaired mobility
The greatest risk to people and populations from earthquake is death and injury. More information on people and populations at risk to earthquake shaking events can be seen in the Hazus scenarios developed for this LHMP and described in the Hazus Analysis section below.

Structures
All structures in the Plumas County Planning Area are vulnerable to earthquakes, depending on the severity and location of the shake. Older buildings, unreinforced masonry and soft story buildings are at a much greater risk to earthquakes. More information on structures at risk can be seen in the Hazus scenarios below.

Critical Facilities and Infrastructure
Earthquakes present a tremendous threat to critical facilities and infrastructure. More information on structures at risk can be seen in the Hazus scenarios below.

Natural, Historic, and Cultural Resources
The 2023 State Hazard Mitigation Plan noted that environmental problems from earthquakes can be numerous. Earthquakes can significantly damage surrounding habitats. It is also possible for earthquakes to reroute streams, which can change the water quality, possibly damaging habitat and feeding areas.

Streams fed by groundwater and/or springs may dry up because of changes in underlying geology. Another threat to the environment from earthquakes is the potential release of hazardous materials. Historical and cultural resources are at risk, often due to their age and construction types. The Hazus scenarios below are relatively silent on the vulnerability to natural, historic, and cultural resources, but impacts to these resources could be long lasting.

Hazus Analysis
Earthquake losses will vary across the Plumas County Planning Area depending on the source and magnitude of the event. To further evaluate potential losses associated with earthquake activity in the Planning Area, four Hazus-MH earthquake scenarios were run for this 2025 LHMP Update as detailed below.

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FEMA’s Hazus model, utilizing GIS technology, estimates potential losses from earthquakes by analyzing the built environment, including buildings, infrastructure, and population, along with hazard data and damage functions. The model incorporates inventory information about buildings, infrastructure (roads, utilities, etc.), and population to assess the potential impact of a hazard. It uses hazard-specific data, such as earthquake intensity and employs damage functions that relate the intensity of the hazard to the level of damage to buildings and infrastructure based on research and historical data. By combining the inventory, hazard, and damage function data, HAZUS estimates the potential losses in terms of direct physical damage, induced damage, and direct losses (e.g., building damage, debris generation, casualty rates, economic impacts). The results of HAZUS analysis can be used to identify vulnerable areas, prioritize mitigation actions, and develop disaster preparedness plans.

Hazus-MH 6.1 was utilized to model earthquake losses for the County. Level 1 analyses were run, meaning that only the default data was used and not supplemented with local building inventory or hazard data. This is a planning level analysis.

HAZUS Level 1 Runs
These events were chosen from data gathered from the Plumas County General Plan Safety Element and the USGS’ seismic scenarios catalog. For deterministic Hazus scenarios, a single scenario event is selected based on a specific fault, the epicenter location, and the potential magnitude of the event.

Table 4-73 Hazus Scenario Data from the USGS Earthquake Hazards Program Catalog Hazus Fault Info Hazus Catalog Information
Earthquake Fault Great Valley 3
Epicenter 38.757000, -121.877000
Epicenter Location
Description
Approximately 80 miles to the southwest of Plumas County Scenario Great Valley 03a Dunnigan Hills
Ruptures Great Valley 03a Dunnigan Hills
Magnitude 6.53
MMI 8.42
Event ID bssc2014greatvalley03adunnig_m6p53_se
URL https://earthquake.usgs.gov/scenarios/eventpage/bssc2014greatvalley03adunnig_m6p 53_se/origin/detail
Earthquake Fault Hat Creek-McArthur-Mayfield
Epicenter 41.001, -121.477
Epicenter Location
Description
Approximately 40 miles to the north/northwest of Plumas County Scenario HC-MAC-MAY 7.29
Ruptures Hat Creek-McArthur-Mayfield
Magnitude 7.29
MMI 8.58
Event ID bssc2014hatcreekmcarthurmayf_m7p29_se

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Hazus Fault Info Hazus Catalog Information
URL https://earthquake.usgs.gov/scenarios/eventpage/bssc2014hatcreekmcarthurmayf_m7 p29_se
Earthquake Fault Honey Lake
Epicenter 40.174400, -120.289001
Epicenter Location
Description
Approximately 5 miles to the northeast of Plumas County Scenario HONEY LAKE 7.03
Ruptures Honey Lake
Magnitude 7.03
MMI 8.72
Event ID bssc2014honeylake2011cfmshaw_m7p03_se
URL https://earthquake.usgs.gov/scenarios/eventpage/bssc2014honeylake2011cfmshaw_m 7p03_se/executive
Earthquake Fault Warm Springs Valley
Epicenter 39.955002, -119.803001
Epicenter Location
Description
Approximately 19 miles to the east of Plumas County
Scenario Warm Springs Valley 6.92
Ruptures Warm Springs Valley fault zone
Magnitude 6.92
MMI 8.74
Event ID bssc20141605_m6p92_se
URL https://earthquake.usgs.gov/scenarios/eventpage/bssc20141605_m6p92_se/executive Source: Hazus 6.1
The earthquake scenarios are selected for analysis based on the earthquake fault availability in the FEMA Hazus-MH 6.1 Tool and the corresponding earthquake scenario availability through the USGS Earthquake Hazards Program Data Catalog.

The earthquake faults of concern noted in the 2020 Plumas County LHMP and the County Safety Element were available in the USGS Catalog but were not available in Hazus and therefore were not available for analysis. These faults include: Almanor, Butte Creek, Fort Sage, Indian Valley, and Mohawk Valley.

These Hazus scenarios can be seen on Figure 4-103 and are captured in totality in Appendix I. Summary Hazus results follow the Figure, and include summary tables of all Hazus scenario runs and a Hazus scenario comparison table.

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Figure 4-103 Plumas County Planning Area – Hazus Earthquake Faults and Epicenter Locations

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Great Valley (03A) Fault 6.53 Hazus Scenario
The results of this deterministic scenario are captured in Table 4-74. Key losses included the following:
➢ Total economic loss estimated for the earthquake was $0, which includes building losses and lifeline losses based on the Hazus-MH inventory.
➢ The total building-related losses were 0.00 (millions of dollars); 0% of the estimated losses were related to the business interruption of the region. By far, the largest loss was sustained by the residential occupancies which made up over 0% of the total loss.
➢ No buildings in the County were at least moderately damaged. No buildings were completely destroyed.
➢ Before the earthquake, the region had 87 hospital beds available for use. On the day of the earthquake, the model estimates that 86 hospital beds (almost 100%) are available for use by patients already in the hospital and those injured by the earthquake. After one week, 100% of the beds will be back in service.

By 30 days, 100% will be operational.
➢ Regardless of the time of the earthquake, none had casualties. ➢ No households experienced a loss of potable water the first day after the earthquake. ➢ No households experienced a loss of electricity the first day after the earthquake.

Table 4-74 Plumas County Planning Area – Great Valley 3 Fault 6.53 Magnitude Scenario Results
Type of Impact Great Valley 3 6.53 Magnitude Deterministic Scenario Total Buildings Damaged
(based on 16,000 buildings)
Slight: 0
Moderate: 0
Extensive: 0
Complete: 0
Building and Income Related Losses $0
Total Economic Losses
(Includes building, income and lifeline
losses)
$1,800,000
Casualties
(Based on 2 a.m. time of occurrence)
Without requiring hospitalization: 0
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 2 p.m. time of occurrence)
Without requiring hospitalization: 0
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 5 p.m. time of occurrence)
Without requiring hospitalization: 0
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Damage to Transportation Systems None with at least moderate damage Damage to Essential Facilities None with at least moderate damage Damage to Utility Systems There are no electrical power facilities with at least moderate damage. 1 potable water line breaks, 1 wastewater line breaks, and 0 natural gas line breaks.

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Type of Impact Great Valley 3 6.53 Magnitude Deterministic Scenario Households without Power/Water Service
(Based on 9,216 total households)
Power loss, Day 1: 0
Power loss, Day 3: 0
Power loss, Day 7: 0
Power loss, Day 30: 0
Power loss, Day 90: 0
Water loss, Day 1: 0
Water loss, Day 3: 0
Water loss, Day 7: 0
Water loss, Day 30: 0
Water loss, Day 90: 0
Displaced Households 0 displaced households
Shelter Requirements 0 persons
Debris Generation 0 tons
Source: Hazus 6.1
Fires often occur after an earthquake. Because of the number of fires and the lack of water to fight the fires, they can often burn out of control. Hazus uses a Monte Carlo simulation model to estimate the number of ignitions and the amount of burnt area. For this scenario, the model estimates that there will be 0 ignitions that will burn about 0.00 sq. mi 0.00 % of the region’s total area.). The model also estimates that the fires will displace about 0 people and burn about 0 (millions of dollars) of building value.

Figure 4-104 shows the peak ground acceleration due to the Great Valley 6.53 earthquake. Figure 4-105 shows the total losses by census tract in Plumas County due to the Great Valley 6.53 earthquake.

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Figure 4-104 Plumas County Planning Area – Great Valley 6.53 Magnitude Peak Ground Accelerations

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Figure 4-105 Plumas County Planning Area – Great Valley 6.53 Magnitude Hazus Total Loss

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Hazus Hat Creek-McArthur-Mayfield Fault 7.29 Hazus Scenario The results of this deterministic scenario are captured in Table 4-75. Key losses included the following:
➢ Total economic loss estimated for the earthquake was $154,050,000, which includes building losses and lifeline losses based on the Hazus-MH inventory.
➢ The total building-related losses were 19.96 (millions of dollars); 15% of the estimated losses were related to the business interruption of the region. By far, the largest loss was sustained by the residential occupancies, which made up over 57% of the total loss.
➢ 72 buildings in the County were at least moderately damaged. No buildings were completely destroyed. ➢ Before the earthquake, the region had 87 hospital beds available for use. On the day of the earthquake, the model estimates that only 82 hospital beds (95%) are available for use by patients already in the hospital and those injured by the earthquake. After one week, 99.00% of the beds will be back in service. By 30 days, 100% will be operational.
➢ Regardless of the time of the earthquake, none had casualties. ➢ No households experienced a loss of potable water the first day after the earthquake. ➢ No households experienced a loss of electricity the first day after the earthquake.

Table 4-75 Plumas County Planning Area – Hat Creek-McArthur-Mayfield 7.29 Magnitude Scenario Results
Type of Impact Hat Creek-McArthur-Mayfield 7.29 Magnitude Deterministic Scenario
Total Buildings Damaged
(based on 16,000 buildings)
Slight: 454
Moderate: 68
Extensive: 4
Complete: 0
Building and Income Related Losses $19,390,000
Total Economic Losses
(Includes building, income and lifeline
losses)
$154,050,000
Casualties
(Based on 2 a.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 2 p.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 5 p.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Damage to Transportation Systems None with at least moderate damage Damage to Essential Facilities None with at least moderate damage Damage to Utility Systems There would be no electrical power facility with at least moderate damage. 17 potable water line breaks, 9 wastewater line breaks, and 3 natural gas line breaks.

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Type of Impact Hat Creek-McArthur-Mayfield 7.29 Magnitude Deterministic Scenario
Households without Power/Water Service
(Based on 9,216 total households)
Power loss, Day 1: 0
Power loss, Day 3: 0
Power loss, Day 7: 0
Power loss, Day 30: 0
Power loss, Day 90: 0
Water loss, Day 1: 0
Water loss, Day 3: 0
Water loss, Day 7: 0
Water loss, Day 30: 0
Water loss, Day 90: 0
Displaced Households 1 displaced household
Shelter Requirements 0 persons
Debris Generation 2,000 tons
Source: Hazus 6.1
Fires often occur after an earthquake. Because of the number of fires and the lack of water to fight the fires, they can often burn out of control. Hazus uses a Monte Carlo simulation model to estimate the number of ignitions and the amount of burnt area. For this scenario, the model estimates that there will be 0 ignitions that will burn about 0.00 sq. mi 0.00 % of the region’s total area.). The model also estimates that the fires will displace about 0 people and burn about 0 (millions of dollars) of building value.

Figure 4-106 shows the peak ground acceleration due to the Hat Creek-McArthur-Mayfield 7.29 earthquake. Figure 4-107 shows the total losses by census tract in Plumas County due to the Hat CreekMcArthur-Mayfield 7.29 earthquake.

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Figure 4-106 Plumas County Planning Area – Hat Creek-McArthur-Mayfield 7.29 Magnitude Hazus Peak Ground Acceleration

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Figure 4-107 Plumas County Planning Area – Hat Creek-McArthur-Mayfield 7.29 Magnitude Hazus Total Loss

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Honey Lake 7.03 Hazus Scenario
The results of this probabilistic scenario are captured in Table 4-76. Key losses included the following:
➢ Total economic loss estimated for the earthquake was $179,640,000, which includes building losses and lifeline losses based on the Hazus-MH inventory.
➢ The total building-related losses were 50.87 (millions of dollars); 15% of the estimated losses were related to the business interruption of the region. By far, the largest loss was sustained by the residential occupancies, which made up over 54% of the total loss.
➢ 17 buildings in the County were at least moderately damaged. No buildings were completely destroyed. ➢ Before the earthquake, the region had 87 hospital beds available for use. On the day of the earthquake, the model estimates that only 81 hospital beds (93%) are available for use by patients already in the hospital and those injured by the earthquake. After one week, 99% of the beds will be back in service.

Table 4-76 Plumas County Planning Area – Honey Lake 7.03 Magnitude Deterministic Scenario Results
Type of Impact Honey Lake 7.03 Magnitude Deterministic Scenario Total Buildings Damaged
(based on 16,000 buildings)
Slight: 1,047
Moderate: 198
Extensive: 16
Complete: 1
Building and Income Related Losses $50,870,000
Total Economic Losses
(Includes building, income and lifeline
losses)
$179,640,000
Casualties
(Based on 2 a.m. time of occurrence)
Without requiring hospitalization: 2
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 2 p.m. time of occurrence)
Without requiring hospitalization: 3
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 5 p.m. time of occurrence)
Without requiring hospitalization: 2
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Damage to Transportation Systems None with at least moderate damage Damage to Essential Facilities None with at least moderate damage Damage to Utility Systems There are no electrical power facilities with at least moderate damage. 38 potable water line breaks, 19 wastewater line breaks, and 6 natural gas line breaks.

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Type of Impact Honey Lake 7.03 Magnitude Deterministic Scenario Households without Power/Water Service
(Based on 9,216 total households)
Power loss, Day 1: 0
Power loss, Day 3: 0
Power loss, Day 7: 0
Power loss, Day 30: 0
Power loss, Day 90: 0
Water loss, Day 1: 0
Water loss, Day 3: 0
Water loss, Day 7: 0
Water loss, Day 30: 0
Water loss, Day 90: 0
Displaced Households 5 displaced households
Shelter Requirements 2 persons
Debris Generation 5,000 tons
Source: Hazus 6.1
Fires often occur after an earthquake. Because of the number of fires and the lack of water to fight the fires, they can often burn out of control. Hazus uses a Monte Carlo simulation model to estimate the number of ignitions and the amount of burnt area. For this scenario, the model estimates that there will be 0 ignitions that will burn about 0.00 sq. mi 0.00 % of the region’s total area.). The model also estimates that the fires will displace about 0 people and burn about 0 (millions of dollars) of building value.

Figure 4-104 shows the peak ground acceleration due to the Honey Lake 7.03 earthquake. Figure 4-109 shows the total losses by census tract in Plumas County due to the Honey Lake 7.03 earthquake.

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Figure 4-108 Plumas County Planning Area – Honey Lake 7.03 Magnitude Hazus Peak Ground Acceleration

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Figure 4-109 Plumas County Planning Area – Honey Lake 7.03 Magnitude Hazus Total Loss

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Warm Springs Valley 6.92 Hazus Scenario
The results of this probabilistic scenario are captured in Table 4-76. Key losses included the following:
➢ Total economic loss estimated for the earthquake was $31,20,000, which includes building losses and lifeline losses based on the Hazus-MH inventory.
➢ The total building-related losses were 15.16 (millions of dollars); 17% of the estimated losses were related to the business interruption of the region. By far, the largest loss was sustained by the residential occupancies, which made up over 52% of the total loss.
➢ 71 buildings in the County were at least moderately damaged. No buildings were completely destroyed. ➢ Before the earthquake, the region had 87 hospital beds available for use. On the day of the earthquake, the model estimates that only 84 hospital beds (98%) are available for use by patients already in the hospital and those injured by the earthquake. After one week, 100% of the beds will be back in service.

Table 4-77 Plumas County Planning Area – Warm Springs Valley 6.92 Magnitude Deterministic Scenario Results
Type of Impact Warm Springs Valley 6.92 Magnitude Deterministic Scenario Total Buildings Damaged
(based on 16,000 buildings)
Slight: 374
Moderate: 66
Extensive: 5
Complete: 0
Building and Income Related Losses $15,160,000
Total Economic Losses
(Includes building, income and lifeline
losses)
$31,200,000
Casualties
(Based on 2 a.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 2 p.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 5 p.m. time of occurrence)
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Damage to Transportation Systems None with at least moderate damage Damage to Essential Facilities None with at least moderate damage Damage to Utility Systems There are no electrical power facilities with at least moderate damage. 14 potable water line breaks, 7 wastewater line breaks, and 2 natural gas line breaks.

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Type of Impact Warm Springs Valley 6.92 Magnitude Deterministic Scenario Households without Power/Water Service
(Based on 9,216 total households)
Power loss, Day 1: 0
Power loss, Day 3: 0
Power loss, Day 7: 0
Power loss, Day 30: 0
Power loss, Day 90: 0
Water loss, Day 1: 0
Water loss, Day 3: 0
Water loss, Day 7: 0
Water loss, Day 30: 0
Water loss, Day 90: 0
Displaced Households 1 displaced household
Shelter Requirements 0 persons
Debris Generation 2,000 tons
Source: Hazus 6.1
Fires often occur after an earthquake. Because of the number of fires and the lack of water to fight the fires, they can often burn out of control. Hazus uses a Monte Carlo simulation model to estimate the number of ignitions and the amount of burnt area. For this scenario, the model estimates that there will be 0 ignitions that will burn about 0.00 sq. mi 0.00 % of the region’s total area.). The model also estimates that the fires will displace about 0 people and burn about 0 (millions of dollars) of building value.

Figure 4-104 shows the peak ground acceleration due to the Warm Springs Valley 6.92 earthquake. Figure 4-109 shows the total losses by census tract in Plumas County due to the Warm Spring Valley 6.92 earthquake.

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Figure 4-110 Plumas County Planning Area – Warm Springs Valley 6.92 Magnitude Hazus Peak Ground Acceleration

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Figure 4-111 Plumas County Planning Area – Warm Springs Valley 6.92 Magnitude Hazus Total Loss

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Hazus Scenario Comparisons
A comparison of the results of the four deterministic scenarios is shown on Table 4-78.

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Table 4-78 Plumas County Planning Area – Comparison of Hazus Scenarios Type of Impact Great Valley 3 6.53 Magnitude
Deterministic Scenario
Hat Creek-McArthur-Mayfield

7.29 Magnitude Deterministic
Scenario
Honey Lake 7.03
Magnitude Deterministic
Scenario
Warm Springs Valley
Magnitude 6.92
Deterministic Scenario
Total Buildings Damaged
(based on 16,000
buildings)
Slight: 0
Moderate: 0
Extensive: 0
Complete: 0
Slight: 454
Moderate: 68
Extensive: 4
Complete: 0
Slight: 1,047
Moderate: 198
Extensive: 16
Complete: 1
Slight: 374
Moderate: 66
Extensive: 5
Complete: 0
Building and Income
Related Losses
$0 $19,390,000 $50,870,000 $15,160,000
Total Economic Losses
(Includes building,
income and lifeline
losses)
$1,800,000 $154,050,000 $179,640,000 $31,200,000
Casualties
(Based on 2 a.m. time of
occurrence)
Without requiring hospitalization:
0 Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 2
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 2 p.m. time of
occurrence)
Without requiring hospitalization:
0 Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 3
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Casualties
(Based on 5 p.m. time of
occurrence)
Without requiring hospitalization:
0 Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 2
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Without requiring
hospitalization: 1
Requiring hospitalization: 0
Life threatening: 0
Fatalities: 0
Damage to
Transportation Systems
None with at least moderate
damage
None with at least moderate damage None with at least moderate damage
None with at least moderate
damage
Damage to Essential
Facilities
None with at least moderate
damage
None with at least moderate damage None with at least moderate damage
None with at least moderate
damage
Damage to Utility
Systems
There are no electrical power
facilities with at least moderate
There would be no electrical power
facility with at least moderate
There are no electrical power
facilities with at least
There are no electrical power
facilities with at least

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Type of Impact Great Valley 3 6.53 Magnitude
Deterministic Scenario
Hat Creek-McArthur-Mayfield

7.29 Magnitude Deterministic
Scenario
Honey Lake 7.03
Magnitude Deterministic
Scenario
Warm Springs Valley
Magnitude 6.92
Deterministic Scenario
damage. 1 potable water line
breaks, 1 wastewater line breaks,
and 0 natural gas line breaks.
damage. 17 potable water line breaks,
9 wastewater line breaks, and 3
natural gas line breaks.
moderate damage. 38 potable
water line breaks, 19
wastewater line breaks, and 6
natural gas line breaks.
moderate damage. 14 potable
water line breaks, 7
wastewater line breaks, and 2
natural gas line breaks.

Households without
Power/Water Service
(Based on 9,216 total
households)
Power loss, Day
1: 0
Power loss, Day
3: 0
Power loss, Day
7: 0
Power loss, Day
30: 0
Power loss, Day
90: 0
Water loss,
Day 1: 0
Water loss,
Day 3: 0
Water loss,
Day 7: 0
Water loss,
Day 30: 0
Water loss,
Day 90: 0
Power loss, Day
1: 0
Power loss, Day
3: 0
Power loss, Day
7: 0
Power loss, Day
30: 0
Power loss, Day
90: 0
Water loss, Day
1: 0
Water loss, Day
3: 0
Water loss, Day
7: 0
Water loss, Day
30: 0
Water loss, Day
90: 0
Power loss,
Day 1: 0
Power loss,
Day 3: 0
Power loss,
Day 7: 0
Power loss,
Day 30: 0
Power loss,
Day 90: 0
Water loss,
Day 1: 0
Water loss,
Day 3: 0
Water loss,
Day 7: 0
Water loss,
Day 30: 0
Water loss,
Day 90: 0
Power loss,
Day 1: 0
Power loss,
Day 3: 0
Power loss,
Day 7: 0
Power loss,
Day 30: 0
Power loss,
Day 90: 0
Water loss,
Day 1: 0
Water loss,
Day 3: 0
Water loss,
Day 7: 0
Water loss,
Day 30: 0
Water loss,
Day 90: 0
Displaced Households 0 displaced households 1 displaced household 5 displaced households 1 displaced household Shelter Requirements 0 persons 0 persons 2 persons 0 persons Debris Generation 0 tons 2,000 tons 5,000 tons 2,000 tons Source: Hazus 6.1

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FEMA NRI Hazard Vulnerability Assessment
The HMPC looked at the FEMA NRI for additional specific data on Plumas County’s vulnerability to earthquakes. The NRI contains data and analysis for 18 separate natural hazards, including earthquakes.

As shown in Table 4-79, earthquakes could cause a total of $5,576,231 in expected annual losses, with a historic loss rating of relatively high. Earthquake has a relatively low rating for social vulnerability and a relatively high community resilience, with a risk index score of 93.6 out of 100. According to the NRI, there have been 0 earthquake events during the 2021 dataset with a 0.63% annual chance per year.

Table 4-79 Plumas County Planning Area – Earthquake Vulnerability Ratings from FEMA NRI
NRI Hazard Hazard Risk Index
Rating
Expected Annual
Loss Values
(total)**
Exposure Value
(total)***
Historic Loss Ratio
Rating
Earthquake 93.6 $5,576,231 $237,969,269,000 Relatively high Source: FEMA NRI Report
*Hazard Risk Index is calculated using data for only a single hazard type, and reflects a communities expected annual loss value, community risk factors, and the adjustment factor used to calculate the risk value. **Expected annual loss represents the average economic loss in dollars resulting from natural hazards each year. ***Exposure is a natural hazard consequence factor that is the representative value of buildings, populations, or agriculture potentially exposed to a natural hazard occurrence.

Additional information on the FEMA NRI can be found in Section 4.2 and Appendix H.

Impacts from Earthquake
Earthquakes can strike without warning and cause dramatic changes to the landscape of an area that can have devastating impacts on the built environment. The greatest impact is to life safety of Plumas County residents and visitors. Earthquakes can cause loss of life and injuries in affected areas. Other impacts to the Plumas County Planning Area would include damages to infrastructure such as roads, bridges, and dams; damages and loss of services to utilities and critical infrastructure, including those related to gas, power, water, wastewater and communication systems; and damages to structures and other development.

Earthquakes can also cause failure of dams, levees, and reservoirs. Facilities and land downslope from dams or water reservoirs or behind levees might be subject to flooding, if the dams, reservoirs, or levees fail as a result of an earthquake. The County has locations with significant flood risk that include facilities downslope from dams or reservoirs or behind levees.

Impacts that are not quantified, but can be anticipated in large future events, include:
➢ Injury and loss of life;
➢ Commercial and residential structural and property damage; ➢ Disruption of and damage to public infrastructure, utilities, and services; ➢ Damage to roads/bridges resulting in loss of mobility; ➢ Significant economic impact (jobs, sales, tax revenue) to the community; and ➢ Negative impact on commercial and residential property values

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Impacts to identified assets at risk to this hazard and the overall vulnerability of the County may be affected in the future by climate change (which was discussed in the hazard profile section above), changes in population patterns, and changes in land use and development. The influencing effects of these factors on this hazard are discussed further in the Future Conditions/Future Development discussion below.

Future Conditions
Future conditions may be affected by climate change, changes in population patterns (migration, density, or the makeup of socially vulnerable populations), and changes in land use and development. Findings on this for Plumas County include the following:
➢ Climate change is likely to have no effect on future earthquake conditions and associated impacts and vulnerability of the County.
➢ Population in Plumas County is expected to shrink. If this changes, any additional growth in the County would place additional populations at risk to earthquake. Vulnerable population groups could face disproportionate effects from an earthquake and should be planned for. Changes in population and population patterns may or may not increase the impacts and vulnerability of the County to this hazard depending on the location and nature of growth and continued planning for future hazard conditions. ➢ Land use planning should be proactive to address future hazard conditions. County building codes are in effect to reduce structure damage, including damage to critical facilities and infrastructure, and should be updated as necessary to continue to address future earthquake conditions. Depending on the location of new development and adherence to protective building codes, changes in land use and development may or may not increase the impacts and associated vulnerabilities of the County to this hazard.

Future Development
Future development considerations should be ongoing and proactive. Ensuring that comprehensive land use planning is conducted, regulatory requirements, such as building codes, are followed, and varying needs for different demographic groups with intersecting identities that reside within the County are considered can help to make for a more resilient community. Although new growth and development corridors would fall in the area affected by earthquake, given the small chance of major earthquake and the building codes in effect, development in the earthquake area will continue to occur.

4.3.11. Flood: 1%/0.2% Annual Chance (w/levee failure)
Hazard Profile
This hazard profile contains multiple sections that detail how this hazard can affect the Plumas County Planning Area. These sections include a hazard/problem description; description of location and extent;

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past occurrences of this hazard; likelihood of future occurrence; and how climate change can affect or influence this hazard.

Hazard/Problem Description
Flooding is the rising and overflowing of a body of water onto normally dry land. Floods are among the costliest natural disasters in terms of human hardship and economic loss nationwide. Floods can cause substantial damage to structures, landscapes, and utilities and can cause life safety issues. Floods can be extremely dangerous. Six inches of moving water can knock over a person given a strong current. A car will float in less than two feet of moving water and can be swept downstream into deeper waters. Floods kill more people trapped in vehicles than anywhere else.

During a flood, people can also suffer heart attacks or electrocution due to electrical equipment short outs.

Floodwaters can transport large objects downstream which can damage or remove stationary structures, such as dam spillways. Ground saturation can result in instability, collapse, or other damage. Objects can also be buried or destroyed through sediment deposition. Floodwaters can also break utility lines and interrupt services. Standing water can cause damage to crops, roads, foundations, and electrical circuits.

Direct impacts, such as drowning, can be limited with adequate warning and public education about what to do during floods. Where flooding occurs in populated areas, warning and evacuation will be of critical importance to reduce life and safety impacts from any type of flooding.

There are three primary types of freshwater flood events in the Plumas County Planning Area: riverine and lake, flash flooding, and urban stormwater. Regardless of the type of flood, the cause is often the result of severe weather and excessive rainfall, either in the flood area or upstream reaches. ➢ Riverine flooding – Riverine flooding, defined as when a watercourse exceeds its “bank-full” capacity, generally occurs as a result of prolonged rainfall, or rainfall that is combined with already saturated soils from previous rain events. This type of flood occurs in river systems whose tributaries may drain large geographic areas and include one or more independent river basins. The onset and duration of riverine floods may vary from a few hours to many days. Factors that directly affect the amount of flood runoff include precipitation amount, intensity and distribution, the amount of soil moisture, seasonal variation in vegetation, snow depth, and water-resistance of the surface due to urbanization.

In the Plumas County Planning Area, riverine flooding is largely caused by heavy and continued rains, sometimes combined with snowmelt, and heavy flow from tributary streams. These intense storms can overwhelm the local waterways as well as the integrity of flood control structures. The warning time associated with slow rise floods assists in life and property protection. ➢ The term “flash flood” describes localized floods of great volume and short duration. In contrast to riverine flooding, this type of flood usually results from heavy rainfall on a relatively small drainage area. Precipitation of this sort usually occurs in the winter and spring. Flash floods often require immediate evacuation within the hour.
➢ Stormwater/Urban flood events have increased as land has been converted from fields or woodlands to roads and parking lots and lost its ability to absorb rainfall. Urbanization increases runoff by two to six times that of natural terrain. This is discussed in Section 4.3.12 below.

The Plumas County Planning Area is also at risk to flooding resulting from dam failure and levee failure.

Dam failure flooding is discussed separately in Section 4.3.8 of this document. Levee failure is discussed

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below. The potential for flooding can change and increase through various land use changes and changes to land surface, resulting in a change to the floodplain. Environmental changes can create localized flooding problems in and outside of natural floodplains by altering or confining natural drainage channels. These changes are most often created by human activity.

Levee Failure
A levee is a raised area that runs along the banks of a stream or canal. Levees reinforce the banks and help prevent flooding by containing higher flow events to the main stream channel. By confining the flow to a narrower steam channel, levees can also increase the speed of the water. Levees can be natural or manmade. A natural levee is formed when sediment settles on the stream bank, raising the level of the land around the stream.

Levees provide strong flood protection, but they are not failsafe. Levees are designed to protect against a specific flood level and could be overtopped during severe weather events or dam failure. Levees reduce, not eliminate, the risk to individuals and structures located behind them. A levee system failure or overtopping can create severe flooding and high-water velocities. It is important to remember that no levee provides protection from events for which it was not designed, and proper operation and maintenance are necessary to reduce the probability of failure.

Levee Failure Extent
A search of the National Levee Database identified 3 leveed areas in Plumas County (as shown on Figure 4-112). These levees include:
➢ Plumas County Levee 1 - near Taylorsville as shown on Figure 4-113). ➢ Plumas County Levee 2 - near Greenville as shown on Figure 4-114). ➢ North Fork Feather River at Chester (near Chester) – East and West levees as shown on Figure 4-115 and Figure 4-116.

None of these 3 levees are certified as providing protection from the 1% annual chance or other flood. As such, no mapping of these areas is included in the Vulnerability Assessment below as part of the DFIRM analysis as no X-protected by levee flood zones are include in that dataset. However, the vulnerability of the County to these levees to is shown in the Vulnerability Assessment below from data obtained from the National Levee Database.

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Figure 4-112 Plumas County – Levee Locations
Source: National Levee Database
Figure 4-113 Plumas County – Levee 1
Source: National Levee Database

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Figure 4-114 Plumas County – Levee 2
Source: National Levee Database
Figure 4-115 Plumas County – North Fork Feather River at Chester East Levee Source: National Levee Database

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Figure 4-116 Plumas County – North Fork Feather River at Chester West Levee Source: National Levee Database
Location and Extent
Floodplains
The area adjacent to the channel is the floodplain (see Figure 4-117). Floodplains are illustrated on inundation maps, which show areas of potential flooding and water depths. In its common usage, the floodplain most often refers to that area that is inundated by the 1% annual chance (or 100-year) flood, the flood that has a one percent chance in any given year of being equaled or exceeded. The 1% annual chance flood is the national minimum standard to which communities regulate their floodplains through the National Flood Insurance Program. The 500-year flood is the flood that has a 0.2% chance of being equaled or exceeded in any given year.

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Figure 4-117 Floodplain Schematic
Source: FEMA
According to the 2005 Flood Insurance Study(FIS) for Plumas County, flooding in Plumas County may be caused by either general rainstorms or cloudburst storms. General rainstorms can occur from late fall to early spring, but mostly in the winter months of December through March. Cloudburst storms can be expected in the spring, summer, and fall. General rain floods resulting from prolonged heavy rainfall over tributary areas are characterized by high peak flows of moderate duration and by a large volume of runoff.

Flooding is more severe when antecedent rainfall results in saturated ground conditions, when the ground is frozen and infiltration is minimal, or when rain on snow in the higher elevations adds snowmelt to rain flood runoff.

Cloudburst storms, sometimes lasting as long as 6 hours in the study areas, are high intensity storms that can produce floods characterized by high peak flows, short duration flood flows, and small runoff volume.

In small drainage basins such as Portola Tributary, cloudbursts can produce peak flows substantially larger than those of general rainstorms.

The FIS also noted the following flood areas in the County. ➢ 1% annual chance flood flows on the Middle Fork Feather River are attributed to combined general rain/snowmelt runoff. The 1% annual chance flooding on Portola Tributary is a result of cloudburst storms.
➢ Flood elevations in Spanish Creek were high enough to necessitate failure scenarios of the embankments along Spanish Creek's right bank. Upstream of the SH 70/89 crossing, the failure of the

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right bank of Spanish Creek causes flow into the Clear Stream drainage. This split flow is less than the natural peak from the Clear Stream drainage. The right embankment overflow downstream of the SH 70/89 crossing flows across the valley as sheetflow until it accumulates in the lower Nugget Creek drainage, creating the highest flow condition in this Nugget Creek reach. The last substantial out-ofbank flow from Spanish Creek fills the valley area just upstream of its confluence with Mill Creek, where the flood storage in this portion of the valley significantly attenuates the peak flows downstream in Spanish Creek. Each of these out-of-bank spills along Spanish Creek was analyzed independently of each other as a stand-alone scenario. This is because the spills could not concurrently occur as a worstcase condition. ➢ The Greenhorn Creek drainage is conveyed through a narrow valley with occasional division of flow at road crossings lacking capacity to convey the entire 100-year flow. Although the main channel does not contain the 100-year and 500-year (0.2-percent-annual-chance) flows, the right and left overbank flows are conveyed parallel and contiguous to Greenhorn Creek, except where a portion of the Greenhorn Creek flows spill into the lower reach of Thompson Creek. This spill is also parallel to Greenhorn Creek, downstream of the SH 70/89 crossing and upstream of the confluence of Greenhorn and Thompson Creeks. Although Greenhorn Creek flooding is not contained in the main channel, the overbank floodplain is generally shallower than Spanish Creek, and the floodplain storage was considered to have negligible effect on the calculated peak flow values. ➢ Gansner Creek flows out-of-bank from upstream of Bucks Lake Road until beyond its confluence with Clear Stream.
➢ Clear Stream spills into the upper reaches of the unnamed tributary at SH 70/89 and continues downstream with less than a 10-year capacity.
➢ Boyle Ravine spills most of its flow over its left bank just upstream of Alder Street and conveys less than a 10-year capacity until just upstream of its confluence with the unnamed tributary, where the Alder Street split flow returns.
➢ Nugget Creek spills overland and recombines several times within its studied reach upstream and downstream of SH 70/89. The worst-case flooding for the lower reach of Nugget Creek is when the right bank of Spanish Creek fails, and the overflow is conveyed through Nugget Creek. ➢ Mill Creek spills significant flows over the left bank at locations upstream of SH 70/89, Lee Road, and Bell Lane. These flows are recombined (ponded) upstream of Quincy Junction Road and are again split before reaching the lowest part of the Mill Creek drainage before combining with Spanish Creek. The worst-case flooding for Mill Creek downstream of Quincy Junction Road is when Spanish Creek flows out-of-bank and inundates the portion of the valley upstream of their confluence. ➢ Thompson Creek splits and recombines several times upstream of SH 70/89; most of its natural drainage spills into Greenhorn Creek just upstream of SH 70/89, before Greenhorn Creek reaches its peak flow.
➢ Chandler Creek and Taylor Creek spill out-of-bank before they reach Chandler Road. The spills then flow into Greenhorn Creek rather than returning to their respective channels. ➢ The unnamed tributary to Boyle Ravine appears to be sized to convey its local drainage area while acting as an outlet (overflow) path for the larger spills from the Clear Stream drainage.

The HMPC noted that the north end of Greenville in the Willow area there are flooding issues. The HMPC also noted that most of the stream channels of interest in this hazard planning have been significantly modified, or wholesale moved, to facilitate drainage, irrigation and/or road crossings. Much of this modification effort occurred between the 1890’s and 1960’s, pre-CEQA. These modifications generally initiated rapid channel incision, which in turn developed numerous gravel deposits and subsequent instream

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gravel mining operations. The gravel mining mostly over-drafted the gravel supplies, leading to additional incision. By the late 1990’s, CDFW monitoring requirements shut down these operations, with the exception of Green Flat northwest of Meadow Valley. These channels are gradually trying to adjust to new sediment inputs, small and regular or large and episodic, often with abrupt catastrophic results.

There are three primary conditions that contribute to major flooding: high antecedent basin moisture; high intensity, long duration rainfall; and high water content snow at all elevations, compounded by high snow levels. 1986 had low antecedent basin moisture; long duration’ high intensity rain, ~25 inches in 6 days; low snow water content, level 7,000 feet. 1997 had fully saturated antecedent moisture; long duration, low/moderate rainfall intensity, ~12 inches in 6 days; and high snow water content, 8” water in 22” of snow in Genesee (personal observation of HMPC members), snow level 10,000 feet. None of these events had all 3 components at max. When these components converge, it will be catastrophic for Plumas County, as well as everything downstream. The 1862 flood is probably the only analog. More information on these events can be found in the Past Occurrences section below.

Hydrologic Regions
According to Cal DWR, California has 10 hydrologic regions. Plumas County sits in the Sacramento hydrologic region. The Sacramento River hydrologic region covers approximately 17.4 million acres (27,200 square miles). The region includes all or large portions of Modoc, Siskiyou, Lassen, Shasta, Tehama, Glenn, Plumas, Butte, Colusa, Sutter, Yuba, Sierra, Nevada, Placer, Sacramento, El Dorado, Yolo, Solano, Lake, and Napa counties. Small areas of Alpine County are also within the region. Geographically, the region extends south from the Modoc Plateau and Cascade Range at the Oregon border, to the Sacramento-San Joaquin Delta. The Sacramento Valley, which forms the core of the region, is bounded to the east by the crest of the Sierra Nevada and southern Cascades and to the west by the crest of the Coast Range and Klamath Mountains. The Sacramento metropolitan area and surrounding communities form the major population center of the region. With the exception of Redding, cities and towns to the north, while steadily increasing in size, are more rural than urban in nature, being based in major agricultural areas.

A map of California’s hydrological regions is provided in Figure 4-118.

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Figure 4-118 California Hydrologic Regions
Source: California Department of Water Resources
The Plumas County Watersheds and Waterway System
Plumas County encompasses multiple rivers, streams, creeks, and associated watersheds. Plumas County is in the Feather River Watershed. Figure 4-119 illustrates the primary watersheds of Plumas County, as well as the primary waterways in the County.

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Figure 4-119 Primary Watersheds and Waterways of Plumas County

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Feather River Watershed
The Upper Feather River watershed encompasses 2.3 million acres in the northern Sierra Nevada, where that range intersects the Cascade Range to the north and the Diamond Mountains of the Great Basin and Range Province to the east. The watershed drains generally southwest to Lake Oroville, the largest reservoir of the State Water Project.

Land ownership in the Feather River Watershed is approximately 64 percent Federal, 1 percent State, and 35 percent private. Federal lands are managed primarily by the U.S. Forest Service (USFS) except for less than 1 percent of the watershed that is within Lassen Volcanic National Park and some Bureau of Land Management lands in the Sierra Valley watershed. The boundary of the watershed largely corresponds to the boundary of Plumas County but also includes portions of six neighboring counties.

Plumas County Flood Mapping
As part of the County’s ongoing efforts to identify and manage their flood prone areas, Plumas County relies on a variety of different mapping efforts. What follows is a brief description of FEMA and CA DWR mapping efforts covering the Plumas County Planning Area.

FEMA Floodplain Mapping
FEMA established standards for floodplain mapping studies as part of the National Flood Insurance Program (NFIP). The NFIP makes flood insurance available to property owners in participating communities adopting FEMA-approved local floodplain studies, maps, and regulations. Floodplain studies that may be approved by FEMA include federally funded studies; studies developed by state, city, and regional public agencies; and technical studies generated by private interests as part of property annexation and land development efforts. Such studies may include entire stream reaches or limited stream sections depending on the nature and scope of a study. A general overview of floodplain mapping is provided in the following paragraphs. Details on the NFIP and mapping specific to the County are in Section 4.3 Vulnerability Assessment.

Flood Insurance Study (FIS)
The FIS develops flood-risk data for various areas of the community that will be used to establish flood insurance rates and to assist the community in its efforts to promote sound floodplain management. The current Plumas County FIS is dated March 2, 2005. This study covers both the unincorporated and incorporated areas of the County.

Flood Insurance Rate Map (FIRM)
The FIRM is designed for flood insurance and floodplain management applications. For flood insurance, the FIRM designates flood insurance rate zones to assign premium rates for flood insurance policies. For floodplain management, the FIRM delineates 1% and 0.2% annual chancer floodplains, floodways, and the locations of selected cross sections used in the hydraulic analysis and local floodplain regulation. The County FIRMs have been replaced by digital flood insurance rate maps (DFIRMs) as part of FEMA’s Map Modernization program, which is discussed further below.

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Letter of Map Revision (LOMR) and Map Amendment (LOMA)
LOMRs and LOMAs represent separate floodplain studies dealing with individual properties or limited stream segments that update the FIS and FIRM data between periodic FEMA publications of the FIS and FIRM.

Digital Flood Insurance Rate Maps (DFIRM)
As part of its Map Modernization program, FEMA is converting paper FIRMS to digital FIRMs, DFIRMS.

These digital maps:
➢ Incorporate the latest updates (LOMRs and LOMAs);
➢ Utilize community supplied data;
➢ Verify the currency of the floodplains and refit them to community supplied basemaps; ➢ Upgrade the FIRMs to a GIS database format to set the stage for future updates and to enable support for GIS analyses and other digital applications; and
➢ Solicit community participation.

DFIRMs for Plumas County have been developed (dated March 2, 2005), are the most current effective DFIRMs for Plumas County, and are being used for the flood analysis for this LHMP Update. The DFIRM is shown in Figure 4-120 (with the individual inset maps shown as Figure 4-121, Figure 4-122, and Figure 4-123), and DFIRM flood zones are shown on Table 4-80.

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Figure 4-120 Plumas County – DFIRM Flood Zones

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Figure 4-121 Plumas County Planning Area – Map Inset 1 DFIRM Flood Zones

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Figure 4-122 Plumas County Planning Area – Map Inset 2 DFIRM Flood Zones

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Figure 4-123 Plumas County Planning Area – Map Inset 3 DFIRM Flood Zones

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Table 4-80 Plumas County – FEMA DFIRM Flood Zones by Jurisdiction Flood Zone Description Present in City of
Portola
Present in
Unincorporated County
A 1% annual chance flooding: No base flood
elevations provided
X X
AE 1% annual chance flooding: Base flood
elevations provided
X X
AE Regulatory
Floodway
1% annual chance flood: Regulatory floodway;
Base flood elevations provided
X X
AH 1% annual chance flood areas of shallow
flooding between one to three feet deep.

Regulatory floodway; Base flood elevations
provided
X
Shaded X 0.2% annual chance flooding: The areas between the limits of the 1% annual chance flood and the

0.2-percent-annual-chance (or 500-year) flood
X X
X (unshaded) No flood hazard X X
Source: FEMA 03/02/2005 DFIRM
California Floodplain Mapping
Also to be considered when evaluating the flood risks in Plumas County are various floodplain maps developed by the California DWR for various areas throughout California, and in the Sacramento-San Joaquin Valley cities and counties. The FEMA regulatory maps provide just one perspective on flood risks in Plumas County. SB 5, enacted in 2007, authorized Cal-DWR to develop the Best Available Maps (BAM) displaying 1% and 0.5% (200-year) annual chance floodplains for areas located within the Sacramento-San Joaquin (SAC-SJ) Valley watershed. This effort was completed by DWR in 2008. DWR has expanded the BAM to cover all counties in the State and to include 0.2% annual chance flood zones.

Different than the FEMA DFIRMs which have been prepared to support the NFIP and generally reflect only the 1% and 0.2% annual chance flood risks, the BAMs are provided for informational purposes and are intended to reflect current 1%, 0.5% (200-year) as applicable, and 0.2% annual chance flood risks using the best available data. The 100-year floodplain limits on the BAM are a composite of multiple 1% annual chance floodplain mapping sources. It is intended to show all currently identified areas at risk for a 100- year flood event, including FEMA’s 1% annual chance flood zones. The BAM are comprised of different engineering studies performed by FEMA, Corps, and DWR for assessment of potential 1%, 0.5%, and 0.2% annual chance floodplain areas. These studies are used for different planning and/or regulatory applications, and for each flood frequency may use varied analytical and quality control criteria depending on the study type requirements.

The value in the BAMs is that they provide a bigger picture view of potential flood risk to the County than that provided in the FEMA DFIRMs. This provides the community and residents with an additional tool for understanding potential flood hazards not currently mapped as a regulated floodplain. Improved awareness of flood risk can reduce exposure to flooding for new structures and promote increased protection for existing development. Informed land use planning will also assist in identifying levee maintenance

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needs and levels of protection. By including the FEMA 1% annual chance flood zone, it also supports identification of the need and requirement for flood insurance. Figure 4-124 shows the BAM for the Plumas County Planning Area.

Figure 4-124 Plumas County– Flood Awareness (Best Available) Map Source: California DWR. Retrieved 3/5/2025.

Legend explanation: Blue - FEMA 100-Year, Orange – Local 100-Year (developed from local agencies), Red – DWR 100-year (Awareness floodplains identify the 100-year flood hazard areas using approximate assessment procedures.), Pink – USACE 100- Year (2002 Sac and San Joaquin River Basins Comp Study), Yellow – USACE 200-Year (2002 Sac and San Joaquin River Basins Comp Study), Tan – FEMA 500-Year, Grey – Local 500-Year (developed from local agencies), Purple – USACE 500-Year (2002 Sac and San Joaquin River Basins Comp Study).

Flood extents are usually measured in depths of flooding and area flooded. Expected flood depths in the Plumas County Planning Area vary. Flood durations in the County tend to be short to medium term, or until either the storm drainage system can catch up, or flood waters move downstream. Geographical extents of DFIRM flood zones for the Plumas County Planning area are summarized in Table 4-81. Details on the City of Portola and each Special District’s DFIRM flood extents can be found in their respective annexes to this LHMP Update.

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Table 4-81 Plumas County Planning Area – Geographical Extent of FEMA DFIRM Flood Zones by Jurisdiction
Jurisdiction /
Flood Zone
Total Acres % of Total
Acres
Improved
Acres
% of Total
Improved
Acres
Unimproved
Acres
% of Total
Unimproved
Acres
City of Portola
1% Annual
Chance Flood
Hazard
168 0.01% 10 0.01% 157 0.01%

0.2% Annual
Chance Flood
Hazard
1 0.0001% 0.1 0.0001% 1 0.00005%
Other Areas 3,323 0.20% 2,227 2.39% 1,096 0.07%
City of Portola
Total
3,491 0.21% 2,237 2.41% 1,254 0.08%
Unincorporated Plumas County
1% Annual
Chance Flood
Hazard
84,609 5.06% 23,036 24.77% 61,573 3.90%

0.2% Annual
Chance Flood
Hazard
1,431 0.09% 667 0.72% 764 0.05%
Other Areas 1,582,631 94.65% 67,072 72.11% 1,515,559 95.97% Unincorporated
Plumas County
Total
1,668,671 99.79% 90,775 97.59% 1,577,896 99.92%
Grand Total 1,672,162 100.00% 93,013 100.00% 1,579,150 100.00% Source: FEMA 03/02/2005 DFIRM
There is not a scientific scale or measurement system in place for levee failure. It is usually measured in area covered and depth of flooding. Maps showing inundation depths due to a levee failure in the County do not exist. The speed of onset is slow as the river rises, but if a levee fails the warning times are short for those in the inundation area. The duration of levee failure risk times can be hours to weeks, depending on the river flows that the levee holds back.

Past Occurrences
Disaster Declaration History
A list of federal and state disaster declarations for Plumas County from flooding, (including heavy rains and storms) is shown on Table 4-82.

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Table 4-82 Plumas County – Federal and State Disaster Declaration from Flood 1950-2025 Disaster Type State Declarations Federal Declarations
Count Years Count Years
Flood (including heavy
rain and storm)
22 1950, 1955, 1958 (twice), 1964,
1963, 1964, 1969, 1970 1980,
1986, 1993, 1992*, 1995
(twice*), 1996, 1997, 2006, 2017
(twice*), 2023 (twice*)
19 1950, 1955, 1958 (twice), 1963,
1964, 1969, 1970, 1986, 1992
(twice), 1995 (twice), 1997,
2006*, 2017 (twice*), 2023
(twice*)
Source: Cal OES, FEMA
*included a landslide component.

NCDC Events
The NCDC tracks flooding events for the County. Events have been tracked for flooding since 1993. Table 4-83 shows events in Plumas County since 1993.

Table 4-83 NCDC Flood Events in Plumas County 1993 to 7/31/2024* Event Type Number
of Events
Deaths Deaths
(indirect)
Injuries Injuries
(indirect)
Property
Damage
Crop
Damage
Flash Flood 3 0 0 0 0 $0 $0
Flood 12 0 0 1 0 $3,140,000 $0
Total 15 0 0 1 0 $3,140,000 $ 0
Source: NCDC
*Note: Losses reflect totals for all impacted areas, much of which fell outside of Plumas County Hazard Mitigation Planning Committee Events
These sections separate events since the 2019/2020 LHMPs and those before 2019.

Events since 2019/2020 LHMP
It was noted that there have been many atmospheric river events since 2017. These are shown on Table 4-84.

Table 4-84 Plumas County Planning Area – Atmospheric River Events and Floods 2017-2025 Date/Timeframe
of Atmospheric
River Event
Portola
Middle
Fork River
Gauge
(feet
above
mean sea
level)
Portola (or
Lake Davis
where
Portola is
not
reported)
Precipitation
(inches)
Spanish
Creek AB
Blackhawk
Creek at
Keddie
Stream
Gauge
(feet)
Spanish
Creek AB
Blackhawk
Creek at
Keddie
Discharge
(ft3/s)
Quincy
Precipitation
(inches)
Greenville
Precipitation
(inches)
Chester
Precipitation
(inches)
2/5/2025 7.90 10.19 8740.00 15.38 10.71 10.50
11/22-24/2024 4.54 7.31 3780.00 15.64 11.68 9.86

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Date/Timeframe
of Atmospheric
River Event
Portola
Middle
Fork River
Gauge
(feet
above
mean sea
level)
Portola (or
Lake Davis
where
Portola is
not
reported)
Precipitation
(inches)
Spanish
Creek AB
Blackhawk
Creek at
Keddie
Stream
Gauge
(feet)
Spanish
Creek AB
Blackhawk
Creek at
Keddie
Discharge
(ft3/s)
Quincy
Precipitation
(inches)
Greenville
Precipitation
(inches)
Chester
Precipitation
(inches)
3/15-16/2023 4854.42 26.47 9.08 6680.00 49.71 37.76 34.49 2/1/2023 (snow
event)

18.98 3.38 540.00 37.20 27.61 26.63
12/26/22 to
1/20/23

10.64 7.67 4370.00 19.85 13.70 14.27
1/3/23 to
1/17/23

7.14 7.67 4370.00 13.05 9.84 10.48
12/26/22 to
1/2/23

3.24 6.53 2950.00 6.00 3.43 3.48
October 17-26,
2021

8.22 6.73 3160.00 12.00 9.72 6.36
2/14/2017-
2/22/2017

3.54 10.27 9000.00 9.38 7.01 5.02
2/1/2017 -
2/11/2017
4856.27 8.93 13.85 16700.00 13.15 11.15 7.55
01/17/2017 -
01/23/2017

2.83 4.77 1470.00 5.27 5.49 4.04
01/02/2017 -
01/13/2017
4853.55 11.07 13.87 14300.00 16.86 11.97 8.28
November 27-
December 3, 2012

7.65 10.44 5000.00 12.25 9.64 6.68
Source: Plumas County Volunteers Active in Disasters
It was also noted that in 2019 Indian Valley had snow on rain events that caused flooding.

The County also noted an event in February to April of 2023. The 2023 Late Winter Storms Feb-Apr which began Feb 27th, 2023. The winter storms included three days of blizzards, many weeks of heavy snow loads, rain-on-snow events, road closures, washouts, flooding, and debris removal, and damaged water systems. FEMA declared a federal disaster declaration for this event (DR-4699).

Due to the storms, the City of Portola and the County both had to lessen the immediate threat to lives and safety by initiating emergency protective measures before, during, and after the incidents. The effects of the storms were beyond our control. The City of Portola suffered a loss of water due to its water system failing, washouts, frozen pipes, water-main breaks, and traffic disruptions due to flooded streets and excessive emergency snow removal expenses. The County also had excessive emergency snow removal expenses, emergency tree, and debris removal and has discovered many sinkholes, flooded roadways and damaged/washed out roads due to the excessive snow and runoff. With the snowpack being 189% to normal, much of the County covered in snow, so not all damage was known.

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These storms affected all the citizens of this County, but especially that of our senior and AFN population who were stuck in their homes many days to a week after as the snow continued to fall. Plumas County’s population of over 65 is 30%- that is over double the State average. Many homes have been damaged with 5 being destroyed. Private property damage was mostly minor and still habitable; however, several businesses that were affected and one destroyed.

Plumas County was still in recovery from the Dixie Fire (DR 4610) which occurred just 20 months earlier The costs of the emergency expenditures, road damages, and water system failures were more than the County can handle at this time. It was also that discovered many homeowners do not have full coverage or all-hazard insurance or only have the California Fair Plan. Overcoming the hardship of the previous disaster has affected all citizens of Plumas County.

Events before 2019
Regional and localized flooding in Plumas County has been a continuous occurrence dating back to at least 1893 when Quincy experienced its first photographed flood, shown in Figure 4-125.

Figure 4-125 1893 Quincy Flooding
Source: Plumas County
Winter storms in 1986, 1995, 1997, 2017, and 2019 caused tremendous flood damage to properties and infrastructure throughout the Upper Feather River Watershed. Discharge values are from the historic USGS gage (#11401500) at the outlet of Indian Valley (1906-1993 period of record). Subsequent values from Plumas Corp and/or USGS extrapolation. Recent flood values of note: 1986 (36,600 cfs); 1995 (24,000

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cfs), 1997 (46,600 cfs); 2017 (greater than 1986, less than 1997). At ~25,000 cfs Arlington bridge is impassible, along with Stampfli Ln. and North Valley Road. Taylorsville and Genesee are isolated. At <30,000 cfs Genesee is isolated from Taylorsville. Those communities have no medical services. Minor floods occurred in the 1990s in burn scar areas of the County. These were prevalent in the eastern margin by the escarpment.
1986
From February 8-20, 1986, a large storm lasting 13 days precipitated rain and snow across Northern California. Plumas County was located within the interior of the storm extent and experienced tremendous rainfall, causing the ground to saturate and allowed surface water to flow freely. As rain fell over the county filling creeks and drainage ditches, it also flowed downhill through the Feather River system, incrementally adding more water to the lower elevation valleys and the river canyons. By the 11th day of the storm the capacity of the hydrologic system was exceeded, and extensive damage was experienced throughout Plumas County. The most visually impressive damage was found in the North Fork Feather River Canyon, along CA-70 and the Railroad, due to the large volume of water that was funneled through the canyon. A member of the HMPC from Viera ranch noted that during this time, 22.08" of rain were recorded at their ranch.

Quincy was effectively cut off. Highway 70 in Feather River Canyon was washed out.

The flood damage was extensive, as numerous bridges were severely damaged or destroyed, large sections of roadway and railroad were wiped out (see Figure 4-126), bridges were destroyed (see Figure 4-127), many houses were flooded with over one foot of water, and debris was deposited in throughout Plumas County. Train service was disrupted for at least 3 days through the Feather River Canyon, and several state highways were temporarily out of commission to public traffic for several weeks. In addition, many residential wells were flooded.

Figure 4-126 1986 Railroad Bed Flood Damages
Source: The Storm of ’86 by Robert Moon, Feather River Publishing, Quincy, CA 1986

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Figure 4-127 Bridges Destroyed by Flooding – Indian Creek Bridge (left) and Mohawk Valley Bridge (right)
Source: The Storm of ’86 by Robert Moon, Feather River Publishing, Quincy, CA 1986 1995
Heavy rains caused flooding in the County.

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Figure 4-128 Plumas County – 1995 Flooding at Arlington Bridge Source: HMPC member Jim Wilcox
1997
Winter storms in late December 1996 through January 1997 poured tremendous amounts of rain throughout Plumas County. This was the biggest flood on record for the County. Such as in 1986, the ground became saturated, and the river system overflowed with excess water. On January 2nd the State declared a disaster and on January 4th a federal disaster was declared. The extent and severity of flooding and related damage exceeded the 1986 event throughout Plumas County, from the high-elevation valleys to the low-elevation river canyons. The type of damage experienced was similar to that in 1986. Examples can be seen in the following figures.

In the first image, the home was not flooded, but Indian Creek moved laterally several hundred feet in less than 24 hours to undermine the main structure and topple it into the channel. The remains of the garage followed a few days later. Homeowners reportedly had no flood insurance. The gravel bar on the left rapidly extended under sediment input from Little Grizzly Creek 1/3 mile upstream. The Indian Creek channel has radically shifted alignment in this area 3 times since 1977.

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Figure 4-129 Erosion from Floodwaters Cause Home to fall into Indian Creek Source: Feather River Bulletin, Wednesday January 29, 1997

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Figure 4-130 Damage to Home in Genesee from 1997 Flood
Source: Feather River Bulletin, Wednesday January 29, 1997 Figure 4-131 Sloat Bridge Damage from 1997 Flood
Source: Plumas County Road Department

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Figure 4-132 Damage to CA-70 near Tobin
Source: Feather River Bulletin, Wednesday January 29, 1997

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Figure 4-133 High Water Marks from 1986 and 1987 in North Fork Feather Canyon Source: Plumas County
2017
January and February of 2017 brought heavy rains from atmospheric rivers that struck Plumas County, resulting in a federal disaster declaration (DR-4301).

Many damages occurred in the County. One area hit hard was the Plumas Eureka Community Services District. The District area saw flood damage to 26 condominiums, and two single family homes (see Figure 4-134). Erosion caused the loss of half a backyard requiring the owner to reinforce the riverbank bordering

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the property (see Figure 4-135). Erosion to roads and water main right of way, damage to sewer pumps, debris removal from fallen trees occurred. Sewer service was shut down for 12 hours. There was also a road closure to flooded areas for 12 hours.

Figure 4-134 Plumas County 2017 Flooding in the Plumas Eureka Community Services District
Source: Plumas Eureka Community Services District
Figure 4-135 Erosion to Backyard of Home during 2017 Floods (left – during flood, right – after flood)
Source: Plumas Eureka Community Services District

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Private property damage in the District came from both flooding and from fallen trees. Amount unknown, but one property owner stated that flood damage related costs exceeded $120,000 of which he received $67,000 for his claim. The District received $12,189 from damages sustained.

Additionally, the HMPC noted that there were issues with flooding of agriculture in 2017 and 2018. In the Sierra Valley and Beckwourth areas, rains caused a 30% decrease in hay production and caused a 25% loss in calves. It was thought that $200,000 in damages to hay and $19,000 in damages to calves was suffered in both 2017 and 2018, respectively. Other areas of the Sierra Valley suffered field erosion and additional hay losses. $1.6 million in hay was lost, and $230,000 in damages was suffered from field erosion.

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Figure 4-136 2017 Flooding – Arlington Bridge
Source: Plumas County Agriculture Commissioner

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Figure 4-137 2017 Road Flooding
Source: Plumas County Agriculture Commissioner
Figure 4-138 2017 Flooding – Flooded Meadow
Source: Plumas County Agriculture Commissioner

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January 2026
Figure 4-139 2017 Flooding
Source: Plumas County Agriculture Commissioner
The HMPC noted that there were washouts near Laporte and Thompson Creek. These can be seen in Figure 4-140 and Figure 4-141.

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Plumas County 4-315
Local Hazard Mitigation Plan Update
January 2026
Figure 4-140 Plumas County – LaPorte Road Washout
Source: Plumas County
Figure 4-141 Plumas County – Thompson Creek Washout
Source: Plumas County

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Additionally, Plumas County Public Works kept a tally of damages from the 2017 storms:
➢ 2017 Storm Damage Debris Removal:
✓ CR 507 Johnsville-Mccrea Road
✓ CR 423 Big Creek Road
✓ Lindan Channel next to CR QU30, Lindan Avenue
✓ CR TV03, Thompson Street
✓ CR 112, North Valley Rd. at Lights Creek (Br. No. 9C-0012) ✓ CR 207, Arlington Road at Hough Creek
✓ CR 206, Stampfli Lane Br. 9C-0053
✓ CR 207C, Old Arlington Road
✓ CR CM08, Wagon Road
✓ CR 317, Rush Creek Road
✓ CR 511, Quincy-Laporte Rd. (LaPorte - to Yuba Co. line) ✓ CR 311, Section - Old Red Bluff Road, Br. 9C-0052
✓ CR 414, Bucks Lake Road at Clear Creek
✓ CR 517, Mt. Tomba Road.
✓ CR 202, Greenville-Wolf Creek Rd.
✓ CR 404A, Oakland Camp Road
✓ CR 511, From the M.F.F.R. to Silver Tip
✓ CR 511, From Silver Tip to Laporte
✓ CR 219, Williams Valley Rd
✓ CR 312, Chester-Warner Valley Road, Br. 9C-0050 at Warner Creek ➢ 2017 Storm Damage Unplug Culverts / Culvert Washouts:
✓ CR 507 Johnsville-Mccrea Road
✓ Mill Creek next to Maintenance yard
✓ CR 423 Big Creek Road
✓ CR 520, Little Bear Road at Bonta Creek
✓ CR CM03, Main St. in Crescent Mills
✓ CR 115, Clio-State 40A Road at Willow Creek
✓ CR 306, Seneca Road at Davis Creek
✓ CR 420, Blackhawk Road
✓ CR 408, West's Ranch Road
✓ CR 529, Gill Ranch Road
✓ CR 308, Humboldt Road
✓ CR 516, Mohawk Vista Drive
✓ CR 403, Mt. Hough-Crystal Lake Road
✓ CR 532, Harrison Road
✓ CR 507, Johnsville-McCrea Road
✓ CR 301, Highlands Road
➢ 2017 Storm Damage Road Washouts:
✓ CR 507 Johnsville-Mccrea Road
✓ CR 420 Blackhawk

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✓ CR 305, Prattville - Butt Reservoir Road
✓ CR 124, Rocky Point Road
✓ CR 103, Dotta Guidici Road
✓ CR 304, Rich Bar Rd.
✓ CR 417, Butterfly Valley - Twain Rd.
✓ CR 303A, Howells Road (February Storm)
✓ CR 510, Radio Hill Road
✓ CR 202A, Setzer Camp Road at Wolf Creek, Br. No, 9C-0131 ✓ CR 306, Seneca Road at Owl Creek
✓ CR402A, Old State Highway
✓ CR 214, North Arm Road, Br 9C-0143 (Peters Creek)
✓ CR 118, Harriet Lane
✓ CR 107, Dyson Lane
✓ CR 108, Beckwourth-Loyalton Road
✓ CR 404, Chandler Road
✓ CR 206, Stampfli Lane (road - not the bridges)
✓ CR 303A, Howells Road (January Storm)
➢ 2017 Storm Damage Bank Failure / Streambank Erosion:
✓ CR 409, Beskeen Lane
✓ CR 509B, Sloat-Poplar Valley Road Bridge Rock Slope Bank Repair Likelihood of Future Occurrence
1% Annual Chance Flood
Occasional— The 1% annual chance flood (100-year) is the flood that has a 1 percent chance of being equaled or exceeded in any given year. This, by definition, makes the likelihood of future occurrence occasional. However, the 100-year flood could occur more than once in a relatively short period of time.

0.2% Annual Chance Flood
Unlikely—The 0.2% annual chance flood (500-year) is the flood that has a 0.2 percent chance of being equaled or exceeded in any given year. This, by definition, makes the likelihood of future occurrence unlikely.

It is likely that climate change will increase the chance of future occurrence as well as future impacts. More information on climate change and flood can be found in the next section. More information on future impacts can be found in the Future Conditions/Future Development section of the Vulnerability Assessment below.

Climate Change and Flood
Climate change and its effect on flooding is discussed using three sources:
➢ 2021 CAS

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➢ Cal-Adapt
➢ National Center for Atmospheric Research
California Climate Adaptation Strategy
According to the 2021 CAS (as well as the 2024 Draft CAS), climate change may affect flooding in California and the Planning Area. While average annual rainfall may increase or decrease slightly, the intensity of individual rainfall events is likely to increase during the 21st century. It is possible that average soil moisture and runoff could decline, however, due to increasing temperatures, evapotranspiration rates, and spacing between rainfall events. Reduced snowpack and increased number of intense rainfall events are likely to put additional pressure on water infrastructure which could increase the chance of flooding associated with breaches or failures of flood control structures such as levees and dams.

Cal Adapt
Cal Adapt future precipitation projections were shown in Figure 4-57 in Section 4.3.4 Severe Weather:

Heavy Rains and Storms. These could affect future flooding in the Plumas County Planning Area.

National Center for Atmospheric Research
Also, according to the National Center for Atmospheric Research in Boulder, Colorado, atmospheric rivers are likely to grow more intense in coming decades, as climate change warms the atmosphere enabling it to hold more water.

Vulnerability Assessment
Vulnerability—High
Historically, the Plumas County Planning Area has always been at risk to flooding during the rainy season from November through April. Historically, the Plumas County Planning Area has been at risk to flooding primarily during the winter and spring months when river systems in the County swell with heavy rainfall and snowmelt runoff. Normally, storm floodwaters are kept within defined limits by a variety of storm drainage and flood control measures. Occasionally, extended heavy rains result in floodwaters that exceed normal high-water boundaries and cause damage. Flooding has occurred both within the 1% and 0.2% annual chance floodplains and in other localized areas. The vulnerability of the County to severe flooding is high as it can result in significant life safety and property damage.

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Local Concerns (unincorporated Plumas County)
The unincorporated County has specific concerns regarding this hazard. These concerns form a portion of the basis for the mitigation strategy and mitigation actions that seek to reduce vulnerabilities to this hazard.

Overall Flood Concerns
Floods have been a part of Plumas County’s historical past and will continue to be so in the County’s future.

During winter months, long periods of precipitation and the timing of that precipitation are critical in determining the threat of flood, and these characteristics further dictate the potential for widespread structural and property damages. Historically, much of the growth in the County has occurred adjacent to rivers or streams, resulting in significant damages to property, and losses from disruption of community activities during periods of flooding. Additional development in the watersheds of these streams affects both the frequency and duration of damaging floods through an increase in stormwater runoff. Other problems connected with flooding and stormwater runoff include erosion, sedimentation, degradation of water quality, losses of environmental resources, and certain health hazards.

Rain on snow events are also a concern in the County.

Wolf Creek Flooding
The County provided a 2025 County memo regarding an area of the County at risk from accelerated bank erosion. This is known locally as the Wolf Creek Phase 4 area. 2025 saw calls by residents in the area to the County stating concern about bank erosion issues in this area. This is a continuation of similar effects since Wolf Creek was captured by 4th Water Trough Creek in 1997. Now a deeply incised, confined channel, Wolf Creek is subject to periodic episodes of mineral and organic debris loading during high intensity rain/saturated soils/wind events. These loads are derived from both upstream and onsite sources, exacerbated by Dixie Fire effects. This is the reason Wolf Creek was included in the original LHMP process and a Hazard Mitigation grant application. This Hazard Mitigation Grant Program (HMGP) grant has been approved by FEMA, but a local match is still being sought. The worry here is a repeat of a previous incident in the County. Figure 4-129 above is a photo of a home that was victim of rapid channel migration of Indian Creek in Genesee in 1997. The channel moved laterally +400 ft. in 24 hrs. The home was never flooded, just undermined by bank erosion until it fell in. This type of local flood disaster is a threat in numerous places in Plumas County.

Two principal conditions stand out: the amount of catastrophic burn area above the reach under discussion, and the width of the original floodplain to the current floodplain (or lack thereof). Over time Wolf Creek will erode and deposit until a similarly expansive floodplain is developed. There are residents located in these areas
Local concerns for Portola and the Special Districts can be found under the Local Concerns heading in each respective jurisdictional annex to this LHMP Update.

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Portola City CouncilJune 10, 2026

Portola City Council
June 10, 2026