Thermalito Afterbay Reservoir Report

Last Updated: December 5, 2025

Thermalito Afterbay, located in Marysville, California, is a state-regulated water resource with a primary purpose of water supply.


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Summary

Completed in 1967, this earth dam stands at a height of 39 feet and spans a length of 42,000 feet, with a storage capacity of 57,041 acre-feet. The afterbay is situated off-stream from the Feather River and covers a surface area of 6,900 acres, serving multiple purposes including hydroelectric power generation, irrigation, recreation, and water supply.

Managed by the California Department of Water Resources (DWR) and regulated by the Federal Energy Regulatory Commission (FERC), Thermalito Afterbay plays a crucial role in water management in the region. With a high hazard potential and a very high risk assessment rating, the afterbay is subject to regular inspections and enforcement measures to ensure its safety and functionality. Additionally, the presence of controlled spillways and emergency action plans highlight the commitment to mitigating potential risks associated with the dam.

As a key component of the water infrastructure in Butte County, Thermalito Afterbay serves as a vital resource for the community, providing essential water supplies for agricultural and municipal needs. With its strategic location and multiple benefits, including recreational opportunities for residents, the afterbay stands as a testament to effective water resource management in the face of climate challenges.

Year Completed

1967

Dam Length

42000

Dam Height

39

River Or Stream

Feather River - Off Stream

Primary Dam Type

Earth

Surface Area

6900

Hydraulic Height

31.5

Drainage Area

13.3

Nid Storage

57041

Structural Height

39

Hazard Potential

High

Foundations

Rock

Nid Height

39
       
Seasonal Comparison

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.