Thermalito Afterbay
Thermalito Afterbay is an earth dam located in Butte County, California, built in 1967. It carries a High hazard potential classification and has a fair condition assessment.
About Thermalito Afterbay
The primary purpose of Thermalito Afterbay is water supply. The dam stores water for municipal, industrial, or agricultural use, helping ensure a reliable supply for the surrounding area. The dam is owned by California Department Of Water Resources (state). It impounds the Feather River - Off Stream near Marysville.
The dam stands 39 feet tall and stretches 42,000 feet across, creates a reservoir covering 6,900 acres, has a storage capacity of 57,041 acre-feet, and collects water from a drainage area of 13 square miles. Completed in 1967, the structure is well into its service life at over 50 years old.
This dam carries a high hazard potential classification, which means that a failure or uncontrolled release of water would likely cause loss of human life. This classification reflects the consequences of failure, not the likelihood — it indicates that people live or work in the area downstream. Its condition is rated fair — the dam has minor deficiencies that are being addressed or do not pose an immediate safety threat. An emergency action plan is in place for this dam. The most recent inspection on record was 08/23/2023.
At 39 feet, this dam is taller than 86% of dams nationwide.
Living Downstream
If you live or work downstream of this dam, consider taking these steps: learn your local evacuation routes, check whether your county emergency management office has a dam failure inundation map, and sign up for local emergency alerts. The California dam safety program can provide more specific information about emergency planning for this structure.
Search for dams near your address to see all dams in your area with hazard classifications and safety data.
Location
| State | California |
| County | Butte |
| Nearest City | Marysville |
| River/Stream | Feather River - Off Stream |
| Coordinates | 39.47710, -121.68710 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 39 ft |
| Dam Height | 39 ft |
| Dam Length | 42,000 ft |
| Year Completed | 1967 (59 years old) |
Storage & Hydraulics
| NID Storage | 57,041 acre-ft |
| Normal Storage | 57,041 acre-ft |
| Surface Area | 6,900 acres |
| Drainage Area | 13 sq mi |
| Max Discharge | 17,000 cfs |
| Spillway Type | Controlled |
Ownership & Safety
| Purpose | Water Supply |
| Owner | California Department of Water Resources |
| Owner Type | State |
| Hazard Classification | High |
| Condition Assessment | Fair |
| Last Inspection | 08/23/2023 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA00042 |
| Federal ID | CA00042 |
| More Info | State Dam Safety Program |