Ralston Afterbay
Ralston Afterbay is a concrete dam located in Placer County, California, built in 1966. It carries a High hazard potential classification and has a fair condition assessment.
About Ralston Afterbay
The primary purpose of Ralston Afterbay is irrigation. The dam stores and diverts water for agricultural irrigation, supporting farming operations in the surrounding area. The dam is owned by Placer County Water Agency (local government). It impounds the Rubicon River near Auburn.
The dam stands 89 feet tall and stretches 560 feet across, creates a reservoir covering 83 acres, has a maximum storage capacity of 2,780 acre-feet, and collects water from a drainage area of 214 square miles. Completed in 1966, 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 89 feet, this dam is taller than 97% of all dams in the United States.
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 | Placer |
| Nearest City | Auburn |
| River/Stream | Rubicon River |
| Coordinates | 39.00430, -120.74640 |
Physical Characteristics
| Dam Type | Concrete |
| NID Height | 89 ft |
| Dam Height | 89 ft |
| Dam Length | 560 ft |
| Year Completed | 1966 (60 years old) |
Storage & Hydraulics
| NID Storage | 2,780 acre-ft |
| Max Storage | 2,780 acre-ft |
| Normal Storage | 2,700 acre-ft |
| Surface Area | 83 acres |
| Drainage Area | 214 sq mi |
| Max Discharge | 159,000 cfs |
| Spillway Type | Controlled |
Ownership & Safety
| Purpose | Irrigation |
| Owner | Placer County Water Agency |
| Owner Type | Local Government |
| Hazard Classification | High |
| Condition Assessment | Fair |
| Last Inspection | 08/23/2023 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA00859 |
| Federal ID | CA00859 |
| More Info | State Dam Safety Program |