Mammoth Reservoir
Mammoth Reservoir is an earth dam located in Placer County, California, built in 1851. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Mammoth Reservoir
Mammoth Reservoir is a dam in Placer, California. The dam is owned by Placer County Water Agency (local government). It impounds the Offstream near Granite Bay.
The dam stands 23 feet tall and stretches 2,360 feet across, creates a reservoir covering 11 acres, has a maximum storage capacity of 103 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 1851, the structure is one of the older dams in the region, with over a century and a half of service.
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 satisfactory, indicating no existing or potential dam safety deficiencies are recognized. An emergency action plan is in place for this dam. The most recent inspection on record was 09/24/2025.
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 | Granite Bay |
| River/Stream | Offstream |
| Coordinates | 38.81860, -121.13900 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 23 ft |
| Dam Height | 23 ft |
| Dam Length | 2,360 ft |
| Year Completed | 1851 (175 years old) |
Storage & Hydraulics
| NID Storage | 103 acre-ft |
| Max Storage | 103 acre-ft |
| Surface Area | 11 acres |
| Drainage Area | 0 sq mi |
Ownership & Safety
| Purpose | Other |
| Owner | Placer County Water Agency |
| Owner Type | Local Government |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 09/24/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA00861 |
| Federal ID | CA00861 |
| More Info | State Dam Safety Program |