Henry J. Mills Reservoir
Henry J. Mills Reservoir is an earth dam located in Riverside County, California, built in 1979. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Henry J. Mills Reservoir
The primary purpose of Henry J. Mills Reservoir 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 Metropolitan Water District Of Southern California (public utility). It impounds the Offstream near Mission Grove.
The dam stands 23 feet tall and stretches 2,038 feet across, creates a reservoir covering 6 acres, and has a maximum storage capacity of 83 acre-feet. Completed in 1979, the structure is 47 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 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 12/09/2025.
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 | Riverside |
| Nearest City | Mission Grove |
| River/Stream | Offstream |
| Coordinates | 33.92080, -117.32160 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 23 ft |
| Dam Height | 23 ft |
| Dam Length | 2,038 ft |
| Year Completed | 1979 (47 years old) |
Storage & Hydraulics
| NID Storage | 83 acre-ft |
| Max Storage | 83 acre-ft |
| Surface Area | 6 acres |
Ownership & Safety
| Purpose | Water Supply |
| Owner | Metropolitan Water District Of Southern California |
| Owner Type | Public Utility |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 12/09/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA01085 |
| Federal ID | CA01085 |
| More Info | State Dam Safety Program |