Upper Oso is an earth dam located in Orange County, California, built in 1979. It carries a High hazard potential classification and has a satisfactory condition assessment.
Dam profile
NID height · 142 ft142 ft tall and 800 ft long, taller than 99% of U.S. dams with a recorded height. Height is drawn to scale against a ten-story building, about 100 ft; the profile is a generic embankment section and the water level is illustrative.
About Upper Oso
The primary purpose of Upper Oso 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 Santa Margarita Water District (public utility). It impounds the Oso Creek near Mission Viejo.
The dam stands 142 feet tall and stretches 800 feet across, creates a reservoir covering 115 acres, has a maximum storage capacity of 3,700 acre-feet, and collects water from a drainage area of 1 square miles. 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 04/03/2025.
At 142 feet, this dam is taller than 99% 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 | Orange |
| Nearest City | Mission Viejo |
| River/Stream | Oso Creek |
| Coordinates | 33.65970, -117.63200 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 142 ft |
| Dam Height | 142 ft |
| Dam Length | 800 ft |
| Year Completed | 1979 (47 years old) |
Storage & Hydraulics
| NID Storage | 3,700 acre-ft |
| Max Storage | 3,700 acre-ft |
| Surface Area | 115 acres |
| Drainage Area | 1 sq mi |
Ownership & Safety
| Purpose | Water Supply |
| Owner | Santa Margarita Water District |
| Owner Type | Public Utility |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 04/03/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA01145 |
| Federal ID | CA01145 |
| More Info | State Dam Safety Program |