San Marcos
San Marcos is an earth dam located in San Luis Obispo County, California, built in 1964. It carries a High hazard potential classification and has a satisfactory condition assessment.
About San Marcos
San Marcos is a dam in San Luis Obispo, California. The dam is owned by Harry E. Blythe Jr. (private). It impounds the San Marcos Creek.
The dam stands 42 feet tall and stretches 502 feet across, creates a reservoir covering 29 acres, has a maximum storage capacity of 325 acre-feet, and collects water from a drainage area of 7 square miles. Completed in 1964, 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 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 05/21/2025.
At 42 feet, this dam is taller than 89% 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 | San Luis Obispo |
| Nearest City | None |
| River/Stream | San Marcos Creek |
| Coordinates | 35.67400, -120.81300 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 42 ft |
| Dam Height | 42 ft |
| Dam Length | 502 ft |
| Year Completed | 1964 (62 years old) |
Storage & Hydraulics
| NID Storage | 325 acre-ft |
| Max Storage | 325 acre-ft |
| Surface Area | 29 acres |
| Drainage Area | 7 sq mi |
Ownership & Safety
| Purpose | Other |
| Owner | Harry E. Blythe Jr. |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 05/21/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA00726 |
| Federal ID | CA00726 |
| More Info | State Dam Safety Program |