Johnston is a gravity dam located in San Mateo County, California, built in 1919. It carries a Low hazard potential classification and has a satisfactory condition assessment.
Dam profile
NID height · 31 ft31 ft tall and 132 ft long, taller than 72% of U.S. dams with a recorded height. Height is drawn to scale against a two-story house, about 25 ft; the profile is a generic concrete gravity section and the water level is illustrative.
About Johnston
The primary purpose of Johnston 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 Peninsula Open Space Trust (private). It impounds the Arroyo Leon.
The dam stands 31 feet tall and stretches 132 feet across, creates a reservoir covering 5 acres, has a maximum storage capacity of 30 acre-feet, and collects water from a drainage area of 7 square miles. Completed in 1919, the structure is over a century old.
This dam has a low hazard potential classification, meaning that failure would cause minimal damage, limited primarily to the dam owner's property, with no expected loss of life. Its condition is rated satisfactory, indicating no existing or potential dam safety deficiencies are recognized. The most recent inspection on record was 01/06/2026.
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Location
| State | California |
| County | San Mateo |
| River/Stream | Arroyo Leon |
| Coordinates | 37.45410, -122.42390 |
Physical Characteristics
| Dam Type | Gravity |
| NID Height | 31 ft |
| Dam Height | 31 ft |
| Dam Length | 132 ft |
| Year Completed | 1919 (107 years old) |
Storage & Hydraulics
| NID Storage | 30 acre-ft |
| Max Storage | 30 acre-ft |
| Surface Area | 5 acres |
| Drainage Area | 7 sq mi |
Ownership & Safety
| Purpose | Water Supply |
| Owner | Peninsula Open Space Trust |
| Owner Type | Private |
| Hazard Classification | Low |
| Condition Assessment | Satisfactory |
| Last Inspection | 01/06/2026 |
| Emergency Action Plan | No |
Identifiers
| NID ID | CA00667 |
| Federal ID | CA00667 |
| More Info | State Dam Safety Program |