San Dimas Dam, Gravity dam in Los Angeles County, United States.

San Dimas Dam is a concrete gravity dam 130.9 feet high and 339.8 feet long in the San Gabriel Mountains within Angeles National Forest. Its reservoir, holding up to 1,515 acre-feet, stays dry most of the year and fills only after major winter storms. The dam regulates San Dimas Creek, a tributary of the San Gabriel River, and diverts floodwater to Puddingstone Reservoir.

The structure was built in 1922 when the county flood control agency constructed it to manage overflows from San Dimas Creek. This project was part of the region's broader effort to address seasonal water challenges.

The dam is named after San Dimas Creek, whose floodwaters it regulates, and it is one of 14 dams launched by the Los Angeles County Flood Control District after the flood of 1914. Its water is diverted along San Dimas Wash to Puddingstone Reservoir, protecting the San Dimas area of the San Gabriel Valley.

The dam sits within Angeles National Forest in the San Gabriel Mountains, reached via San Dimas Creek drainage; there are no visitor facilities or marked viewpoints, and the structure is a flood-control facility rather than a public attraction. The reservoir is dry most of the year and holds water only after significant winter storms, so the basin is usually an empty channel. Check current county access rules before approaching.

San Dimas Reservoir stays dry most of the year and holds water only after major winter storms. The dam's regulation instead sends floodwater from San Dimas Wash into Puddingstone Reservoir, protecting the San Dimas area of the San Gabriel Valley.