San Gabriel Dam is a rockfill dam on the San Gabriel River in the San Gabriel Mountains, in Los Angeles County, California, within the Angeles National Forest. The dam is situated on the main stem of the San Gabriel about 2.5 mi (4.0 km) downstream from the confluence of the river's East and West Forks, which drain a fair portion of the San Gabriel Mountains of the Transverse Ranges.
San Gabriel Reservoir is the name of the lake impounded by the dam. It stretches nearly 3 miles (4.8 km) upstream, extending up a bit of each fork, at full pool. The reservoir stores 44,183 acre·ft (54,499,000 m3) of water when full, creating one of Southern California's larger reservoirs. Its capacity was 53,344 acre·ft (65,799,000 m3) when the dam was first built, but sedimentation has since reduced its volume by about 17%. The reservoir is long and narrow, stretching in a southwesterly direction along a steep gorge.