Pacoima Dam, Concrete arch dam in the San Gabriel Mountains, Los Angeles County, USA

Pacoima Dam is a 371-foot (113 m) concrete arch dam on Pacoima Creek in the San Gabriel Mountains, about 1 mile northeast of Sylmar above the San Fernando Valley. Its arch curves between the canyon walls, and the reservoir it creates, Pacoima Reservoir, holds 3,777 acre-feet (4,659,000 m³).

Built by the Los Angeles County Flood Control District, later part of the Department of Public Works, Pacoima Dam was completed in 1928. At 371 feet (113 m) high, it was then the tallest arch dam in the United States. It serves as a flood control structure, restraining stormwater released from the reservoir to compensate for hydraulic deficiencies downstream.

The dam's name comes from the Pacoima Creek it blocks; "Pacoima" is a Tongva place name. Roy W. Carlson, hired by Los Angeles County when construction began, developed the world's first strain meter that could be embedded in concrete here, along with an adiabatic calorimeter and electrical-resistance thermometers to study curing heat and cracking.

The dam sits about 1 mile northeast of Sylmar, above the San Fernando Valley, reached by mountain roads in the San Gabriel range. Access to the structure can be limited by security measures, so check current conditions before setting out. The County of Los Angeles releases stormwater flows from the dam to compensate for downstream hydraulic deficiencies, so the creek below can rise during storms.

Roy W. Carlson, hired by Los Angeles County as concrete and soil testing engineer when construction began, developed the world's first strain meter that could be embedded in concrete for this dam. He also built an adiabatic calorimeter and electrical-resistance thermometers to learn why curing concrete heated up and how to limit cracking.