Castaic Dam is a dam near the city of Castaic, California. It is an earth-fill dam, though its surfaces are covered with boulders and cobble-sized rocks to prevent erosion. Although located on Castaic Creek and forming Castaic Lake, Castaic Creek provides little of its water. The lake is the terminus of the West Branch of the California Aqueduct, part of the State Water Project. The dam was built by the California Department of Water Resources and construction was completed in 1973. The lake has a capacity of 325,000 acre feet (401,000,000 m3) and stores drinking water for the western portion of the Greater Los Angeles Area.
The 11 MW Foothill Feeder hydroelectric power plant is located at the base of the dam and generates electricity when water is needed in Los Angeles. The 1,495 MW (Nominal) Castaic Power Plant is located at the upper end of the West arm of Castaic Lake. Castaic Lake serves as the lower reservoir, while Pyramid Lake serves as the upper. When demand for electricity is high, usually during the afternoon, water is withdrawn from Pyramid Lake and released into Castaic Lake. At night, when demand is low, water is pumped back into Pyramid Lake. The sale of peak electricity reduces the Department of Water Resources' overall electric costs for operating the California Aqueduct.