Castaic Dam is an embankment dam in northwestern Los Angeles County, California, in the unincorporated area of Castaic. Although located on Castaic Creek, a major tributary of the Santa Clara River, 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 and stores drinking water for the western portion of the Greater Los Angeles Area. Dimensions Castaic is an earth-fill dam with its surfaces covered with boulders and cobble-sized rocks to prevent erosion. The dam is high above the streambed, above the foundations, and long, containing of material. The maximum thickness of the base is . Flood waters are released through an ungated, concrete overflow spillway on the west side of the dam, emptying into a stilling basin called Castaic Lagoon (colloquially referred to as the "Lower Lake", while the larger Castaic Lake is called "Upper Lake"). The total storage capacity of Castaic Lake is , of which is considered active capacity and are considered inactive. The inactive capacity is only used during periods of extended drought or interrupted water delivery, most recently in 2014. At maximum water elevation of AMSL, the lake covers , with of shoreline. The much smaller Castaic Lagoon can hold and covers . Operations Water supply Castaic Lake is the lower and larger of two main storage reservoirs for the West Branch of the California Aqueduct. Water drawn from the Sacramento–San Joaquin River Delta is transported down the San Joaquin Valley via the California Aqueduct and pumped over the Tehachapi Mountains, where it splits into the East Branch – providing water for Riverside and San Bernardino and eastern Los Angeles Counties – and the West Branch, which supplies western Los Angeles and parts of Ventura County. The West Branch first enters Pyramid Lake, formed by Pyramid Dam, before traveling through the Angeles Tunnel to the upper end of Castaic Lake. Together, the two reservoirs can store , or about a year's supply of water. During normal operations, Castaic Lake serves as a regulatory reservoir for water delivered through the California Aqueduct, releasing it at times of peak demand. However, the dam and lake was also built to provide a pool of "emergency storage" that can be