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. 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 drawn down if water deliveries from northern California are interrupted, whether due to construction, equipment malfunction or severe drought. Below the dam, the majority of the water flows to Los Angeles via a system known as the Foothill Feeder, operated by the Metropolitan Water District of Southern California. The water travels about south via a diameter pipeline to the Jensen Filtration Plant near San Fernando, where it connects to the municipal water system. The mostly underground, pre-stressed concrete pipe has walls nearly thick. Water from the Foothill Feeder is stored in the smaller Los Angeles Reservoir in the San Fernando Valley. The water continues south via the Sepulveda Feeder, which provides water to Los Angeles proper and other municipalities in south Los Angeles and Orange Counties. The main customer water agencies are the Central Basin Municipal Water District, West Basin Municipal District, and Municipal Water District of Orange County. As many as 12 million people in these areas receive their full or supplemental water supply from Castaic Lake and the feeder system. A smaller portion of the water supply is distributed by the Castaic Lake Water Agency. The service area covers about in Ventura and north Los Angeles counties, providing water to about 287,000 people. The main constituents of the agency include the Los Angeles County Waterworks District No. 36, Newhall County Water District, Santa Clara Water Division, and Valencia Water Company.
Power generation
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. In 2009, the Foothill Feeder plant generated 49 million kilowatt hours. The 1,495 MW Castaic Pumped-Storage Plant is located at the upper end of the west arm of Castaic Lake. The Elderberry Forebay Dam separates the upper arm from the rest of Castaic Lake, maintaining a small pool for power generation known as the Elderberry Forebay, serving as the lower reservoir of the pumped-storage operation. Pyramid Lake, located to the west, 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. In 2009, the Castaic pumped-storage plant generated a net 465 million KWh.
Safety
Due to seismic vulnerabilities, the dam has one of the lowest safety ratings in Los Angeles County.
