Castaic Power Plant, also known as the Castaic Pumped-Storage Plant, is a seven unit pumped-storage hydroelectric plant, operated by the Los Angeles Department of Water and Power, which provides peak load power from the falling water on the West Branch of the California State Aqueduct. It is a cooperative venture between the LADWP and the Department of Water Resources of the State of California. An agreement between the two organizations was signed on September 2, 1966, for construction of the project.
California State Aqueduct water en route to Southern California is pumped up through the Tehachapi Mountains (by the Edmonston Pumping Plant) to the Tehachapi Afterbay. There the aqueduct divides into East and West Branches. The West Branch water is again pumped at the Oso Pumping Plant to Quail Lake, near Highway 138 East of Gorman. From here it flows through a permanent pipeline waterway to Warne Power Plant, which is located at the inlet to Pyramid Lake. The Warne Plant was constructed by the state of California in 1983. Pyramid Lake has a surface area of 1,380 acres (560 ha) and a storage capacity of 180,000 acre-feet (220,000,000 m3) with a maximum water surface elevation of 2,578 feet (786 m) above sea level. Pyramid Lake is the upper forebay for the Castaic Power Plant.
The State of California had planned to build a tunnel17 feet (5.2 m) in diameter. Under the cooperative development, this tunnel was enlarged to 30 feet (9.1 m) in diameter. The 7.2-mile (11.6 km) long tunnel, including the penstocks, drops water 1,060 feet (320 m) between Pyramid Lake and the hydroelectric power facilities, and carries over five times the flow previously contemplated for the 17-foot (5.2 m) tunnel.