Located about 10 miles south between Lake Pyramid and Castaic Lake, the Castaic Powerplant is a pumping-generating plant on the West Branch owned and operated by the Los Angeles Department of Water and Power and jointly developed with the California Department of Water Resources. Unlike other facilities along the West Branch, it recovers power from the lift over the Tehachapis and has pumped-storage capability, generating electricity during peak hours and pumping water from Elderberry Forebay back to Lake Pyramid during off-peak hours for later use. Construction began in 1969 and was completed in 1973. Elderberry Forebay and Dam, at the upper end of Castaic Lake, provides regulatory storage for pumpback during off-peak hours, permits submergence for the pump-generator when the lake is at its lowest operating levels, and reduces daily and weekly fluctuations in Castaic Lake; it was constructed between 1965 and 1974 and is operated by the Los Angeles Department of Water and Power. Between Pyramid Lake and the powerplant, the Angeles Tunnel, built with the cooperation of the Los Angeles Department of Water and Power, is 30 feet in diameter and, with the penstocks, drops water 1,060 feet to the plant. At the South Portal of the Angeles Tunnel, the Surge Chamber acts as a shock absorber during emergency shutdowns by relieving excess pressure in the tunnel and penstocks.