El Capitan Reservoir is a reservoir in central San Diego County, California about 30 miles (50 km) northeast of the city of San Diego and two miles north of the town of Alpine. The reservoir is formed by El Capitan Dam on the San Diego River and has a capacity of 112,800 acre feet (139,100,000 m3). The 237-foot (72 m) dam is composed of hydraulic fill and was completed in 1934. The dam is owned by the city of San Diego and its primary purpose is to supply drinking water. It is the largest of the city's reservoirs.
The amount of runoff that enters the reservoir varies considerably. During a 25-year period, it ranged from 1,000 to 70,000 acre feet (86,000,000 m3) per year (39 to 2,700 L/s). The water in the reservoir usually consists of runoff from above the dam, but in years of drought, water is sometimes transferred to it from San Vicente Reservoir, which is the terminus of the First San Diego Aqueduct.