For decades, it was a source of drinking water for the San Marcos area. Today, the South Lake Reservoir has new life as a local wildlife habitat.

South Lake was built with an earthen dam and provided drinking water to the Lake San Marcos area, most of downtown San Marcos, and the Coronado Hills area. Updates to potable water treatment had detrimental effects on the lake ecology, so the Vallecitos Water District stopped using the lake as a drinking water source in 1984, although it remained an emergency water supply for another decade.