Role of Snowpack, Rainfall, and Reservoirs in Buffering California Against Drought Effects [open pdf - 1MB]
"California's vast reservoir system, fed by annual snow-and rainfall, plays an important part in providing water to the State's human and wildlife population. There are almost 1,300 reservoirs throughout the State, but only approximately 200 of them are considered storage reservoirs, and many of the larger ones are critical components of the Federal Central Valley Project and California State Water Project. Storage reservoirs, such as the ones shown in figure 1, capture winter precipitation for use in California's dry summer months. In addition to engineered reservoir storage, California also depends on water 'stored' in the statewide snowpack, which slowly melts during the course of the summer, to augment the State's water supply."
U.S. Geological Survey: https://www.usgs.gov/