In 1960, California voters approved financing for construction of the initial features of the State Water Project (SWP). The project includes some 22 dams and reservoirs, a Delta pumping plant, a 444-mile-long aqueduct that carries water from the Delta through the San Joaquin Valley to southern California. The project begins at Oroville Dam on the Feather River and ends at Lake Perris near Riverside. At the Tehachapi Mountains, giant pumps lift the water from the California Aqueduct some 2,000 feet over the mountains and into southern California.
The 1,440-mile-long Colorado River passes through parts of seven states, several Indian reservations and the Republic of Mexico. California is entitled to 4.4 million acre-feet of water annually from river. Most of that water irrigates crops in the Palo Verde, Imperial and Coachella valleys, located in the southeastern corner of the state, but the Colorado also is a vital source of water for urban southern California. Urban supplies are distributed by the Metropolitan Water District of Southern California through its Colorado River Aqueduct.
A number of large population centers in California have developed their own extensive water projects. The Hetch Hetchy Project transports Tuolumne River water 156 miles from the Central Sierra to San Francisco and peninsula cities.
The East Bay Municipal Utility District supplies cities on the east side of San Francisco Bay with Mokelumne River water.
About 30 percent of California’s total annual water supply comes from groundwater in normal years, and up to 60 percent in drought years. Local communities’ usage may be different; many areas rely exclusively on groundwater while others use only surface water supplies. Contrary to popular opinion, groundwater does not exist in underground lakes. Groundwater fills pores (spaces) between sand, gravel, silt and clay in water-bearing formations known as aquifers.