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.