Public agencies and private water developers have built nearly 1,400 reservoirs in California to capture seasonal runoff, protect against floods and allocate water supplies throughout the year. These reservoirs hold about 42 million acre-feet of water when full.

Most of the state’s rainfall occurs from December through April, but the greatest demand for water is during the dry summer months. The reservoirs capture a large portion of runoff from the Sierra snow pack, which is the state’s largest reservoir of fresh water. In addition to water supply, many of the reservoirs also must provide downstream flood protection. Consequently, large volumes of runoff pass through the dams in order to reserve storage capacity for flood control. Once the flood season passes, the remaining runoff is stored in the reservoirs for later use.

Before 1900, water development in California was principally undertaken by individuals and private companies. As the population and economic activity of the state grew, cities, irrigation districts, public utilities and large municipal agencies took on the responsibility for developing water supplies for their jurisdictions. Over time, large distribution networks were developed to pipe treated surface and groundwater to homes, businesses, parks, schools and other facilities.