Water Storage & Supply

On average, California receives about 200 million acre-feet of water per year in the form of rain and snow. However, we rarely experience an average year. California has the most variable weather conditions in the nation, often fluctuating between extreme drought and extreme flood. Climate change may intensify that variability. 

The Department of Water Resources built the State Water Project (SWP) to increase statewide water supply reliability. Constructed in the 1960s and 1970s, the SWP is a network of dams, canals, and pumping plants that store and deliver water to people, farms, and industry.

We operate and maintain this complex water storage and supply system, transporting water more than 700 miles from California’s northern Sierra Nevada mountains south to Los Angeles and beyond. 


The SWP delivers water to 29 water contractors in the state. These water contractors, in turn, sell water to their customers. The SWP supplies water to almost 27 million Californians and about a million acres of farmland.

Key features of this engineered water supply system include:

Dams – Dam construction is the centerpiece of a water supply system that defies the seasonal and annual variability of our climate. By engineering structures that strategically dam the flow of our rivers, we create reservoirs of water that can be tapped throughout the dry summer months and during drought. Because of the important function that dams serve in our modern water supply system, they are thoroughly monitored and carefully maintained. DWR manages 21 dams across the state. DWR’s Division of Safety of Dams monitors the safety of 1,250 dams across the state and is recognized as a national leader in dam safety.

California Aqueduct – We operate and maintain the California Aqueduct, which snakes through the Central Valley from the Sacramento-San Joaquin Delta down to Los Angeles and Riverside counties. The aqueduct relies on gravity to move water, and is periodically elevated by pumping plants. We operate 24 pumping plants, including the Edmonston Pumping Plant, which pumps water 1,926 feet over the Tehachapi Mountains.

The Sacramento-San Joaquin Delta – Water from Lake Oroville flows down the Feather and Sacramento rivers to the Sacramento-San Joaquin Delta, the hub of the state’s water delivery system. At the southern end of the Delta, giant pumps of the SWP and federal Central Valley Project feed aqueducts carrying water to the rest of the state. With our partners, we monitor the health of the complex Delta ecosystem to manage water resources adaptively.



DWR manages above-ground reservoirs and surface water. We also provide technical assistance for the management of underground reservoirs, also known as groundwater aquifers, through the execution of the 2014 Sustainable Groundwater Management Act

Above ground storage – We manage 17 reservoirs:

  • Antelope Lake
  • San Luis Reservoir  (shared with federal, Central Valley Project)
  • Bethany Reservoir
  • Castaic Lake
  • Silverwood Lake
  • Lake del Valle
  • Elderberry Forebay
  • Frenchman Lake
  • Lake Davis
  • O’Neill Forebay (shared with Central Valley Project)
  • Lake Oroville
  • Lake Perris
  • Pyramid Lake
  • Quail Lake
  • Tehachapi Afterbay
  • Thermalito Afterbay
  • Thermalito Forebay

Lake Oroville, built in 1968, is the largest reservoir in the State Water Project. It receives water from the Feather River Watershed, which is released as needed to feed the Sacramento River and meet municipal, agricultural, and environmental needs downstream. Our reservoirs serve multiple benefits: flood protection, recreation, and habitat for fish and wildlife.  

Underground storage
 – Underground reservoirs, or groundwater aquifers, are critical to the management of California’s precious water resources, especially during drought. 

Unlike surface water, underground reservoirs have not historically been regulated. In 2014, Governor Edmund G. Brown Jr. signed into law the 
Sustainable Groundwater Management Act to bring depleted aquifers into balanced levels of pumping and recharge. DWR was tasked with providing assistance to local communities to achieve that goal. 

Groundwater recharge is a means of replenishing underground reservoirs either directly through injection, or by allowing water to percolate into the ground, spreading basins, or some stormwater capture. 

Surface storage can be operated with groundwater storage to increase opportunities for groundwater recharge during high flow periods.

Looking Ahead

DWR continues to advance new water storage and supply projects. Identifying a climate-resilient water conveyance solution through the Delta that protects and enhances the environment while increasing the reliability of water deliveries to farms and homes is a top priority.