California's Water: Storing Water

If you live in California, chances are your community enjoys some of the many benefits provided by surface water reservoirs in the state. Since rain falls only during winter months in California and annual precipitation can vary wildly from year to year, surface water reservoirs are critical to providing reliable water supplies to communities, farms and the environment year-round.

This segment of “California’s Water” focuses on surface water reservoirs and the multiple roles these man-made structures play in providing flood control, hydroelectricity, water supply, water quality improvements, and other benefits. The segment takes viewers to Shasta Dam, the state’s largest reservoir, and looks at newer storage projects that are helping to improve water quality and meet environmental needs. Background on the Issue

California has more than 1,000 surface storage reservoirs. Ranging from the very small to the very large, these structures play a key role in meeting the state’s water needs. In addition to water supply, surface storage reservoirs can provide flood control, hydropower, water quality improvements, water system flexibility benefits, as well as emergency water supplies in case of drought or catastrophic events.

The 200 biggest reservoirs in the state have a combined capacity of more than 41 million acre-feet. That’s enough water for more than 80 million families for a year.

California has many kinds of surface storage reservoirs. On-stream reservoirs feature dams that were constructed across active rivers or streams. Off-stream reservoirs, on the other hand, are located away from streams, and are filled by diverting water into them from a nearby river or water source.

The state and federal governments built many of California’s most important reservoirs 40 to 60 years ago as part of two statewide water systems – the State Water Project and the federal Central Valley Project.

In recent decades, however, local and regional agencies have developed and built surface storage reservoirs that provide a number of benefits while minimizing impacts on the environment. These reservoirs have improved local and regional water supply reliability, improved water quality and provided a key source of emergency water if needed in times of drought or other catastrophe.

One example is Los Vaqueros Reservoir in Contra Costa County. Built by Contra Costa Water District and completed in December 1997, the off-stream reservoir holds up to 100,000 acre-feet of high quality water and provides a one-to-three-month emergency supply for the district’s 500,000 customers.

The reservoir’s main purpose is to improve the quality of water provided to Contra Costa’s customers by allowing good quality water that is low in salinity to be captured and stored during the spring and summer months. The water is then delivered to customers when salt levels in the Sacramento-San Joaquin River Delta rise.

Another example is Diamond Valley Reservoir in Southern California. The 800,000 acre-foot off-stream reservoir was built by Metropolitan Water District of Southern California to improve the region’s water supply reliability and reduce the threat of water shortages during droughts and periods of peak summer use.

The reservoir, completed in December 1999, nearly doubled Southern California’s total surface water storage capacity, and helped secure six months’ emergency storage for the region. It is capable of receiving and storing water from both the Colorado River Aqueduct and the State Water Project.

Future Projects

The latest California Water Plan Update points to a need for additional surface storage to meet future needs and improve the flexibility of the state’s water system. State and federal agencies are studying the feasibility of several potential storage projects. A decision on which, if any, will be constructed is expected some time in 2007. Collectively, these projects could add as much as 4.5 million acre-feet to the state’s total storage capacity.

The projects under study include: proposed enlargement of Shasta Reservoir; proposed construction of Sites Reservoir near Maxwell in the Sacramento Valley; proposed expansion of Los Vaqueros Reservoir; and proposed development of storage in the upper portion of the San Joaquin River basin.