Valley Water prepared to continue ensuring a reliable water supply to Santa Clara County during project to strengthen Anderson Dam

December 14, 2020

Valley Water began lowering water levels in Anderson Reservoir on Oct. 1 as part of our effort to strengthen the existing dam so it can safely withstand a large earthquake. This project will help keep the public safe and eventually allow Valley Water to fill Anderson Reservoir in Morgan Hill and store more water.

While Valley Water does this critical work, we will not be able to use Anderson Reservoir to store water until the project is completed. But Valley Water has a plan to make sure we continue to provide safe, clean water to Santa Clara County.

About 55% of Santa Clara County’s water supply comes from hundreds of miles away. Snow and rain in the Sierra Nevada range of northern and eastern California eventually runs into rivers that flow into the Sacramento-San Joaquin River Delta. Often called “imported water,” this water is brought into Santa Clara County through the complex infrastructure of the State Water Project, the federal Central Valley Project and San Francisco’s Hetch Hetchy system.

Valley Water will rely more on these imported water supplies during the next decade.

In Santa Clara County, the largest water storage reservoir rests beneath our feet. Our local underground basins can hold more water than all 10 of Valley Water’s surface water reservoirs combined. Currently our groundwater basins have healthy storage levels.

Valley Water also has an emergency supply of water stored in an underground aquifer in Kern County. Currently, our portion of this Semitropic Groundwater Storage Bank is about 339,000 acre-feet of water. On average, one acre-foot of water supplies two households in Santa Clara County for one year.

Of course, our region could experience a drought or another unforeseen event during the next decade that has an adverse effect on our water supply. Valley Water has a plan in place to respond to such a shortage, which could include asking residents and businesses to conserve. Valley Water already has several programs to help residents and businesses save water.

So why is Anderson Reservoir being taken out of service?

On Feb. 20, 2020, the Federal Energy Regulatory Commission ordered Valley Water to lower water levels in Anderson Reservoir to reduce the risk to the public should Anderson Dam fail during a major earthquake. Valley Water has made sure to use most of the water in Anderson Reservoir by sending it to our water treatment plants for delivery to our water retailers and releasing it into Coyote Creek and percolation ponds to replenish our groundwater supplies.

We estimate the work at Anderson Dam will take about 10 years. Valley Water will build a new outlet tunnel and rebuild the dam and spillway to modern-day safety levels.

While we work to build a safer Anderson Dam, Valley Water continues to carefully manage the water supply for Santa Clara County.

Please visit valleywater.org/anderson-dam-project for more information and to sign up to receive project updates. To read some commonly asked questions and answers, click here.

Valley Water manages an integrated water resources system that includes the supply of clean, safe water, flood protection and stewardship of streams on behalf of Santa Clara County's 2 million residents. Valley Water effectively manages 10 dams and surface water reservoirs, three water treatment plants, an advanced recycled water purification center, a state-of-the-art water quality laboratory, nearly 280 acres of groundwater recharge ponds and 350 miles of waterways. We provide wholesale water and groundwater management services to local municipalities and private water retailers who deliver drinking water directly to homes and businesses in Santa Clara County.