California’s largest Anderson Dam in Santa Clara County is set to begin reconstruction in early 2027.
Located on the Coyote Creek Fault on the eastern side of Morgan Hill, parallel to the Calaveras Fault, the dam has been deemed to have unstable risks due to earthquakes and soil liquefaction since an independent review in 2008, prompting plans for safety reinforcement projects.
According to a press release from the Santa Clara Valley Water District on July 28th, officials have recently approved a construction contract worth $1.88 billion to strengthen the 235-foot-high dam to withstand major earthquakes. Upon completion, the reservoir is expected to reach full capacity by the end of 2033, as its water level has only been at 3% of its total capacity since mid-December 2020.
The water district stated: “This project will enhance water supply reliability, support groundwater recharge, reduce flood risks, and restore recreational activities around Anderson Reservoir and the Coyote Creek watershed.” Built in 1950, the reservoir is the largest among the district’s 10 reservoirs with a full capacity of approximately 900,000 acre-feet, surpassing the total capacity of the other 9 reservoirs.
A white paper released by the organization in March of this year revealed plans to increase the dam’s height by 9 feet to store more water during storms; a new spillway raised by 7 feet will replace the existing concrete spillway to enhance flood discharge capacity during heavy rainfall. The seismic reinforcement project for Anderson Dam began in 2012, with a total project cost of around $2.8 billion. The white paper noted that subsequent research had identified previously unidentified fault zones, resulting in the delayed completion timeline.
The press release indicated that the construction team will commence the reconstruction of the majority of the dam structure and spillway in January 2027, installing new outlet pipes, and completing other significant safety upgrades.
The water district reported that most of the preliminary work has been completed, including improving the flow management of the Coyote Creek and preparing for the dam reconstruction. The Anderson Dam tunnel project is expected to be completed later this year, with a new 13-foot diameter outlet pipe replacing the original 4-foot diameter outlet pipe to allow more water flow into the river channel during construction, thus ensuring resilience to future heavy rains.