Officials Order Reservoir Be Drained Over Earthquake Risk

Anderson Reservoir must be drained by October
By Newser Editors and Wire Services
Posted Feb 26, 2020 1:36 PM CST
Officials: Quake Could Collapse Big Dam Near Silicon Valley
In this Feb. 21, 2017 file photo, water from the Lake Anderson Reservoir gushes down a spillway in Morgan Hill, Calif.   (AP Photo/Marcio Jose Sanchez, File)

Worried that an earthquake could collapse a big dam south of San Francisco, officials have ordered its reservoir to be completely drained. The 240-foot-high earthen Anderson Dam, built in 1950 between San Jose and the community of Morgan Hill, poses too great a risk of collapse and must be fully drained by Oct. 1, according to the Federal Energy Regulatory Commission, which regulates dams. Since 2009, the dam's water level has been kept at a maximum of 74% of capacity because of an assessment that it could fail in a 7.2 quake and send floodwaters into Silicon Valley, reports the AP. The Anderson Reservoir, which is built along the Calaveras Fault, was 29% full as of Monday. A seismic retrofit project has been underway because of its state classification in 2017 as being an "extremely high" downstream hazard.

The Santa Clara Valley Water District said in a statement Monday that officials agree with the commission's assessment that the reservoir should be drained ahead of the retrofit project, which is "complicated and time consuming." If the dam collapsed, runoff could damage cities and rural areas from the San Francisco Bay through Monterey Bay to the south, including much of Silicon Valley. Complicating the issue: California may be heading into a new drought, and stored water is an important part of the south San Francisco Bay Area's supply. "While residents have done an excellent job of conserving water since 2013, another drought during this time frame could require everyone to significantly decrease their water use," Valley Water CEO Norma Camacho said.

(More dam stories.)

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