Despite California nearing drought, reservoir near San Jose ordered to be drained. Here's why.

Jorge L. Ortiz, USA TODAY

Amid the threat of another California drought, federal regulators have ordered that a large reservoir south of San Jose be drained because of concerns that its dam may collapse in an earthquake, leading to a massive release of water that could flood much of Silicon Valley.

Local authorities agree with the need to mitigate that risk, but not on the approach to doing so.

Because the 240-foot-high earthen Anderson Dam could fail if a major earthquake strikes on the Calaveras Fault that sits next to it, the process of emptying the reservoir should begin no later than Oct. 1, the Federal Energy Regulatory Commission said.

“It is unacceptable to maintain the reservoir at an elevation higher than necessary when it can be reduced, thereby decreasing the risk to public safety and the large population downstream of Anderson Dam,” David Capka, director of the FERC’s Division of Dam Safety and Inspections, wrote in a letter Thursday to the Santa Clara Valley Water District, which owns the reservoir.

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Anderson Reservoir, also known as Anderson Lake, was built in 1950 between San Jose and the community of Morgan Hill to the south. The lake can hold close to 90,000 acre-feet of water, accounting for more than half the district’s capacity of 170,000 acre-feet in all 10 of its reservoirs.

Since 2009, the dam’s water level has been kept at a maximum of 74% of capacity because of an assessment that it could be damaged in a magnitude 6.6 earthquake centered at the fault or a 7.2 quake with an epicenter as far as a mile away. On Monday, as the possibility of another drought loomed in California, Anderson Reservoir was just 29% full.

More than 60% of the state is experiencing abnormally dry conditions during what’s usually the rainy season, and there’s no precipitation in the forecast until March.

A buoy on the ground next to the dam at Anderson Reservoir on Monday reflects the low water level at a facility that's at 29% of capacity. Nevertheless, federal regulators have mandated the reservoir be drained because of high risk to the public in the event of a major earthquake.

In a statement posted Monday, water district CEO Norma Camacho said the reservoir is already operating at levels below the requirements set by the FERC and the state’s Division of Safety of Dams.

She also noted the adverse impact draining the reservoir could have on residents, wildlife and the dam’s infrastructure.

“The demand to empty Anderson Reservoir could result in unsafe consequences,’’ Camacho wrote. “A top concern is the potential to damage the intake structure, which would give us no way to control water flows out of the reservoir, potentially impacting downstream communities.

“Valley Water is also concerned about the environmental impacts of these new requirements. With the draining of the reservoir, experts would expect fish die-offs.’’

Camacho said the preferred solution would be completing the reservoir’s seismic retrofit project, which has been in the works for years but complicated by additional safety and design requirements. Construction is expected to begin in 2022.

If the dam collapsed, runoff could damage cities and rural areas from the San Francisco Bay through Monterey Bay to the south, the water district said. In 2017, more than 14,000 residents had to be evacuated as water from swollen Coyote Creek, downstream of Anderson Reservoir, flooded homes and temporarily shut down a portion of a freeway.

Contributing: The Associated Press

This article originally appeared on USA TODAY: Feds order draining of California reservoir despite ongoing drought