Despite a dry winter that has California nearing drought-like conditions, federal regulators have ordered that a large reservoir in the Bay Area be drained due to concerns that its dam could collapse in a strong earthquake and release a flood that would inundate much of Silicon Valley.
The Federal Energy Regulatory Commission ordered that the Anderson Reservoir be completely drained by October 1st over fears that the 240-foot high earthen dam could fail if a major earthquake strikes the Calaveras Fault, which is located next to the dam.
“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.
Anderson Reservoir, also known as Anderson Lake, was built in 1950 and has the capacity to hold close to 90,000-acre feet of water - making it the largest reservoir in Santa Clara County. It also accounts for more than half of the water district's total capacity of 170,000 acre-feet in all 10 of its reservoirs scattered across the county.
Local officials have kept the dam at 74% of capacity since 2009 after a study showed that an earthquake of 6.6 or greater could trigger a failure in the dam. The water district has been working to retrofit the Anderson Dam since 2017, because California classified the dam as being an "extremely high" downstream risk.
"Studies have shown a large earthquake could damage Anderson Dam, causing an uncontrolled release of water that could inundate cities and rural areas from San Francisco Bay south to Monterey Bay, including much of Silicon Valley," the water district said on its website about the seismic retrofit.
Valley Water produced a startling video of the flooding that could occur if the reservoir was filled and the dam failed in the event of a major earthquake. Water can be seen traveling throughout Silicon Valley all the way through downtown San Jose to the San Francisco Bay.
Because California has been experiencing a drier winter than on average, the Anderson Dam is only filled to about 29% of its capacity.
Valley Water says that while they agree with federal authorities that the reservoir should be drained ahead of the retrofit project, the demand to empty Anderson Reservoir could "result in unsafe consequences."
"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 CEO Norma Camacho said.
"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. The inability to keep a consistent flow in Coyote Creek downstream of the dam year-round would significantly impact sensitive native fish, amphibians, reptiles, wetlands, and riparian habitats. Water quality could also be significantly impacted downstream of the dam," she added.
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