Anderson Reservoir Must Be Drained Over Quake Danger, Feds Say

MORGAN HILL, CA — The federal government has ordered Santa Clara County to completely drain Anderson Reservoir, the largest water source in the county, over fears that its dam could collapse during an earthquake, Patch has confirmed.

The order to begin emptying the reservoir by Oct. 1 could have a significant impact on Silicon Valley's environment and water supply.

The directive by the Federal Energy Regulatory Commission came in the form of a letter Thursday by David Capka, director of the FERC Division of Dam Safety and Inspections, to the Santa Clara Valley Water District, first reported Monday by the Mercury News.

"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," Capka wrote to Valley Water, which operates the dam. "Your actions to date do not demonstrate an appropriate sense of urgency regarding the interim conditions at the project."

In a statement, Valley Water CEO Norma Camacho acknowledged the need to better secure the dam, but said the government's order to completely empty the reservoir present its own safety risks.

"The demand to empty Anderson Reservoir could result in unsafe consequences," Camacho said. "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."

The order is also likely to affect the county's water supply. Anderson Reservoir is by far the county's largest, holding 89,278 acre feet of water — enough to fill Valley Water's nine other dams. Camacho said the district is already exploring other sources of water from outside the county, since the draining of Anderson will likely affect other groundwater basins in San Jose and South County.

A new water shortage could make matters worse, Camacho said.

"While residents have done an excellent job of conserving water since 2013, another drought during this timeframe could require everyone to significantly decrease their water use," she wrote.

Environmental impact

The risks surrounding the 70-year-old dam have been known for years. A 2009 seismic study found that the 240-foot earthen dam could collapse in a major earthquake, resulting in flooding across Silicon Valley. In 2011, the district began planning a five-year, $550 million project to rebuild the dam to reduce earthquake risks, with work set to begin in 2022.

In the statement, Camacho said the government's order is likely to harm the local environment, killing fish in the reservoir basin and harming amphibians, reptiles and wetland habitats by reducing the flow in Coyote Creek, downstream from the dam.

"Water quality could also be significantly impacted downstream of the dam," Camacho said.

As of Monday, Anderson Reservoir was only 29% full, but still held a crucial water supply — enough for at least 130,000 people annually plus an emergency supply, according to the Mercury News.

The district said it is sponsoring a bill introduced in the state legislature to expedite the regulatory process for the dam's rebuilding.