Anderson Reservoir to be drained due to earthquake risk
SAN JOSE — In a dramatic decision that could significantly impact Silicon Valley’s water supply, federal dam regulators have ordered Anderson Reservoir, the largest reservoir in Santa Clara County, to be completely drained starting Oct. 1.
The 240-foot earthen dam, built in 1950 and located east of Highway 101 between Morgan Hill and San Jose, poses too great of a risk of collapse during a major earthquake, the Federal Energy Regulatory Commission, which regulates dams, has concluded.
“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,” wrote David Capka, director of FERC’s Division of Dam Safety and Inspections, in a letter to the Santa Clara Valley Water District on Thursday.
Anderson Reservoir is owned by the Santa Clara Valley Water District, a government agency based in San Jose. When full, it holds 89,278 acre feet of water — more than all other nine dams operated by the Santa Clara Valley Water District combined.
In a statement Monday, Norma Camacho, the water district’s CEO said the impacts of draining the largest reservoir in Santa Clara County will be significant.
“With these new requirements, we expect to see an impact to groundwater basins that are replenished with water released from Anderson Reservoir, including South County and southern San Jose,” Camacho said. “Staff is already exploring other sources of water that will have to come from outside of the county. 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.”
Camacho also said that draining the reservoir starting in seven months is likely to kill wildlife downstream in Coyote Creek, including endangered steel head trout, amphibians and reptiles. Coyote Creek flows from the dam through downtown San Jose to San Francisco Bay.
Complicating the issue, California may be heading into a new drought. On Monday, amid a dry winter, Anderson Reservoir was just 29% full. Nevertheless, the 26,133 acre feet of water stored there is an important part of the South Bay’s water supply — holding enough water for the annual needs of at least 130,000 people, and what the district considers an emergency supply.
The water district, a government agency based in San Jose, became aware of the dam’s problems a decade ago.
In December 2008, an engineering consultant found that a 6.6 magnitude quake centered on the Calaveras Fault directly at Anderson Reservoir, or a 7.2 quake centered one mile away, could cause the reservoir’s huge dam to fail.
Although unlikely, if that occurred when the reservoir was full, such as during a wet winter, it could send a wall of water 35 feet high into downtown Morgan Hill within 14 minutes, and 8 feet deep into San Jose within three hours, potentially killing thousands of people.
The largest earthquake recorded on the Calaveras Fault was a 6.5 in 1911. But the U.S. Geological Survey has estimated the Calaveras Fault can produce a quake of up to 7.2.
During test borings 2008, consultants found that the dam’s foundation was not built on solid bedrock in 1950s. Rather, there is some sand and gravel under it, which could liquefy in a big quake, causing the dam to potentially slump and fail.