Project to drain region’s largest reservoir set to begin this week
$576 million earthquake retrofit will take 10 years
Santa Clara County’s largest reservoir will soon be nearly empty and stay that way for the next 10 years.
Under orders from federal dam regulators, the Santa Clara Valley Water District will begin a project to drain Anderson Reservoir on Thursday, the first
step in a $576 million effort to tear down and rebuild its aging dam.
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, concluded in February when it issued the order.
During construction, Anderson Lake County Park will remain closed to boating and fishing. A few trails and day use picnic areas, mostly downstream of the dam along Coyote Creek, will remain open, but the main parking lot and boat ramp area will be off limits, probably until 2030.
“This project will take a long time,” said John Varela, a water district board member. “But I can tell you that when the work is done our communities in Morgan Hill and along Coyote Creek will be safer. It will be worth it.”
The district provides drinking water and flood control to 2 million people in Santa Clara County.
Anderson Reservoir is seven miles long. When full, it holds 89,278 acre feet of water — more than all other nine dams operated by the water district combined.
In January, largely due to a dry winter, the reservoir was only 30% full. Over the summer, the water district has drawn its level down to 18% full, delivering the water to homes and businesses, and using it to recharge underground aquifers. Starting this week, the agency will further lower the level to just 3% full. That could be achieved as soon as December if the weather is dry. If it rains significantly this winter, that accomplishment could take as long as next June, according to the water district.
Even at 3% full, the lake
will still hold about 3,000 acre feet of water, roughly the same as Stevens Creek Reservoir when full, or seven times the size of Vasona Lake in Los Gatos. That ‘ dead pool’ will be enough to keep a fair number of fish and other aquatic animals that live in the reservoir now alive, although it will be completely gone in four years.
Most important, the agency’s water managers said Tuesday that they don’t expect the loss of the water in Anderson Reservoir to cause major shortages for Silicon Valley in the coming years.
There are three reasons, they said. First, the district normally draws about half of its water from groundwater wells in Santa Clara County. Those aquifers are nearly full, following the deluge rainfall of 2017, and regular work to replenish them.
Second, the district has 340,000 acre-feet of water, more than a year’s supply, that it has been banking for years in underground aquifers in Kern County in the Semitropic Water Storage District. It plans to draw on some of that water.
And third, district officials say, if there were a multi-year drought, water levels at Anderson would fall anyway.
“In some years during the drought — like 2015 — there was actually more evaporation from the reservoir than we were gaining in storage,” said Aaron Baker, deputy operating officer for the water district’s raw water division.
“Realistically in the overall picture of things, yes, the reservoir definitely has its benefits,” Baker said. “But during a drought it’s the initial storage that we feel very comfortable that we can offset with our local supplies, and our groundwater banking and imported transfers.”
The water district already has moved 70 steelhead trout, an endangered species, from Coyote Creek to Upper Penitencia Creek. To stop Coyote Creek, a major body of water that flows through downtown San Jose, from going completely dry during construction, the district plans to release imported water through a pipeline into the creek. Some also will be piped into percolation ponds near Coyote Creek Golf Course, to boost local groundwater supplies.
Ma ny nei g hbors of the project aren’t happy. Some worry about landslides when the reservoir is drained. Others are concerned about the loss of their property values, or the loss of reservoir water for firefighting helicopters.
“I’ve felt helpless,” said Lynne Meyer, one of about 500 residents of Holiday Lake Estates community adjacent to the reservoir. “Some people are moving because of this. Our neighbors down the street just moved because of it.”
Federal regulators criticized the water district for taking so long to undertake the project. The agency first learned of the seismic problems in 2009, and subsequently released engineering studies showing that a 6.6 magnitude quake on the Calaveras Fault directly at Anderson Reservoir, or a 7.2 quake centered one mile away, could cause the reservoir’s huge earthen dam to slump and fail.
Although the chances of that are extremely slim, if it had happened when the reservoir was full, it could have sent a 35-foot wall of water into downtown Morgan Hill in 14 minutes. The waters would have been 8 feet deep in San Jose in three hours, potentially killing thousands of people.
“The amount of damage that would be done in the event of a catastrophic failure is quite substantial,” said Chris Hakes, the district’s deputy operating officer of dam safety and capital delivery. “From a life-safety perspective it is critical to get this facility retrofitted.”