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Anderson Dam: Plans to drain the largest reservoir in Santa Clara County

Three months after federal dam protection regulators directed Anderson Reservoir, Santa Clara County ‘s largest reservoir, to be drained due to earthquake issues, fresh information are emerging about what’s going to happen to all that water, the fish that rely on it, and the Silicon Valley water sources.

The Santa Clara Valley Water District, which operates the 7-mile-long reservoir situated east of Highway 101 between Morgan Hill and San Jose, has proposed proposals to start emptying it as of October 1.

The large lake would be drained at a rate of about 200 acres, or 65 million gallons a day — the equivalent of 98 Olympic pools per 24 hours. If the environment this fall and early winter is warm, so by mid-December or early January the reservoir will be all but empty. However, if the weather is rainy it could take full drainage until April, district water engineers say. On 26 May, a 42-page drainage proposal was authorized by the district council. The proposal involves constructing a 1,700-foot-long pipe, up to 24 miles in diameter, at a expense of $220 million, on the left side of the dam starting early next year.

The amount at which water will be released during severe storms or during an earthquake which could destroy the dam is projected to rise by five times. The pipe, which was expected to be finished in late 2023, was part of the initial proposal to restore the 70-year-old Anderson Dam until the project was speeded up by federal decree. It was first 12 years earlier that seismic issues were found at the dam. Yet the difficulties in the district restoring the 240-high Earthen dam, first installed in 1950, triggered the Federal Energy Regulatory Commission’s unprecedented action in February that stunned water managers throughout California.

“This is our number 1 priority,” said Chris Hakes, deputy operating officer of the district, also known as Valley Water.

“It is a public safety project first,” he said. “It’s a long time coming. We have good momentum. We have hit every roadblock we could along the way. I feel like we have our arms around the situation at this point. Everybody is committed to getting this done.”

The research would mean that much, if not all, of Anderson Lake County Park will be inaccessible for up to 10 years, water district officials claim.
Now the reservoir is 29 per cent complete. The district plans to use the water in it for human consumption over the summer so that it will be only 11 per cent full by Oct. 1 when draining starts. Even then, Hakes said, much of that water will not simply flow out of the reservoir into San Francisco Bay, down Coyote Creek. Some will be stored in submerged local aquifers. And some will be diverted for human use, if the quality remains sufficient as the lake level drops.

“We don’t want to waste anything,” Hakes said.

So what about all of the fish? The reservoir is drained down to only 3 percent full, a point known as the “dead pool” below the outlet pipes. There will be some fish that could live there. The dead pool would remain relatively significant—about 2,300 acre feet, about five times the size of Los Gatos’ Vasona Dam. Other fish that are trapped in smaller pools around the reservoir, including rainbow trout, catfish and large mouth bass, will be caught in nets and moved upstream to creeks, or possibly placed in Coyote Reservoir or San Luis Reservoir.

The district also aims to save threatened steelhead trout live only downstream from the dam in Coyote Creek, and relocate them to Upper Penitencia Creek.
The district plans to use imported water after the reservoir is drained to stop Coyote Creek, a major body of water which flows through downtown San Jose, from going completely dry. In deep aquifers in Kern County, Semitropic Water Storage Area, the system maintains nearly 350,000 acre-feet of storage, about a year’s worth. Some will be pumped into Coyote Creek by pipes and others will be piped through percolation ponds around Coyote Creek Golf Course, holding levels of local groundwater high.

Hakes acknowledged local groundwater sources are nearly complete in Santa Clara City. And the district has recovered water from the River, plus contracts with state and federal power.

“Through this year and into next, it looks really good from a water supply perspective,” he said. “We always knew that Anderson would be taken off line so we have been reaching out for some additional imported sources.”

Hakes acknowledged local groundwater sources are nearly complete in Santa Clara City. And the district has recovered water from the River, plus contracts with state and federal power.

“Generally speaking, in a drought we don’t have a lot of supply to put into Anderson,” Hakes said. “It’s kind of no harm no foul.”

The water district plans to hold a public hearing on the plan June 23.

In December 2008, engineers found that a 6.6-magnitude quake at Anderson Reservoir’s Calaveras Fault, or a 7.2-mile quake, could cause the dam to fail, sending a 30-foot water wall into downtown Morgan Hill within 10 minutes, and 10-foot deep into San Jose in three hours, potentially killing thousands of people, the latest studies suggest. Environmentalists remember that federal dam authorities cracked down on protection after the spillway of Oroville Dam collapsed three years ago, forcing 188,000 residents across Butte County to be relocated in the emergency.

“FERC is throwing down the gauntlet against agencies that are slow to respond to dam safety emergencies or circumstances that require new investments for dam safety,” said Ron Stork, a senior policy advocate at Friends of the River, in Sacramento. “We’ve seen that big time, since the soul searching after Oroville.”

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