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A correction to an earlier version of this article has been appended to the end of the article.
Three months after federal dam safety regulators ordered Anderson Reservoir, the largest reservoir in Santa Clara County, to be drained due to earthquake concerns, new details are emerging on what will happen to all that water, the fish that depend on it, and the water supply for Silicon Valley.
The Santa Clara Valley Water District, which owns the 7-mile-long reservoir located east of Highway 101 between Morgan Hill and San Jose, has drawn up plans to begin emptying it starting Oct. 1.
The massive lake will be drained at the rate of about 200 acre feet, or 65 million gallons a day — the equivalent of 98 Olympic swimming pools every 24 hours. If the weather this fall and early winter is dry, the reservoir will be all but empty by mid-December or early January. If the weather is rainy, however, full draining could take until April, district water engineers say.
On May 26, the district’s board approved a 42-page draining plan. The plan includes building, at a cost of $220 million, a 1,700-foot-long tunnel, up to 24 feet in diameter, on the left side of the dam starting early next year. It is expected to increase by five-fold the rate at which water can be released during major storms or after an earthquake that could damage the dam. That tunnel, scheduled to be completed in late 2023, was part of the original plan to rebuild the 70-year-old Anderson Dam before the federal order sped up the process.
Seismic problems at the dam were first identified 12 years ago. But the district’s delays in rebuilding the 240-high earthen dam, first constructed in 1950, prompted the dramatic order from the Federal Energy Regulatory Commission in February that shocked water managers across 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 work will mean the closure of most, if not all, of Anderson Lake County Park for up to 10 years, water district officials say.
The reservoir is currently 29% full. Over the summer, the district plans to use the water in it for human consumption so that by Oct. 1 when draining starts, it will be just 11% full. Even then, Hakes, said, much of that water won’t simply flow out of the reservoir down Coyote Creek into San Francisco Bay. Some will be stored in local underground aquifers. And some, if the quality remains sufficient as the lake level drops, will be diverted for human use.
“We don’t want to waste anything,” Hakes said.
And what of all the fish? The reservoir will be drawn down to just 3% full, a level called “dead pool” that is below outlet pipes. Some fish will be able to live there. The dead pool will still be fairly significant — about 2,300 acre feet, or five times the capacity of Vasona Lake in Los Gatos. Other fish, including rainbow trout, catfish and large mouth bass, that are trapped in smaller pools around the reservoir will be caught in nets and moved to creeks upstream, or possibly put in Coyote Reservoir or San Luis Reservoir.
The district also plans to rescue endangered steelhead trout that live in Coyote Creek just downstream from the dam and move them to Upper Penitencia Creek.
To stop Coyote Creek, a major body of water that flows through downtown San Jose, from going completely dry, the district plans to use imported water after the reservoir is drained. The district holds roughly 350,000 acre-feet of water, more than a year’s supply, in underground aquifers in Kern County in the Semitropic Water Storage District. Some will be released through pipelines into Coyote Creek, and some will be piped into percolation ponds near Coyote Creek Golf Course, keeping local groundwater supplies up.
Hakes noted that local groundwater supplies in Santa Clara County are nearly full. And the district has recycled water, and contracts for state and federal water from the Delta.
“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.”
After the tunnel is finished, the district plans to begin construction on the new dam in 2024 and finish by 2030. The entire project is estimated to cost $576 million. And what if there’s a drought?
“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 on the Calaveras Fault at Anderson Reservoir, or a 7.2 quake a mile away, could cause the dam to fail, sending a wall of water 30 feet high into downtown Morgan Hill within 10 minutes, and 10 feet deep into San Jose in three hours, potentially killing thousands of people, according to the most recent studies.
Environmentalists note that federal dam officials have been cracking down on safety after Oroville Dam’s spillway failed three years ago, causing the emergency evacuation of 188,000 people around Butte County.
“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.”
Correction: June 8, 2020 An earlier version of this article incorrectly described the length of a proposed tunnel as 1,700 feet high. It should have read 1,700 feet long.
