Scott Dam gates to stay open, meaning Lake Pillsbury diversions to remain at drought levels
Pacific Gas & Electric says it intends to keep the gates open at Scott Dam from now on in deference to seismic safety concerns, meaning Lake Pillsbury in Lake County will never completely fill again, even in a wet year like this one.
The utility usually closes the dam gates in April, allowing spring runoff and snowmelt to raise the water level for summer recreation and water releases during the later, drier parts of the year.
But the company says updated seismic analysis of the dam suggested a higher level of risk than previous evaluations, prompting a change in operations. Instead, more water will be allowed to flow into the Eel River this spring instead of keeping it behind the dam.
“While risks to the dam remain very low, by reducing water levels in the reservoir we can mitigate against risk,” Jan Nimick, PG&E’s vice president of power generation, stated in a news release. “Storing less water in the reservoir (a 26% reduction compared to a full reservoir) lowers the water load on the dam, thereby significantly reducing the risk during or after a seismic event.”
That means summer and fall releases for federally protected fish and for diversions into Lake Mendocino and the Russian River will never be higher than they were in the 2020 and 2021 drought years, raising questions about water supply availability for downstream users.
“We had a good weather year,” Sonoma County Water Agency General Manager Grant Davis said. “We’re in the midst of it. But the uncertainty that this is going to bring now in perpetuity — PG&E’s decision to keep the gates down and reduce the flow — is going to have an impact.
“This will be the foreseeable future, where you have even further reduced diversions into the Russian River and make us even less resilient, as a result,” he said.
Lake Pillsbury plays a low-profile but critical role in the water supply system for hundreds of thousands of people in Sonoma and Mendocino counties who obtain their water from Sonoma Water contractors or from smaller agencies and municipalities.
Though most of their water is stored in Lakes Mendocino and Sonoma, some of the water in Lake Mendocino comes each year from Eel River water that goes first into Lake Pillsbury and is then diverted through PG&E’s Potter Valley power plant into the East Branch Russian River, about 80 miles northeast of Santa Rosa.
Historically, those diversions were sent through a mile-long tunnel to turn hydroelectric turbines and were byproducts of the power plant’s main purpose. But over time, Russian River waters users came to depend on the augmentations to Lake Mendocino stores.
At one point, those contributions over the course of a year accounted for more water than could be stored at a given time in the reservoir, which at full water supply level holds 111,000 acre-feet. Annual diversions once measured about 150,000 acre-feet, Sonoma Water Deputy Chief Engineer Don Seymour said.
About a decade ago, the pass-through water was reduced to around 60,000 acre-feet per year, he said. (An acre-foot is equal to 325,851 gallons, or about the amount of water needed to flood most of a football field one foot deep. It can supply the indoor and outdoor needs of three water-efficient households for a year.)
The volume was further reduced by drought in 2020 and in 2021 when a transformer bank on the power house blew and water supplements were limited to what could be sent through a bypass channel.
By then, PG&E already had decided not to renew its license for the aging, inefficient hydroelectric plant, raising uncertainties about water supplies that were only amplified by the equipment failure. The utility at one point said it would repair the transformer, but in December announced it was reassessing that decision.
The decision to keep the dam open, which requires authorization from federal regulators, means permanently low supplements to Lake Mendocino that, if not this year, in the next more moderate rainfall year could make a difference, Davis said.
“It’s not just Sonoma Water,” he said. “It’s all water users along the Russian River that could be affected by this.”
Among them is the Mendocino County Russian River Flood Control and Water Conservation Improvement District.
“This is a devastating blow to water right holders, reservoir operators and all Russian River watershed stewardship efforts,” said District Manager Beth Salomone. “We hoped to have years to prepare for this level of water supply reduction.”
At the same time, the proposal will likely serve to fast-track long-running discussions about the future of diversions through the soon-to-be-surrendered power plant and removal of the dam altogether — a key goal of environmental groups seeking to reopen miles of the upper Eel River to salmon and steelhead trout, Sonoma County Supervisor James Gore said.
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