Shrinking Lake County reservoir prompts North Bay supply concerns

Water supplies for Sonoma, Mendocino and Marin counties this summer could hinge partly on the dwindling storage in a remote, drought-starved reservoir on the Eel River that serves as a cornerstone to the region’s water system.

Water managers, fisheries biologists, environmentalists and PG&E have their eyes on Lake Pillsbury, a diminishing Lake County reservoir where storage has dropped 30 percent in the past three months, leaving it at less than 55 percent of capacity. Barring a change in water policy, the situation could lead to a string of empty reservoirs by year’s end, officials said Friday.

“We get to a place where we’re threatened with dry lakes,” said Janet Pauli, a Mendocino County grape grower who has long served on a local irrigation district that depends upon the reservoir’s supply.

Lake Pillsbury’s decline most immediately affects about 300 ranchers in Potter Valley, but its repercussions could reach Lake Mendocino near Ukiah, and ripple down the Russian River to eventually touch the 660,000 customers in Sonoma and Marin counties whose water is drawn from the river by the Sonoma County Water Agency.

“This is an unprecedented situation,” said Grant Davis, the agency’s general manager.

It underscores the need, he said, for communities that depend on Russian River water to boost conservation efforts and develop off-river alternatives, such as recycled wastewater.

The Water Agency hosted a meeting Thursday involving the major stakeholders - including the Potter Valley Irrigation District, state Department of Fish and Wildlife, National Marine Fisheries Service and PG&E - who all agreed that Lake Pillsbury must make it to the end of December with at least 10,000 acre-feet of water, or about 13 percent of its capacity.

The agencies also agreed, as an immediate step, to support PG&E’s request for federal permission to reduce next week the flow of Eel River water released from Lake Pillsbury. The plan would curb the amount diverted through a mile-long tunnel to Potter Valley from 90 cubic feet per second to about 75 cfs. Of that amount, 50 cfs would flow into the irrigation district’s canals and 25 cfs would flow down the East Fork of the Russian River into Lake Mendocino.

Flows down the Eel River would be ramped down 9 cfs under the plan, which requires the Federal Energy Regulatory Commission to designate a “dry condition” for PG&E’s Potter Valley powerhouse, which generates electricity from water diverted from the Eel to the Russian River.

Pauli and Davis said that PG&E obtained the same standard last year, and both the Potter Valley district and the Water Agency made do.

But PG&E caught the water suppliers by surprise with a proposal late last month to seek a “critical condition” designation from the federal agency, reducing the Potter Valley diversion to 30 cfs.

At that rate, Potter Valley crops would be damaged or fail and little, if any, water would reach Lake Mendocino, officials said.

“We’ve never gone to critical before,” Pauli said.

PG&E determined the need for change this year in April, as Lake Pillsbury was dropping from a peak of 58,900 acre-feet on Feb. 7 toward the level it reached Thursday, 40,500 acre-feet.

Pauli said the Potter Valley district received notice in late April that PG&E intended to seek a critical condition designation, setting off a scramble among local water agencies and state and federal biologists intent on protecting the endangered fish in the Russian River. The response led to Thursday’s meeting at the Water Agency.

For the Water Agency, the main hedge to any loss of water from Lake Mendocino and Lake Pillsbury is the supply from Lake Sonoma, the system’s largest reservoir, which is at about 86 percent capacity.

But the Water Agency’s ability to offset losses from the northern reservoirs, however, is limited by constraints on the daily flow permitted along Dry Creek, which carries Lake Sonoma’s water to the Russian River.

Assuming federal regulators approve PG&E’s request, attention will be fixed this summer on Lake Pillsbury, which must stay above 30,000 acre-feet, or 17 percent of capacity, to stave off deeper cuts in the Eel River supply.

At its current outflow, the small reservoir would most likely fall below that level by midsummer, hence the agreement by all stakeholders to back PG&E’s plan, which McKannay said would be submitted next week.

The federal agency could take 30 days to approve the reduced release from Pillsbury. Pauli said she wished it could start immediately.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.