Lake Sonoma at lowest level in history, but there’s still enough to get us through another year

Lake Sonoma, the region’s largest water storage reservoir, has reached the lowest level in its history after three years of punishing drought with no end in sight.

But there remains plenty of water to get regional users through this winter and even into next, said Sonoma Water Deputy Chief Engineer Don Seymour.

“There’s no imminent risk of Lake Sonoma going dry, and that would only be if we really had another very dry year,” he said.

Lake Sonoma currently holds less than 42% of its water storage capacity after falling continuously since Jan. 21, when it held 152,474 acre feet of water. (An acre foot is about 326,000 gallons or enough water to flood most of a football field a foot deep in water. Estimates vary, but it can serve roughly one to three average homes for a year, depending how careful they are with their water use.)

It now holds about 102,600 acre feet, a decline of about one-third in 10 months.

The lake had drawn near to this point in October a year ago, reaching 105,509 acre feet. Then, an atmospheric river brought record rainfall and boosted supplies significantly. Occasional rain and runoff afterward mostly raised the level until January, when the situation reversed.

Seymour noted that last year’s record storm was an anomaly, bringing so much precipitation and coming so early in the year.

“We usually never see any storage improvement until November, December, January,” he said. “I don’t anticipate any change until later in the winter.”

There’s tremendous uncertainty about the region’s future water supply situation. Forecasters are anticipating a continued La Niña atmospheric influence through the beginning of next year, which likely means very dry conditions in Southern California and the American Southwest and extremely wet conditions in the Pacific Northwest.

But the signal for Central and Northern California could go either way, the National Weather Service and other meteorologists say.

Welcome rain that hit the region Sunday evening is expected to bring up to 1½ inches of rain in the valleys by Wednesday and up to 2 inches in the coastal range, National Weather Service meteorologist Jeff Lorber said. There is a small chance of rain this weekend.

With the landscape so dry, whatever comes is likely to be sucked up by the ground rather than run into creeks and reservoirs — though restoring the groundwater is a good thing, too, Seymour said.

The weather service’s Climate Prediction Center also is calling for above normal rainfall in the area during the next eight to 14 days. But there is no guarantee of normal seasonal rainfall to make up the deficit in Lake Sonoma.

The reservoir was created through construction of Warm Springs Dam across Dry Creek, a tributary of the Russian River.

Dry Creek is the second largest Russian River tributary, draining about 217 square miles of land. The dam was primarily a flood control structure when it was started in 1981 — an attempt to hold back accumulations of floodwater that would hit the lower Russian River at a narrow pass through the coastal mountains and flood local communities.

But it also holds critical water supplies that augment those in the upper Russian River when released into the river near Healdsburg.

Average October storage in the lake is 181,975 acre feet, or about 74.3% of water storage capacity, according to Sonoma Water.

Nick Malasavage, chief of operations and readiness for the U.S. Army Corps of Engineer’s San Francisco District, said the lake has dropped low enough to reveal two water inlets, the lowest of which has only been visible once before: last year.

Even during the last multiyear drought, the lake was still significantly above its current storage, reaching a low of 136,553 acre on Nov. 11, 2014.

Previously, the lake dropped to 125,389 on Feb. 26, 1991.

Lake Mendocino, at about 37,400 acre feet, is still above its historic low level of 12,864 acre feet, reached last October.

That’s about 34% of the reservoir’s storage capacity but closer to 67% of a special target supply curve calculated by Sonoma Water.

Malasavage said Lake Sonoma looks similar to how it looked in October 2021, months after trees long underwater had begun to poke up the lake’s surface in some of the narrow, shallower arms.

“It’s still quite deep here at Lake Sonoma,” Malasavage said. “Despite being this low, the lake is still quite deep.”

You can reach Staff Writer Mary Callahan (she/her) at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.