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Improved North Bay reservoir levels a hopeful sign for 2022

Other North Bay reservoirs fill up

Lake Hennessey, Napa County: 28,169 acre-feet, 91% capacity

Lake Berryessa, Napa and Solano counties: 1.02 million acre-feet, 63.7% capacity

Marin County reservoirs: 73,714 acre-feet, 92.7% capacity (122% of average)

Sources: Napa County, Solano County Water Agency and Marin Water

Winter rains have bolstered water storage in the region’s two key public reservoirs, reversing months of decline and starting off 2022 with hopes for a less-uncertain year ahead.

A lot depends on how the remainder of the rainy season shakes out. After rain this week, the forecast calls for dry weather later this month, followed by months in which the North Coast stands an equal chance of above-normal and below-normal precipitation under La Niña atmospheric conditions.

National Weather Service meteorologist Brian Garcia said there were some promising signs for normal rainfall later this month into mid-February that could “lock in some of the early winter gains.”

But already, there’s more water in Lake Mendocino than there has been for over a year, while Lake Sonoma has risen to 60% of its water supply capacity.

“So far, things are encouraging,” said Don Seymour, principal engineer with Sonoma Water, a wholesale supplier providing water to more than 600,000 people in Sonoma and northern Marin counties.

The picture at the two reservoirs is quite different, given their relative locations and size. Lake Mendocino is smaller and collects from a smaller watershed, where about a third less rain has fallen this year so far. As always, it remains in a more precarious situations than its larger cousin.

Lake Sonoma is well over twice the size and is fed by the coast range, which consistently receives some of the area’s highest rainfall totals.

But two years of extreme drought strained the entire region considerably, forcing conservation measures on consumers and depriving agricultural practitioners of adequate supplies, as personal reserves ran out and access to public resources was limited.

Both lakes Sonoma and Mendocino started the summer at the lowest levels they had ever reached for that time of year. State regulators eventually curtailed withdrawals from the Russian River, primarily to preserve storage held behind Coyote Dam in Lake Mendocino amid fears the smaller reservoir would run dry if no rain came.

The lake was approaching its lowest level in history, when an atmospheric river came through in late October, drenching the region and reintroducing residents to the concept of a wet winter.

The water year, which starts Oct. 1, has so far delivered about 21½ inches of rain to the Santa Rosa area ― more than fell in all of 2019-20 or 2020-21, when 19.35 inches and 13.01 inches, respectively, fell between Oct. 1 and Sept. 30, measured at the Charles M. Schulz ‐ Sonoma County Airport.

Lake Sonoma now holds about 147,000 acre feet of water ― about what it did in early May, before the punishing, warm summer months began. Two years ago, the reservoir was above 218,000 acre feet. (An acre foot is 325,851 gallons, or enough water to cover an area about the size of a football field with a foot of water. It would supply almost 3½ water-efficient California households for a year, according to the Water Education Foundation.)

Water storage graphs for lakes Sonoma and Mendocino as of Jan. 3, 2022. (Sonoma Water)
Water storage graphs for lakes Sonoma and Mendocino as of Jan. 3, 2022. (Sonoma Water)

It’s reached 60% of its water supply capacity already, gaining almost a third of its current storage since rain resumed in Oct. 24.

Lake Mendocino is a much more complicated situation, since it receives some inflow from the Eel River through the Potter Valley power plant, in addition to rainfall. It is operated under strict regulations governing dam releases to sustain streamflows in the upper Russian River as well, contingent on lake conditions.

Those flows from Potter Valley are low this year, because of damage in the plant. And the Ukiah area so far has received just 14.73 inches of rain since Oct. 1, about what it did in each of the last two rainfall years Oct. 1 to Sept. 30.

But, for the moment, Lake Mendocino holds more water than it did at any point after Sept. 23, 2020, when it began approaching 41,000 acre feet and continued to drop, coming close a year later to matching the all-time-low of 12,081.

Water storage graphs for lakes Sonoma and Mendocino as of Jan. 3, 2022. (Sonoma Water)
Water storage graphs for lakes Sonoma and Mendocino as of Jan. 3, 2022. (Sonoma Water)

That’s about 67% of the water storage limit set for the early winter months, when the goal is to maintain abundant flood control space in the event of a deluge.

Recent rain has added 30 feet to the lake level, allowing it to spread out and cover what had been bare lake bottom, said Nick Malasavage, chief of readiness and operations for the U.S. Army Corps of Engineers’ San Francisco Division, which operates the two reservoirs in partnership with Sonoma Water. It’s even allowed the Army Corps staff to envision the possibility of a normal recreational season, he said.

“We’re thankful for what we’ve gotten so far, and we’re hopeful for more,” he said.

If it were to stop raining, Seymour said, Lake Sonoma is sufficiently stored to get through a couple of years, as is.

Those dependent on Lake Mendocino would need to conserve water in an efficient and disciplined manner. “We could get through,” he said, though not with stream flows adequate to serve the fish and the environment to the level desired.

“It’s great to get this drink,” Malasavage said. “But like most experts, and even citizens around the nation and around the state in general, I fully acknowledge this doesn’t dig us out of the hole we’re in. ”

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

Other North Bay reservoirs fill up

Lake Hennessey, Napa County: 28,169 acre-feet, 91% capacity

Lake Berryessa, Napa and Solano counties: 1.02 million acre-feet, 63.7% capacity

Marin County reservoirs: 73,714 acre-feet, 92.7% capacity (122% of average)

Sources: Napa County, Solano County Water Agency and Marin Water

Mary Callahan

Environment and Climate Change, The Press Democrat

I am in awe of the breathtaking nature here in Sonoma County and am so grateful to live in this spectacular region we call home. I am amazed, too, by the expertise in our community and by the commitment to protecting the land, its waterways, its wildlife and its residents. My goal is to improve understanding of the issues, to find hope and to help all of us navigate the future of our environment. 

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