Lake Mendocino was about 10,000 acre-feet above the threshold for critical conditions on April 24. Operators have already cut Coyote Dam outflow by about 65%.
Lake Mendocino is getting tight, and Russian River releases are dropping
Lake Mendocino entered the last week of April with less room than water managers want heading into summer.
On April 24, the reservoir held about 84,400 acre-feet of water. That was about 10,000 acre-feet above the level that would push the upper Russian River into “critical” water conditions. By June 1, that line rises to 76,000 acre-feet.
That narrowing cushion is already changing how the river is being run.
On April 16, Sonoma Water moved the upper Russian River from “normal” to “dry water supply conditions” for the first time in several years. The designation lowers the amount of water that must be kept flowing in the upper river under state rules.
In a normal spring, the upper Russian River must carry at least 185 cubic feet per second. Under dry conditions, the minimum drops to 75. If Lake Mendocino falls into critical conditions, the minimum drops again to 25.
That change gave operators room to hold more water in Lake Mendocino. Between April 14 and April 21, total outflow from Coyote Dam fell from about 133 cubic feet per second to 47, a drop of roughly 65%.
The river is still running above its required minimum. The reservoir still has a cushion. The move into dry conditions means operators have started saving water before the long dry season begins.
The cut at Coyote Dam
For the first half of April, water left Lake Mendocino at a steady spring pace. Those releases help meet downstream water needs, support fish habitat and keep the Russian River moving below the dam.
Then operators cut back.
The change showed up quickly near Hopland, where a federal gauge tracks the upper Russian River below Coyote Dam.
A storm pushed the Hopland gauge to 441 cubic feet per second on April 12. By April 17, the river had fallen to 196. By April 24, it was at 144.
That is still well above the new 75-cubic-foot-per-second floor. The flow at Hopland includes more than water coming through the dam. Creeks, runoff and travel time all add water to the river between Lake Mendocino and the gauge.
Farther downstream, spring storms made the Russian River look flush for a few days. Flows near Guerneville reached about 1,850 cubic feet per second on April 13 and 1,540 on April 22.
Those storms helped the lower river. They did less for Lake Mendocino. The basin north of Ukiah, which feeds the reservoir, did not get the same benefit.
Why Lake Sonoma does not settle this
Sonoma County’s largest reservoir is in good shape.
Lake Sonoma, the big reservoir above Healdsburg, is above its water-supply target and supplies much of Sonoma Water’s drinking water. For many Sonoma County households, that is the reservoir that matters most.
Lake Mendocino plays a different role.
It is smaller. It sits farther north. It carries more weight for Ukiah, the upper Russian River, agriculture around Hopland and Cloverdale, and fish that need cold, moving water through the dry season.
That is why the April 16 dry call matters even with Lake Sonoma looking strong.
The Russian River system has more than one bucket. The big bucket is full. The smaller one serving the upper river is getting tighter.
The line to watch
The key number right now is 74,000 acre-feet.
That is the current threshold for critical conditions at Lake Mendocino. If storage drops below that line, the upper-river minimum flow falls from 75 cubic feet per second to 25. On June 1, the threshold rises to 76,000 acre-feet.
An acre-foot is about 326,000 gallons, enough to cover one acre of land one foot deep. A 10,000-acre-foot cushion sounds large. For a reservoir heading into summer and early fall, it can shrink quickly.
Lake Mendocino has already lost ground this spring. In recent wet years, the lake often gained water in April as rain and runoff beat releases. This year, the reservoir mostly fell through the month. A late-April storm and lower releases helped slow the drop.
They did not erase it.
Who feels this first
The first pressure points are in the upper watershed.
Ukiah depends on Lake Mendocino. So do farms and vineyards along the Hopland and Cloverdale corridor. Salmon and steelhead also depend on enough cool water moving through the upper river.
Those users feel a tight Lake Mendocino before most Sonoma County water customers do.
This is also where the 2021 drought memories are sharpest. Lake Mendocino fell to crisis levels that year. Shoreline turned to mud. Boat ramps stopped reaching water. Emergency flow cuts followed.
Current storage is far better than that. Ukiah rainfall also is not in collapse; as of April 22, it was near 31 inches for the water year, below full-season normal and much better than a severe drought year.
The concern now is spring trajectory. Lake Mendocino is entering the dry season with operators already trimming releases to protect storage.
What comes next
Sonoma Water is checking Lake Mendocino storage every two weeks through June.
A wet May could move the upper Russian River back to normal conditions. Continued dry weather could pull the lake closer to the critical line. Crossing that line would lower the required upper-river flow again and could lead Sonoma Water to seek more flexibility from the state.
The practical picture is straightforward: Lake Sonoma is strong, Lake Mendocino is tight, and the upper Russian River has shifted into a lower-flow setting before summer.
The next few weeks will show how much cushion Lake Mendocino carries into the hottest part of the year.