Lake Mendocino is drying up, and 1,600 Russian River water rights are about to be halted

Water supplies in Lake Mendocino are shrinking at a faster rate than they were a month ago, raising the risk the reservoir could be depleted by fall and setting regulators up to halt diversions from the Russian River for about 1,600 water rights holders before the end of July.

Two-thirds of those diverters, including farmers, ranchers, vineyard operators, rural residents and municipal suppliers, already were put on notice a month ago, when authorities said not enough water existed in the upper river to support irrigation, household use and the watershed’s imperiled fish species.

Now, amid the hottest and driest months of a deepening, two-year drought, regulators are poised to move forward with a more aggressive crackdown, called a curtailment of water rights.

It is likely to be unprecedented in scope, affecting hundreds of property owners along the upper river as well as public agencies, including the cities of Healdsburg and Cloverdale, that rely partly on the river for drinking water.

The action is meant to preserve minimal flows in the driest parts of the Russian River’s main stem and dwindling supplies in Lake Mendocino, which, like larger Lake Sonoma, hit record seasonal lows even in early spring.

Supplies behind Coyote Dam at Lake Mendocino are now dropping close to a key threshold that will trigger regulators’ move to halt downstream diversions.

Even as that grim scenario is unfolding, withdrawals from the river remain high between Cloverdale and vineyard-dense Alexander Valley.

That use has surprised water managers, especially after more than 900 water rights holders in the area were told starting last month that there was not enough in the upper river to sustain diversions for irrigation and household use. The notice stopped short of official curtailment but came with a host of potential fines — up to $1,000 a day or more based on quantity of water used.

“It was supposed to go the other way,” said Don Seymour, principal water engineer with Sonoma Water. “We were supposed to see a lot less water jumping out of there. And in some areas, we’re seeing that. And in some areas, we’re not seeing really any change from last year.”

River gauges show that consumption is heavy particularly between Geyserville and Jimtown, a lightly settled but heavily cultivated grape growing area north of Healdsburg.

Twice in recent weeks, Seymour said, dam managers at Lake Mendocino have had to send more water downstream to make up for flows that have gone missing along that particular stretch of the river and to ensure federally mandated minimum levels for the river’s diminished salmon and steelhead trout populations.

Those releases have made it likely that the all-important threshold at Lake Mendocino will be hit in the next 10-12 days, Seymour said.

Reduced diversions from the Eel River into Lake Mendocino account for part of the difference in reservoir storage, he said. Evaporation and uptake of the water streamside plants and trees are other factors, he said.

New state rules that took effect Monday allow state regulators ultimately to suspend the water rights of up to 2,400 landowners, ranchers, grape growers and others throughout the Russian River watershed, if conditions get bad enough.

The roughly 1,600 diverters in the upper river watershed would be affected first to safeguard supplies in Lake Mendocino.

The reservoir on Friday stored almost 9.2 billion gallons of water, or 28,120 acre-feet, down 195 acre-feet from Thursday. (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.)

Under the emergency regulation, curtailments are triggered if the lake falls to 26,109 acre-feet by Aug. 1.

During the month of June, daily storage was declining by an average of about 145 acre-feet per day.

During the first week of July, the daily loss rose to an average 175 acre-feet.

If the state threshold is hit, water right holders would be allowed only enough water necessary to supply each domestic user with 55 gallons per person per day to meet minimum health and safety standards. Many still have access to groundwater wells or have other water rights permitting them to continue drawing on the river.

Ailene Voisin, spokeswoman for the State Water Board’s Division of Water Rights, said the agency was “encouraged” by the fact that storage levels in Lake Mendocino had remained above threshold targets so far.

“That being said, we’re cautiously watching depletions and releases from the reservoir, and note that both depletions and releases have picked up over the last week or so,” Voisin said. “We’re now entering the time of the year when depletions and use are greatest, and based on projections there’s a significant chance that the threshold targets could be reached by the end of the month.”

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