The Sonoma County Water Agency has cut flows in the Russian River by about 30 percent since Tuesday in an effort to preserve dwindling supplies in Lake Mendocino.

State regulators granted the agency permission to cut flows late Tuesday and the agency promptly dropped from releasing about 100 cubic feet per second at the dam, or about 748 gallons, to 90. On Thursday, the agency dropped that number down to 70 cfs, and could go lower this week or next.

"We don't like to take it down really quickly," agency Assistant General Manager Pam Jeane said.

That means flows in the river downstream could fall to below 55 cfs at its lowest point by early next week. During a normal rain year, the agency would need to keep that flow at around 150 cfs at the lowest point; even on a moderately dry year, it can't cut the flow lower than 75 cfs.

But as of Thursday morning, the reservoir had just 26,280 acre feet of water in reserve, about 8.6 billion gallons, or just 38.7 percent of its capacity.

The region is coming out of a record dry year, with just 7.67 inches of rain falling in the upper reaches of the Russian River, as measured at Ukiah; the area usually sees at least 34 inches of rain.

With no major rain predicted for the early weeks of January, the agency is preparing to face a second straight dry year. Without cutting the flows dramatically now, there is a risk of the reservoir effectively running dry next summer should the drought continue, the agency said.

The State Water Resources Control Board agreed, saying cutting the flow now will affect downstream farmers, cities, and wildlife less than if the reservoir were allowed to dry out next summer.

"Any effects associated with such flow reductions would not be unreasonable, considering the potential catastrophic impacts on fish, wildlife and other instream beneficial uses" should the reservoir fall too low to pump any water into the system during the summer, the agency wrote in its order approving the slashed flows.

The immediate practical effect of the lower flows will fall on the cities that draw water from the Russian River. Water managers in both Healdsburg and Ukiah say they are not worried yet because they have alternative wells, but the city of Cloverdale is scrambling because it relies entirely on wells recharged by the river.

Cloverdale City Manager Paul Cayler plans to update the City Council next week and ask members to approve an ordinance authorizing a system of mandatory water conservation measures later in the month, a first for the city. He hopes to have that ordinance in place by early spring, ready to force curtailed water use if the drought continues.

Water supplies further south on the Russian River are in good shape for the moment, the Water Agency says, because the much larger Lake Sonoma reservoir remains at around 70 percent capacity. That should give communities south of Healdsburg about a year's worth of capacity even if the conditions remain dry.

To stretch that out, however, the Water Agency plans to roll out an unusual mid-winter ad campaign next week urging sharp voluntary water conservation measures by consumers in southern Sonoma and northern Marin counties.

In the cities north of Lake Sonoma that rely on Lake Mendocino, the Water Agency is hoping to convene a first-ever summit meeting of local water managers to discuss coordinated conservation efforts, Jeane said. It hopes to set that meeting in the next several weeks.