Lake Mendocino level now about 75% of what water managers had hoped

The storage level in Lake Mendocino was on pace to drop below 15,000 acre feet on Saturday, meaning a quarter of the supply water managers had hoped to keep in store by Oct. 1 already has been released.

The rapid shrinkage of the reservoir after two years of historic drought raises unsettling questions about the future for a range of consumers along the upper Russian River, whose supplies already are heavily restricted.

Meteorologists say predicted La Niña conditions are expected to produce drier than usual weather for much of central and southern California this winter, but that Sonoma County’s location in a transition zone means it has equal chances of being wet.

Those whose water comes from the lower Russian River, including more than 600,000 people supplied by Sonoma Water, draw most of their storage from Lake Sonoma, which is much larger and still held 109,157 acre feet Friday. Lake Mendocino stood at 15,008 acre feet and had been diminishing by an average 123 acre feet per day over the previous week.

An acre foot is a unit of measure equal to 325,851 gallons or enough water to flood most of a football field 1 foot deep. It would supply almost 3½ water-efficient California households for a year, according to the Water Education Foundation.

Water managers had hoped to prevent the level in Lake Mendocino from falling below 20,000 acre feet to ensure enough water was available in the event that winter brings insufficient rainfall.

The State Water Resources Control Board has prohibited hundreds of water right holders in the upper river from withdrawing water in recent months, though it’s not clear everyone has complied, given continued reductions in stream flow, particularly during the summer. Diversions in some areas, particularly around Alexander Valley and Healdsburg, actually went up during many weeks, compared to the same time a year earlier.

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