Lakes Sonoma, Mendocino exceed seasonal water supply levels

A strong start to the rainy season has filled the two main reservoirs that serve Sonoma County beyond their seasonal supply levels, giving local officials some reason to hope the region might see its wettest winter in years.

As of Monday, Lake Sonoma stood at 100.3 percent of its water supply capacity for this point in the year, while Lake Mendocino was at 120.1 percent of its targeted supply level. Above-average rainfall over the last few months has filled the reservoirs, which together help provide water to some 600,000 North Bay residents, to levels not seen this early in the season since 2012, said Brad Sherwood, spokesman for the Sonoma County Water Agency.

“So far, it’s been pretty darn perfect,” Sherwood said of the rainy season. “There’s been this nice pattern where you’ll have heavy rainfall and then you’ll have a break. The sun will come out and allow the basin to drain, so no severe flooding, and then another system will come in. So you have this nice pattern that’s been set up, which is exactly what we like to see.”

With Lake Mendocino now well beyond its targeted supply level, the Water Agency is hoping to hold on to as much of the extra water as possible.

Sherwood described the capacity of Lake Mendocino like a “two-layered cake,” with the lower portion used for water supply and the top portion reserved for flood control. When the reservoir is filled beyond its targeted supply level, the Army Corps of Engineers can release extra water into the Russian River.

But the Water Agency on Dec. 16 asked the Army Corps to keep more than 5,800 acre-feet of water beyond its targeted supply level through mid-March. An acre foot of water is about enough to cover a football field 1-foot deep.

Sherwood said the Army Corps held back about the same amount last year, providing a “tremendous asset” during the summer that helped the reservoir get into “great shape” going into the fall season.

The Water Agency should receive a response from the Army Corps sometime in January, Sherwood said.

The request comes as much of the region has experienced rainfall well above average so far this season. For example, since Oct. 1, Santa Rosa has received 18.42 inches of rain - more than 150 percent of average rainfall for this time of year, according to the National Weather Service.

Most of Sonoma County is now among the roughly 15 percent of California that’s experiencing no drought conditions at all, according to the U.S. Drought Monitor.

Sherwood was optimistic a “great, wet winter” would continue but cautioned that water resource officials will want to see strong rainfall continue through March.

“We’re always mindful that in 2012, at this exact same time period, we were in somewhat of a similar situation, and then the spigot turned off,” Sherwood said. “That’s why I say it’s great that we have a wet winter now, but what’s really critical are those late winter, early spring rainfalls.”

Santa Rosa can expect more wet weather as soon as Friday, which will bring a chance for some light rain, according to meteorologist Steve Anderson of the National Weather Service. Anderson forecast a low of 29 degrees this morning, thanks to continued cold air from Canada, but he expected low temperatures rising into the 30s later in the week.

You can reach Staff Writer J.D. Morris at 707-521-5337 or jd.morris@pressdemocrat.com. On Twitter @thejdmorris.