Russian River’s seasonal dam coming down again amid heavy rain, runoff

Two days of above-average spring rainfall in the North Bay have forced Sonoma County officials to begin deflating the seasonal dam across the Russian River, an about-face that comes less than a week after the rubber dam was fully inflated to serve the region’s drinking water system.

The move was made in anticipation of a spike in river flows over the weekend amid runoff from the latest storm system set to last through Sunday.

The higher flows, projected at up to 6,000 cubic feet per second by Sunday night - up from less than 800 cfs a week ago - could damage the inflatable dam, said Barry Dugan, a spokesman for the county’s water agency.

The decision to lower the dam was made to avoid any damage, he said.

“It is very unusual for us to do that, as is it unusual to have these storms in May,” Dugan said.

The storm has dropped more than 2 inches of rain on Santa Rosa since Wednesday, with higher amounts in the hills. The totals already have eclipsed historic averages for rainfall throughout the month of May.

The Russian River at Guerneville reached nearly 8 feet by 9 a.m. Friday, and is expected to peak at 12 feet by the storm’s end on Sunday night, according to the National Oceanic and Atmospheric Administration.

Flood stage in Guerneville is ?32 feet, but Sonoma Water’s protocol for the seasonal dam downstream of Wohler Bridge calls for lowering the dam when river flows exceed ?2,000 cfs, a mark that has already been surpassed, Dugan said.

The summertime dam, which spans 175 feet, has only been in place for about 11 days, and been fully inflated for a lesser amount of time. The dam creates a lagoon that Sonoma Water uses to fill ponds and supply groundwater that’s pumped to serve 600,000 North Bay residents

When the dam is in place, boaters are required to portage around the area.

Once river flows subside to a more manageable level, around 800 cubic feet per second, work to raise the dam can resume, Dugan said.

Rainfall is expected to return Friday night and last through Sunday, with more rain to follow next week leading into the Memorial Day weekend, National Weather Service meteorologist Anna Schneider said.

You can reach Staff Writer Alexandria Bordas at 707-521-5337 or alexandria.bordas@pressdemocrat.com. On Twitter @CrossingBordas.