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Cordel Stillman was on Wohler Bridge no more than five minutes Friday before two people had stopped to ask the engineer why construction crews are erecting a dam across the Russian River near Forestville.

Changes to the river, especially dams, are always guaranteed to draw attention and concern.

Stillman, whose orange safety vest lent him an air of authority, explained to his inquisitors that the dam is necessary as part of the Sonoma County Water Agency’s $12 million project that will allow migrating fish to travel more safely and efficiently past the seasonal Mirabel Dam downstream of the historic bridge.

“Oh, good,” said a woman who had stopped riding her bicycle to talk with Stillman.

She added, with regard to the big diggers working in the riverbed below, “This is every kid’s dream, to watch something like this.”

Construction of the temporary dam is the most visible aspect of the fish passage project, which stems from a 2008 biological opinion by the National Marine Fisheries Service that found that two spinning screen cylinders at the Mirabel site can trap tiny juvenile fish, including threatened salmonid species.

Stillman, the Water Agency’s deputy engineer, said the screens were designed in the 1970s to prevent fish from getting sucked up into submerged pumps that draw water from the river for filtration and delivery to 600,000 customers in Sonoma and Marin counties.

But some smaller fish can get trapped against the screens, Stillman said. He said the new screens will be spread across a wider area to reduce the speed of water flowing through them. Crews also will install a new fish ladder the little critters can use to get around an inflatable dam that is deployed in low-flow months to raise the water level and protect supplies.

The project also includes a subsurface viewing gallery for more than 3,000 students and other visitors who tour the site each year.

“It’s a big project, but a necessary one because of the biological opinion we entered into in 2008,” Stillman said.

F&H Construction of Lodi is handling the work; Sonoma County supervisors, acting in their role as the Sonoma County Water Agency board, authorized the $12,032,000 bid in June.

The California Department of Fish and Wildlife contributed $1.2 million in grant funding toward the cost of construction and an additional $250,000 for planning.

Stillman said the Water Agency is paying for the remainder of the project.

The temporary dam is being installed to back water up so that it will recharge aquifers upstream of Wohler Bridge.

The inflatable dam downstream at the Mirabel site has been lowered partway to begin the process of creating a diversion around the crews working on the new fish passage.

Water will continue to flow through holes in the Wohler Bridge dam, which is being constructed out of sheet metal.

Fish also will be able to get around the structure using a swale that opens on the river’s western bank and snakes downstream of the dam.

Friday, crews dumped large rocks into the swale to help slow the speed of the water that eventually will flow through, as well as for erosion control, Stillman said.

Stillman said the dam will remain in place until the river swells from winter rains, likely late November or early December. In the meantime, boaters will have to take their vessels around the structure, a distance of about 200 feet.