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Corps Changes Flow Operations At Bonneville Dam To Reduce High Descaling Levels In Sockeye Juveniles
Posted on Friday, May 18, 2012 (PST)

The U.S. Army Corps of Engineers on Wednesday opted for an operational change at the lower Columbia River’s Bonneville Dam intended to reduce what has been a high level of descaling of juvenile sockeye salmon passing via the hydro project’s Powerhouse No. 2.

 

Descaling of examined sockeye below the dam ranged as high as 23.6 percent during the week of May 8-14 as compared to a high of 9.1 percent at the John Day Dam, the second dam upstream from Bonneville.

 

Mortality estimates for salmon during the same period were “in the 10 percent range for the past three days,” NOAA Fisheries’ Paul Wagner told the Technical Management Team Wednesday. “They are really showing signs of wear.” TMT is made up of federal, state and tribal fish and hydro managers who mull potential operational actions that might improve salmon survival.

 

The level of descaling for spring chinook, steelhead and coho juvenile migrants at Bonneville’s’ second powerhouse during the same period was 8 percent or less.

 

“If you’re going to descale a fish, you’re going to descale sockeye,” Wagner said of a species that has proven to be particularly susceptible to injury during passage through dam turbines and mechanical passage devices.

 

Salmon managers on Wednesday requested that flows through Powerhouse 2 be held within the lower range (50 and/or 25 percent) of the 1 percent range of peak power efficiency that is considered best for fish.

 

The lower the percentage within that 1 percent range means lesser flows are being drawn toward the turbines. The salmon managers’ goal is to reduce velocities in the Powerhouse 2 gatewell, where turbulence is believed to cause problems for the young sockeye. The same issue has in the past caused increased mortality of subyearlng fall chinook released in springtime into the Bonneville pool from Spring Creek National Fish Hatchery.

 

The gatewells are the portals of the powerhouse and where screens are also in place to steer as many fish as possible toward the facility’s mechanical fish bypass system.

 

The vast majority of the passing sockeye are unlisted fish from the Okanogan River basin of central Washington and southeastern British Columbia. Also passing are lesser numbers of juvenile sockeye from central Washington’s Wenatchee River and central Idaho’s Sawtooth Valley. The Idaho fish are listed under the Endangered Species Act as endangered. NOAA Fisheries is charged with protecting listed fish.

 

Wagner said the time was right to offer the sockeye some accommodation.

 

“Sockeye have a tendency to pass in a dense group,” Wagner said. And that outmigration is building, with more than 200,000 estimated passing McNary Dam early this week, and numbers climbing into the 100,000 range in recent days at Bonneville. McNary is the fourth dam upstream in the Columbia and is located just below the confluence with the Snake.

 

“The peak of the run is passing right now,” Wagner said.

 

The lowering of flows at Powerhouse 2 during high flow times, which now exist, forces a shifting of flows to Powerhouse 1 and/or to the dam’s spill gates. Spilling is a limited factor, since the Corps, which operates the dam, is obliged to maintain when possible total dissolved gas levels below prescribed water quality threshholds. Spill implants gas in the water below; turbine-passed water does little to stir up TDG.

 

The Corps on Wednesday agreed to switch operations through Monday with flows from Powerhouse 2 (as much as 30,000 cubic feet per second) for the most part shifted to Powerhouse 1 where an “open geometry” strategy (as opposed to operating within the 1 percent range) would be employed. The open geometry largely sends water through the turbines near the powerhouse’s hydraulic capacity.

 

Little testing has been done of fish condition or survival under an open geometry condition. But available information indicates young salmon fare relatively well, the Corps’ Doug Baus said.

 

“It suggests benefits,” Baus said of modeling done to evaluate the open geometry flow regime.

 

The Columbia River Inter-Tribal Fish Commission’s Tom Lorz, who represents the Umatilla Tribe at TMT meetings, said that the planned Powerhouse 1 operation seemed an attempt to keep the overall operation “generation neutral,” meaning that the generation foregone at Powerhouse 2 would be made up via the open geometry operation at Powerhouse 1.

 

Fish managers, such as the state of Idaho’s Russ Kiefer, suggested that the effects on fish of an open geometry operation are largely untested, unknown. He asked whether it might be better to operate Powerhouse 1 within that 1 percent range of peak efficiency, which is known to be most beneficial for fish.

 

That, however, would force higher flows through spillbays, which would cause TDG exceedances and also poses the specter of delaying passage of spawning adult salmon and increasing rates of spawner fallback – the phenomenon of fish that have passed the dam falling back through the spill gates and being forced to climb the fish ladder again.

 

Despite the concerns, none of the TMT members objected to the Corps’ planned operation.

 

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