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Biologist: New fish weir installed in Foster Dam will facilitate fish travel


March 14, 2018

FISH BIOLOGIST Fenton Kahn, center, explains the workings of the dam and the new fish weir to local residents Tom Pankalla, left, and Davis Olsen, who walked over to find out what was going on.

After a two-week delay U.S. Army Corps of Engineers staffers installed a new fish weir in Foster Dam Tuesday, March 6.

It was clearly a big moment for Corps Fish Biologist Fenton Khan of the Portland office, and various others who helped design and install the giant metal sleeve that will allow young salmon and steelhead to cross the dam and proceed down the river to the ocean.

"How cool," Kahn said, smiling to himself, as he watched lake water shoot through a slot in the weir, which is designed to propel young fish farther away from the dam to a softer landing on the spillway. "That's awesome, dude."

The $400,000 project replaces a weir that provided a 200- to 250-cubic-feet-per-second of flow from Foster Lake into the South Santiam River below.

An earlier attempt to install the weir in late February ended in postponement due to a number of problems, Kahn said.

The weir is designed to help attract fish with a stream-like flow from the surface of the reservoir and make the fall from the reservoir less harmful for the fish.

The weir replaces an older one, installed in 1984 to help improve downstream passage for juvenile salmon and summer steelhead, Khan said. The Corps used the weir for about one month each year in the spring until 2012.

The National Marine Fisheries Service issued a new biological opinion in 2008 that recommended the improvement of downstream passage at Foster Dam, Khan said. The Corps has been conducting research at the dam since 2012 to determine the best way to move the fish downstream at the dam. The old weir was not very efficient at moving fish, he said.

Kahn said he worked with a structural engineer, hydraulic engineer, cost engineer "and some other folks" over several years to develop the new weir design.

Researchers sent sensor "fish" through the power generation turbines at the dam and over the weir at different pool levels to measure the forces affecting the fish as they travel past the dam. That data gave researchers on what was necessary to achieve the lowest mortality and injury rates.

Data from research at the dam informed scientists that the Corps needed to build a new weir because the existing one was not very efficient at moving fish, Khan said. Fish were still been finding their way into the turbines, and he and Corps engineers went to work designing a new weir.

WORKMEN maneuver the new fish weir as a crane llowers it into a slot in Foster Dam Road, where the weir slides into position to rest on stop logs in the dam.

"The research we did from 2012 informed this. How much flow did we need to put through, the shape of it? I told hydraulic engineer how much water velocity we wanted and he calculated how to make that shape."

Salmon, and especially steelhead, are surface-oriented fish and the motion of the water caused by the weir will encourage them to use the weir instead of the turbine inlet.

Kahn said Foster Lake has been kept about 2 feet lower than normal in recent years during the spring and fall, when young fish head downstream. He said the new weir will help avoid the need for that.

The new weir sits on three new stop logs, which help maintain pool elevation. The weir itself is 47 feet long, about 10 feet tall and 3 feet 7 inches thick. The stop logs and weir fit in slots in the spillway through an opening in Foster Dam Road.


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