Reservoir drawdown part of fish recovery
August 22, 2012
BLUE RIVER: Salmon are now checking in at a truck stop after making their way from the Pacific Ocean to the South Fork of the McKenzie River. In the past they only got as far as the base of Cougar Dam, a 400 foot tall barrier between them and the high quality spawning habitat above it.
The U.S. Army Corps of Engineers recently completed construction of a $10.4 million permanent adult fish collection facility to help them out. It consists of a fish ladder leading from the base of the dam to a collection and sorting area. From there, adult salmon, bull trout and other resident fish species are loaded onto trucks and transported above Cougar Reservoir to release sites.
Fisheries biologists believe that reconnecting fish to the upstream waters will substantially support recovery of endangered fish populations. They estimate the habitat above the dam once supported more than 4,000 returning adult spring Chinook. This year close to 500 are expected to show up.
“We’re now putting fish up there that have gone through the system,” says Greg Taylor, a Corps fisheries biologist. Researchers are on the site daily monitoring the returns during May through October, when spring Chinook enter the South Fork. They record the size weigh and sex of the returnees as well as whether they are hatchery bred or from wild stocks. The latter are showing up in increasing numbers.
“Ultimately there is a downward trend of hatchery versus wild so they will come back,” notes Scott Clemans, a Corps Public Affairs specialist. Researchers take scale samples so, “We know who the parents were of every fish that comes back,” he said.
Temperature control of South Fork waters
The U. S. Army Corps of Engineers built Cougar Dam in the 1960s. Original construction included both adult and juvenile fish passage facilities to help move fish past the dam. However, due to downstream changes in river temperature resulting from the dam, adult fish no longer migrated to its base.
The solution, completed in 2005, is a temperature control tower that draws water from differing depths within the reservoir, mixing it to a temperature that more closely replicates pre-reservoir downstream temperatures. By mimicking natural conditions, salmon now return to the area at the same time of the year they did before the dam was constructed.
Clemans refers to the new collection facility, in combination with the cooling tower, as two parts of a three legged stool to support a complete fish lifecycle over long stretches of the South Fork McKenzie River. Part one draws the fish to the dam site. Part two gets them past it. Still missing is a safe way for juvenile fish to make their way back downstream before heading out to sea.
They have two choices – going through the turbines or out the dam’s regulating outlet. On the Columbia River, tests have shown 95% of fish passing through the turbines survive. Taylor said it’s a different situation at Cougar, where a combination of a high head dam and faster spinning turbines increases the mortality rate to 50%.
During December and January the Corps will experiment with ways of creating the “third leg” solution by dramatically increasing the drawdown of the reservoir when juvenile fish are making their move. It’s worked before when tested at Fall Creek, where the lake was completely drained, resulting in a “run of the river” situation with the dam not holding anything back. “We’ve not made that decision yet,” Clemans said. “There are some environmental concerns because it will probably be scouring out a lot of sediment from the bottom of the reservoir. When we did that at Fall Creek downriver of the dam it looked like the chocolate river in Willie Wonka’s Chocolate Factory.”
Taylor said it’s likely the Corps won’t do the full drawdown this year. “We’d like to reduce the size of the reservoir to get fish closer to the intake.”
Typically, the juveniles tend to gather at a depth of about 30 feet. With a 400 foot dam that places them pretty far away.
“We’d like to see if we can get more fish at a lower elevation – it helps them find the outlet and improves their survival,” Taylor said. “Plus this is the first time we’ll be doing this at Cougar.”
“Each project is unique,” Taylor notes. Fall Creek for instance, is a non-power project and is much smaller than Cougar Dam.
Blue River Dam not included
Although at one time there was talk of adding a temperature control tower to the Blue River Dam that is no longer in the offing. Money allocated for that work was reassigned to building the adult fish collection facility at Cougar Dam instead. “The historic returns here were much higher,” Taylor notes. The reason? Studies showed that Blue River had a natural barrier – a waterfall (now underwater) that blocked most upstream fish passage.
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