Cougar Reservoir goes deep
The U.S. Army Corps of Engineers is performing a "deep drawdown" of Cougar Reservoir to test a possible method of improving downstream fish passage past Cougar Dam.
The Corps is lowering the reservoir’s surface to 1,500 feet above sea level, about 32 feet below the usual minimum flood control elevation and 16 feet below the minimum water level required to operate the dam’s powerhouse.
Reservoir regulators will try to hold the reservoir at that elevation for several weeks, depending on juvenile fish counts and flood control operations.
Corps fishery biologists believe lower water levels will give juvenile spring Chinook salmon listed under the Endangered Species Act a better chance of finding the dam’s regulating outlets and passing through them safely.
"Juvenile salmon generally prefer to swim in the top 20-30 feet of the reservoir. Cougar Dam’s regulating outlets are at 1,478 feet above sea level," said Greg Taylor, aquatic stewardship supervisor for the Corps’ Willamette Valley Project. "So this operation should put many more fish closer to the dam’s most efficient passage route."
This Cougar drawdown is not as deep as those the Corps conducts at Fall Creek Reservoir, where water levels are lowered all the way to the historical streambed.
"This operation may make the river look and act a little differently, but downstream residents will probably not notice much out of the ordinary," Taylor said. He added that all of Cougar Reservoir’s boat ramps have been closed due to low water since mid-October, so impacts to recreation upriver of the dam is minimal.
The drawdown will not affect Cougar Dam’s ability to accomplish its flood damage reduction mission. Unseasonably dry weather that leads to lower than expected inflows to the reservoir may compel the Corps to cancel the special operation and/or use the dam’s diversion tunnel as a water passage route to ensure minimum flow targets for South Fork McKenzie River are met.