About the Cougar Dam Adult Fish Collection Facility

Picture of the Cougar Dam temperature control towerProblem: The U. S. Army Corps of Engineers built Cougar Dam on the South Fork McKenzie River in the 1960s. Biologists estimate the habitat above the dam once supported more than 4,000 returning adult spring Chinook. However, water released from the dam was cooler than pre-dam river temperatures in the spring and summer and warmer in the fall and winter. These temperatures resulted in reduced migration of adult fish and early emergence of juveniles.


Solution: The Corps modified the dam’s intake tower by adding adjustable weir gates and a wet well. The new features allow dam operators to selectively draw water from various depths of the reservoir and mix it to a temperature more closely matching pre-dam downriver conditions.

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Total cost of the project from preconstruction work in 1999 to completion in 2005 was $50.5 million. The first phase of the construction began in June 2000 when the diversion tunnel used to redirect the river during initial dam construction was excavated, reinforced and gated. The diversion tunnel was opened in April 2003, drawing the reservoir down to about one-third its normal size to allow the south face of the intake tower to be fitted with its new wet well and adjustable weir gates.


The Corps began refilling the reservoir in January 2005. Intake tower modifications and other associated work were completed and the temperature control tower came on line in May 2005.
There are three vertical slots running almost the full length of the face of the wet well, each with a set of three overlapping weir gates. Since water temperature within a reservoir changes with increasing depth, the gates can slide up and down as needed to create openings at various levels in the lake. Water drawn in through the openings is mixed to the proper temperature in the wet well and then routed through the penstock to the powerhouse or regulating outlet downriver.

While not part of the 2008 biological opinions issued by the National Marine Fisheries Service and U.S. Fish and Wildlife Service, the facility will help the Corps and other federal agencies meet requirements to prevent harmful impacts to spring Chinook salmon and bull trout listed under the Endangered Species Act.

 Diagram showing the function of the temperature control tower

Wild adult spring Chinook salmon began returning to the South Fork McKenzie River almost as soon as the temperature control tower came on line. Returns to the new adult fish collection facility downriver of the dam since it opened in 2010 have ranged between 250 and 525. We expect similar returns in the future until a long-term solution is implemented to help improve survival of juvenile salmon attempting to pass the dam on their way downstream from their spawning grounds to the ocean.