Corps to lower Cougar Reservoir for temperature control tower repairs

Graham Hilson, general maintenance manager for the Corps’ Willamette Valley Project, sizes up debris that has accumulated outside of Cougar Dam’s temperature control tower. (Photo by Scott Clemans, Public Affairs Office)

Graham Hilson, general maintenance manager for the Corps’ Willamette Valley Project, sizes up debris that has accumulated outside of Cougar Dam’s temperature control tower. (Photo by Scott Clemans, Public Affairs Office)

 

By Scott Clemans, Public Affairs Office

The U.S. Army Corps of Engineers will lower Cougar Reservoir to about 80 feet below its usual winter elevation by the end of March to allow for urgent debris removal and repairs in Cougar Dam’s temperature control tower.

Cougar Dam and Reservoir are on the South Fork McKenzie River about 40 miles east of Eugene, Oregon. The Corps normally begins filling the reservoir on Feb. 1 each year, with the goal of reaching its maximum summer elevation by early May.

Recent inspections found three trash rack panels – 2 feet by 5 feet sections of heavy-duty grating that prevent debris from entering the dam’s water intake – were dislodged and lying on the floor of the temperature control tower.

Inspections also found a large amount of debris in the penstock that feeds water to the dam’s two hydropower turbine units, and in the units themselves.

Removal of the debris and repairs to the temperature control tower’s trash rack must be accomplished as soon as possible. The debris affects the dam’s ability to accomplish several of its authorized missions.

The Corps has shut down both generating units, and cannot accomplish the dam’s hydropower generation mission. Generation flows are also essential to attracting endangered spring Chinook to the adult fish collection facility immediately downstream of the powerhouse, which is a key part of the dam’s fish and wildlife mission.

“To minimize impacts to the project’s missions – particularly our fish and wildlife mission – we really want to have this work complete by April 1, before the spring Chinook migration gets underway in the McKenzie Basin,” said Greg Taylor, supervisory fish biologist for the Corps’ Willamette Valley Project.

The debris could affect operation of the dam’s regulating outlets – the alternate route for passing water through the dam – which are important to the dam’s flood control mission.

“This is an operational emergency, in that our ability to accomplish some of our missions has been impacted,” said Willamette Valley Project operations manager Erik Petersen. “However, this is not a dam safety emergency; there is no risk to the structure or to lives or property downstream.”

The repair work will delay refill of Cougar Reservoir and may result in lower than usual summer elevations.

“If we are able to start refilling the reservoir in early April and have a fairly average water year, we will be able to refill the reservoir to about 1,640 feet – 50 feet below the maximum summer elevation,” said Tina Teed, Willamette Valley Projects reservoir regulator.

That would seriously curtail availability of the reservoir’s two boat ramps, but the Corps should be able to meet its downstream flow and water temperature targets for the summer and fall, Teed added.

In comparison, the reservoir did not fill above 1,605 feet during the 2015 drought.