The U.S. Army Corps of Engineers is reviewing public comment submitted on its plan to build a downstream fish-passage facility at Cougar Dam in the upper Willamette River basin, about 42 miles east of Eugene, Ore.
A final environmental assessment on the proposal is expected out later this year, followed by a “finding of no significant impact” decision in 2020, and the start of construction in 2021 if a FONSI is issued.
A 2008 biological opinion identified required actions to avoid jeopardizing ESA-listed Chinook in the Willamette River, including providing either structural modifications or major operational changes for downstream fish passage at Cougar Dam on the South Fork McKenzie River. That population of Chinook is considered to be at moderate risk of extinction, and the Corps considered 30 alternatives for downstream passage.
According to the assessment, the Corps is proposing to install a floating surface screen fish collector, and use trucks to haul trapped fish downstream from the dam. The design will also accommodate other species, including cutthroat, rainbow and bull trout. Fish would be transported at least once a day, and as many as eight times a day, depending on fish collection numbers.
Alternatives were evaluated based on biological efficiency, constructability, environmental impact, operation and overall cost, the assessment says. The proposal would result in negligible to moderate impacts due to construction, and would result in improvements to survival of fish that spawn above the dam.
Cougar Dam is a 25-MW high-head hydroelectric project on the McKenzie River, and one of 13 dams operated by the Corps within the Willamette Project. It restricts access to historical spawning and rearing areas for ESA-listed Chinook salmon.
The dam’s construction included both upstream and downstream facilities, but both were deemed inadequate and eventually closed. The Corps has transported hatchery-origin adult Chinook above the dam since 1993, and began transporting natural-origin adult Chinook above the dam in 2010, when an adult fish-collection facility was completed.
A downstream passage facility is expected to boost returns, the analysis says.