Deep reservoir drawdown at Green Peter Dam causes ‘barotrauma,’ death for thousands of kokanee salmon

The Oregon Department of Fish and Wildlife determined that the fish died from barotrauma, a condition caused by a rapid pressure reduction as fish pass from deep below the surface on one side of the dam to the other side near the surface level. Among divers, this decompression effect is known as “the bends.”

Kokanee salmon, which are not endangered, are particularly sensitive to the pressure change.

“We typically see dead Kokanee every year when the Corps draws reservoir levels down in the fall,” said Greg Taylor, supervisory fish biologist with the Portland District, U.S. Army Corps of Engineers. “However, we’re seeing greater Kokanee losses this year due operational changes required by a court injunction.”

At this time of year, fish would historically pass through the dam using a higher-elevation outlet called a penstock, but a 2021 court injunction outlined operations for moving fish through a lower-elevation regulating outlet this fall. The goal of this change was to improve passage through the dams for Endangered Species Act-listed juvenile spring chinook and steelhead.

“Salmon are surface-oriented fish, so we’re hoping they'll key into the increased surface outflows from the reservoir [during the drawdown] and swim downstream, where they need to go,” said Kathryn Tackley, a physical scientist at the Portland District who is working to implement injunction measures.

Officials with ODFW confirmed at least 8,000 dead kokanee, which are landlocked sockeye salmon that never leave fresh water and spawn in the tributaries of their home lake. The kokanee aren’t native to the Green Peter Reservoir.

Per the court order, Green Peter Reservoir is gradually being drawn down to 780 feet, where it will held at that elevation from mid-November through mid-December. As the reservoir is drawn lower, the difference in pressure the fish experience when passing through the dam is reduced, which will reduce the barotrauma.

Neither the fish die-off nor the reservoir drawdowns will impact the water quality for purposes such as municipal water supply.

Additional information about the kokanee deaths is available at https://www.nwp.usace.army.mil/Locations/Willamette-Valley/Green-Peter/Green-Peter-Kokanee/.