The Great Kokanee Massacre: Marion County Warns of Severe Fish Mortality if Detroit Lake is Drained

SALEM, OR — Marion County has submitted two additional comment letters urging the U.S. Army Corps of Engineers to revise its Draft Supplemental Environmental Impact Statement (SEIS) for the Willamette Valley System, citing major deficiencies in the agency’s analysis of water quality, local infrastructure impacts, and fish mortality if Detroit Lake is drained below normal levels. This includes the risk of a kokanee salmon die‑off similar to the mass mortality event at Green Peter Reservoir in 2023.

In its January 13 submissions, the County emphasized that the Army Corps’ plans in the SEIS fail to fully assess and mitigate the foreseeable environmental consequences of a draining Detroit Lake. One of the letters focuses on the severe impacts to municipal drinking‑water systems, particularly slow‑sand filtration systems used by the cities of Salem and Stayton. The County warned that the fine sediment released during a “deep drawdown” that drains the lake below normal levels could cause irreversible damage to these systems, potentially shutting down potable water production for entire cities.

A second letter highlighted the severe fish mortality risks associated with draining—risks the SEIS acknowledges but does not prevent. The County pointed to the 2023 Green Peter Reservoir drawdown, where more than one million kokanee salmon were killed due to barotrauma as fish were forced through drastic pressure changes at the dam due to low water levels.

Photo: A picture from a U.S. Army Corps of Engineers press release showing some of the many kokanee salmon killed by the 2023 deep drawdown at the Green Peter Reservoir.

“The Corps’ own fisheries biologists have already documented the catastrophic consequences of deep drawdowns,” the County wrote. “The SEIS must fully account for these impacts before any similar action is taken at Detroit Lake.”

The County also noted that the Army Corps did not evaluate how large‑scale fish death could affect ESA‑listed species that rely on those fish as part of their diet, including the endangered gray wolf, which is a required component of federal environmental review.

“The loss of over a million Kokanee salmon at Green Peter wasn’t theoretical,” said Commissioner Colm Willis, Chair. “It was real, it was catastrophic, and it was preventable.

“Detroit Lake should not be the next site for a preventable kokanee massacre. The Corps should revise its plans to ensure our communities and environment are protected.”