A federal judge says he’ll issue a ruling by the end of October in a lawsuit that’s pitting concerns about the drinking water of 230,000 people in Marion County against efforts to save imperiled Chinook salmon.

At issue: Whether the U.S. Army Corps of Engineers will be allowed to lower reservoir levels at Detroit Lake this fall, in a move that officials in Salem, Stayton and Marion County say could churn up so much sediment that it would jeopardize local water systems.

Those communities, along with a Santiam Water irrigation district, have sued over the Corps’ plans to start regularly lowering fall water levels in the Detroit Lake reservoir so more young Chinook can survive their passage beyond its dams.

But since the complex case could take years to resolve, they’re also asking for a preliminary injunction to prevent the Corps from lowering Detroit Lake water levels as they sort through the legal issues.

In arguments before U.S. District Judge Mustafa T. Kasubhai in Eugene on Tuesday, lawyers for the Corps of Engineers defended their plan, argued that sediment won’t be too much for local drinking water filters — and said the agency is required to act under the Endangered Species Act.

While a one-year delay in the Corps’s plans to draw down Detroit Lake reservoir levels each winter might not be catastrophic, the federal agency’s lawyers argued that if it is ultimately not able to follow its plan to start lowering water levels, yearling Chinooks and steelhead populations in the Santiam River will probably go extinct.

Fish that would spawn upriver then swim downstream without human-made impediments face a challenge every year: When reservoirs are too full, they don’t swim deep enough to reach safe passageways in the dams, and so they risk getting churned and killed by the dam’s turbines in their instinctive quest to go downstream, according to the Corps of Engineers.

Lowering the reservoir allows them to access safer passage. But the Corps of Engineers’ multiyear plan calls for deep drawdowns at Detroit Lake starting as soon as Nov. 15 — allowing the reservoir to refill again in the spring, and then lowering water levels even further next fall.

It’s a plan environmental groups, including the Native Fish Society and Northwest Environmental Defense Center, have also embraced. Both have signed on to join the Corps of Engineers’ side in its legal defense.

But while attorneys for the Corps and these environmental advocates argued they’re required to take this action under the Endangered Species Act, attorneys for communities that rely on Detroit Lake for their drinking water cited a different law: the Clean Water Act.

Water filtration systems that rely on Detroit Lake can only handle so much sediment in the water for so long, Steve Elzinga, attorney for Marion County, argued Tuesday. Lawyers for Salem and Stayton joined him as he put forth a case that the Corps of Engineers’ plan does not adequately address this risk.

They said they believe sediment will be worse than the federal agency’s calculations expect. They argued that El Niño weather patterns are altering how and when rain falls this winter, meaning the Corps’ calculations are overly optimistic about how quickly Detroit Lake’s reservoir will refill. And they said they’re worried that too much sediment in the water, or too little water in the reservoir, could affect access to clean drinking water in their communities.

The Corps disagrees — and says it’s adequately planned for the risk. But arguments over the eventual fate of the federal agency’s long-term plans for Detroit Lake will likely take months, or even years, to resolve.

Tuesday’s hearing was focused on the more immediate future: Whether the Corps of Engineers can start its multiyear effort to restore threatened fish populations this fall. That will require determining if allowing that plan to proceed will cause “irreparable harm” — to communities that rely on Detroit Lake for their drinking water, Judge Kasubhai said at the end of Tuesday’s hearing. It’s a question with a precise legal meaning, and the answer will depend both on federal law and a detailed review of the evidence.

News Source : https://www.opb.org/article/2026/10/06/detroit-lake-reservoir-drinking-water-chinook-salmon/