County says the federal plan threatens drinking water and the Santiam Canyon economy, while the Army Corps says its phased approach includes safeguards for downstream communities.
Marion County is asking a federal judge to block the U.S. Army Corps of Engineers from carrying out a new deep-drawdown plan for Detroit Lake, escalating a dispute over drinking water, fish passage and the economic future of Santiam Canyon communities.
The county filed a motion for a preliminary injunction Monday, Aug. 10, as part of a federal lawsuit it originally filed May 13. The latest motion seeks to stop implementation of the Corps’ plan before the first deep drawdown begins later this year.
The Army Corps finalized its new Willamette Valley dam operating plan in June. At Detroit Reservoir, the agency plans to lower the lake to 1,425 feet sometime after Nov. 1 and before Dec. 31. The reservoir could remain at that level for about two weeks before refilling.
In later years, the Corps plans to gradually lower Detroit Reservoir toward 1,395 feet, allowing juvenile Upper Willamette River spring Chinook salmon and winter steelhead to move through a lower dam outlet as part of federal efforts to recover threatened fish populations.
Marion County argues the deeper operations could stir fine sediment from the reservoir bottom, increase turbidity in the North Santiam River and put downstream drinking-water systems at risk. County officials also warn that insufficient winter rainfall could prevent the reservoir from fully refilling, affecting municipal water, irrigation, recreation and businesses that depend on Detroit Lake.
“Risking the drinking water of our communities while harming thousands of fish is patently absurd,” Marion County Commission Chair Colm Willis said.
County officials point to Green Peter Reservoir as a warning. During a court-ordered deep drawdown in 2023, the Army Corps confirmed at least 8,000 kokanee salmon deaths caused by barotrauma. Oregon drinking-water officials have also documented high turbidity at public water-system intakes associated with Green Peter drawdowns and resulting operational disruptions.
The Army Corps says Detroit will not simply repeat the Green Peter operation.
The agency said July 28 that the 1,425-foot target planned for 2026 was previously reached in 2015 and 2019 without reported drinking-water impacts. It also said water quality will be monitored in real time and the operation could be modified, paused or stopped if turbidity threatens downstream drinking-water systems.
The legal fight is especially significant for Salem. The city, which supplies drinking water to about 220,000 people, filed its own federal lawsuit July 8 challenging the Detroit plan. Salem says it has invested nearly $8 million over the past 18 months in groundwater wells, aquifer storage and other backup supplies in preparation for the planned drawdown.
The county has also cited weather uncertainty. NOAA currently gives the developing El Niño a 97% chance of continuing into early spring 2027, including an 81% chance of becoming a very strong event during October through December.
With the Corps saying the 2026 deep drawdown will not begin before Nov. 1, the court fight now moves closer to the deadline when federal operators would begin implementing the disputed plan.
