A federal judge is expected to decide by the end of October whether the U.S. Army Corps of Engineers can begin lowering Detroit Lake this fall, as Marion County, Salem, Stayton and a local irrigation district argue the plan could threaten drinking water for about 230,000 people. The dispute centers on a proposed series of deep reservoir drawdowns intended to help young Chinook salmon and steelhead pass downstream of Detroit Dam. The communities are seeking a preliminary injunction that would prevent the Corps from starting while their broader lawsuit proceeds, a process that could take months or years. At a hearing Tuesday in Eugene, attorneys for the Corps said lowering the reservoir would give juvenile fish access to safer passageways. When the reservoir is high, the fish may not reach those routes and can be swept into dam turbines, according to the agency. The Corps also argued that the Endangered Species Act requires it to act to protect the fish. The multiyear plan could begin as soon as Nov. 15. It calls for lowering the reservoir in winter, allowing it to refill in spring and drawing it down further the following fall. Environmental organizations including the Native Fish Society and Northwest Environmental Defense Center have joined the Corps’ defense of the plan. Local governments counter that the drawdown could stir up sediment beyond what drinking-water filtration systems can handle. Steve Elzinga, an attorney for Marion County, said the systems can tolerate only a limited amount of sediment for a limited time. Attorneys for the communities also argued that the Corps’ estimates may be too optimistic about how quickly the reservoir will refill, citing concerns about changing rainfall patterns this winter. The Corps disputes that assessment and says it has accounted for the risks. Its attorneys also warned that delaying the plan for a year may put yearling Chinook and steelhead populations in the Santiam River at risk of extinction if the agency ultimately cannot carry out its proposed approach. U.S. District Judge Mustafa T. Kasubhai said the immediate question is whether proceeding would cause “irreparable harm” to communities that depend on the lake for drinking water. His decision will address the requested pause, not necessarily resolve the broader legal challenge to the Corps’ long-term plan.