Detroit Reservoir deep drawdown to proceed; here’s what you need to know

PORTLAND, Ore. (KTVZ) -- The U.S. Army Corps of Engineers is preparing to begin a deep drawdown of the Detroit Reservoir in late November following a decision by the U.S. District Court in Eugene. The operation will lower the reservoir water level to an elevation of 1,425 feet this fall.

The temporary drawdown represents the initial phase of fulfilling legal requirements outlined in the National Marine Fisheries Service's 2024 Biological Opinion for Detroit Dam. Fisheries scientists with the National Oceanic and Atmospheric Administration determined that lower water elevations help endangered young salmon and steelhead in the Santiam River basin navigate past the dam's lower outlets to reach the ocean.

Drawing the reservoir down to 1,425 feet is expected to take up to two weeks. The Army Corps of Engineers plans to maintain that elevation for two weeks before refilling, unless a storm event requires filling it sooner. Modeling and historical data indicate the reservoir will reliably refill to meet community and environmental needs.

The fall operation uses a multi-year step-down approach rather than lowering the water level immediately to the ultimate target elevation of 1,395 feet. By incrementally lowering the water level over several autumns, officials aim to limit the amount of previously submerged sediment exposed to river flows in a single year.

Water levels at 1,425 feet match elevations reached during previous drawdowns in 2015 and 2019, neither of which resulted in reported drinking water impacts or downstream turbidity issues. Historically, the agency reduced the reservoir level to 1,450 feet each autumn to maintain capacity for seasonal flood risk management. Current low water levels are due to seasonal drought, though water levels remain within normal ranges.

To protect downstream water quality, the agency will monitor environmental conditions before, during and after the drawdown. If drinking water emergencies occur, an active Emergency Response and Communication Plan allows the Army Corps of Engineers, the Oregon Health Authority, the Oregon Department of Environmental Quality and local water providers to modify, pause or stop the operation.

The agency will host virtual public information sessions on November, 18th, to explain the drawdown schedule, duration and coordination with state agencies regarding drinking water. Specific times for the meetings will be published within the week.