Environmental

Corps facing another challenging water year in Willamette Basin

Corps facing another challenging water year in Willamette Basin

The U.S. Army Corps of Engineers said that despite substantial help from recent rain and snow events, as of April 21 the Willamette Valley reservoir system in Oregon was 53% full, which is 33% below the rule curve.

The Portland District operates 13 dams in the Willamette River basin. Each dam contributes to a water resource management system that provides flood risk management, power generation, water quality improvement, irrigation, fish and wildlife habitat and recreation for the Willamette River and many of its tributaries. Nine of the Willamette Valley dams generate hydroelectricity. Eight of these facilities are owned and operated by the Corps of Engineers, and one is a private facility licensed by the Federal Energy Regulatory Commission.

Corps water managers say they are bracing for another challenging year as they work to refill the 13 reservoirs for the upcoming conservation season.

The Willamette Valley Project reservoirs depend on spring and early summer rainfall to refill, and minimal precipitation is making it challenging to fill multiple reservoirs. In addition, the Corps did not begin to refill Fall Creek Reservoir until the middle of April, and it won’t begin refill of Cougar and Foster reservoirs until May to improve downstream passage conditions for juvenile salmon and comply with a court order.

“Extended drought conditions, variability of snowpack in the Cascades, and modified dam operations to support downstream passage of juvenile salmon are impacting refill of the system,” said Erik Petersen, Willamette Valley Project operations project manager. “This will be a tough water year with less overall system storage, limiting our supplies to augment for water quality and fisheries habitat downstream during the summer and fall.”

Diminished water storage during the summer months means that reservoirs with more water may drop in elevation faster than normal as the Corps draws from them to meet downstream needs.

Year-to-date precipitation across the Willamette was 96% of normal, as of April 21. The snowpack is 128% of median for the Willamette. Snowmelt helps keep reservoir elevations up in the summer if it lasts and matches outflows, but it only accounts for less than 10% of the system’s storage.

The Corps manages reservoir inflows based on a water control diagram, more commonly called the “rule curve,” which is the authorized maximum elevation on a given day to balance flood risk management and storage for other authorized purposes, such as recreation, hydropower, and irrigation supply. The Willamette Valley System’s reservoirs are kept lower in the winter to reduce downstream flooding and are refilled in the spring to prepare for recreation and adequate flows for fish and water quality.