Portland, Ore. -- The start of spring signals something different to the Portland District – the halfway mark of refill season. The U.S. Army Corps of Engineers is in the process of refilling its 13 Willamette Valley reservoirs in preparation for the spring and summer conservation seasons. Refill season generally begins in February and ends in May.
System-wide, the valley is forecasted to be better off than it was in 2015, but there is a continued need for more rain. The Willamette Valley has seen a considerably dry winter, and the National Weather Service has forecasted a warm and dry spring season ahead.
Early season spring rain is crucial for the system to adequately fill before summer. The system largely depends on rainfall to drive and sustain inflows. Snowpack equates to less than 10% of the system’s storage. The valley’s snow event in late February helped boost Willamette Basin snowpack to 107% above normal, which will help sustain inflows throughout the summer conservation season.
“Right now, we’re trying to capture as much of the precipitation coming into the system, while balancing our other purposes that require downstream flows to support hydropower production, water quality, and Endangered Species Act (ESA) listed fish and related habitat,” said Erik Petersen, Willamette Valley Project operations project manager.
The systems’ reservoirs are kept lower in the winter in an effort to reduce downstream flooding and refilled in the spring to prepare for recreation and adequate flows for fish. The 13 Willamette Valley dams and reservoirs support a variety of the Corps’ authorized purposes to include flood risk management, ESA-listed fisheries habitat, hydropower generation and recreation.
The Corps manages inflows based on the “rule curve,” or the authorized maximum elevation on a given day, except during flood events, to balance flood risk and storage for authorized purposes. Cottage Grove, Dorena, Lookout Point and Hills Creek in the southern half of the valley have received more rain and are closer to the rule curve. The northern end, which includes Detroit, Big Cliff, Green Peter and Foster along the North and South Santiam River, are further behind in terms of narrowing the gap between current water levels and the rule curve.
“We remain hopeful for more rain in the valley system,” said Petersen. “An atmospheric river event, for example, could fill the system in a matter of days. This rain-driven system needs to see significantly more precipitation before May.”
For more information about water management, visit: http://www.nwp.usace.army.mil/water/.