Brownlee Reservoir levels are dropping despite higher-than-normal inflows from the Snake River. That’s because Idaho Power, under direction from the U.S. Army Corps of Engineers (USACE), is lowering the reservoir to make room for spring runoff.
At 58 miles long, the reservoir extends from Brownlee Dam in Hells Canyon upstream to Farewell Bend, Oregon. Water stored there is critical to Idaho Power’s hydroelectric system, which is the company’s largest source of electricity.
Each year, beginning in January, Idaho Power lowers the reservoir as part of the Columbia Basin regional flood control plan directed by the USACE. How far and how fast the reservoir must be drafted depends largely on mountain snowpack that feeds the Snake and other rivers in the Columbia Basin.
The preliminary drawdown requirement is 2,045 feet by the end of February, which is 20 to 25 feet below the current elevation and 32 feet below full pool. The official end of February drawdown level will be set in early February. March and April elevation targets are set later in the year as the snowpack develops. As of the second week of January, snowpack is below normal upstream of Brownlee Reservoir, which could mean a less dramatic drawdown than what was seen last year.
The drawdown comes at a time when streamflows in the mid-Snake River are higher than normal for this time of year, as upstream reservoirs with above-average carryover release water to meet winter storage targets and make room for spring runoff. The result is higher than normal flows below Hells Canyon Dam, where daily average flows are expected to range between 18,000 and 25,000 cubic feet per second through January.