Driven by spring snowmelt, the Pacific Northwest saw above-average hydroelectric generation in May to keep year-to-date figures well in excess of the 10-year average.
According to information from the Northwest River Forecast Center, the much region saw below-normal precipitation coupled with above-normal temperatures during the month. During the month, much of the lingering snowpack melted into the region's streams and rivers.
Data from the U.S. Army Corps of Engineers show that total net generation at 23 hydroelectric plants across the Pacific Northwest was 20.4% higher than the year-ago level and 12.2% higher than the 10-year average as it reached 8.84 million MWh. That figure is up 4.3% from April.
Year-to-date hydropower production through May totaled 40.54 million MWh, up 15.0% versus the year-ago level and up 20.2% from the 10-year average for the period.
In the upper Columbia River Basin, the Grand Coulee Dam, the largest of the region, generated 2.78 million MWh, up 47.8% versus the year-ago month and up 28.7% versus the 10-year average for the period. The Chief Joseph Dam generated 1.04 million MWh, down 3.1% versus May 2016 and down 9.8% versus the 10-year average for the month.
May flows in the lower Columbia River Basin were also higher versus the year-ago period. Located on the Washington-Oregon border, the Bonneville Dam produced 438,671 MWh, up 7.4% versus May 2016 and 2.6% higher versus the 10-year average for the month. The Dalles Dam, 50 miles downstream, produced 672,032 MWh, up 11.6% from the year-ago month but down 0.5% from the 10-year average for May.
Looking to the south, much of the snowpack in California's Sierra mountains had also melted, but the state's reservoirs remained at healthy levels relative to historical averages.
As of June 6, Shasta Lake and Lake Oroville, two of the state's largest reservoirs, were at 95% and 68% of capacity, respectively, holding 4.3 million acre-feet and 2.4 million acre-feet of water.