May 2018 brought above-average temperatures to much of the Pacific Northwest, leading to rapid snowmelt in the mountains of Montana, Idaho and Canada that brought about well-above-average hydroelectricity production.
During a June 7 water supply briefing, Northwest River Forecast Center senior hydrologist Ryan Lucas said runoff conditions had increased across the board from April to May, mostly due to the warm weather.
That said, the government's water supply forecast predicts that not all parts of the Pacific Northwest will see above-normal runoff during the water year. Government hydrologists predict the Upper Columbia River Basin and the Upper and Lower Snake River basins will see above-normal water supply during the April-to-September runoff period, while southern areas including the Middle Snake tributaries and the Willamette River in Oregon will see below-normal runoff.
May data from the U.S. Army Corps of Engineers shows that total net generation at 23 hydroelectric plants across the Pacific Northwest was 5.0% higher than the year-ago level and 17.2% above the 10-year average as it reached 9.29 million MWh. That figure is up 28.6% from April.
Year-to-date hydropower production through May totaled 40.15 million MWh, down 1.0% versus the year-ago level but up 17.3% from the 10-year average for the period.
In the upper Columbia River Basin, the Grand Coulee Dam, the largest of the region, generated 3.07 million MWh, up 10.5% versus the year-ago month and up 39.1% versus the 10-year average for the period. The Chief Joseph Dam generated 1.24 million MWh, up 18.5% versus May 2017 and up 9.5% versus the 10-year average for the month.
May flows in the lower Columbia River Basin were mixed versus the year-ago period. The Bonneville Dam on the Washington-Oregon border produced 437,439 MWh, down 0.3% versus May 2017 but 3.1% higher versus the 10-year average for the month. The Dalles Dam, 50 miles downstream, produced 729,252 MWh, up 8.5% from the year-ago month and up 7.6% from the 10-year average for May.

