The Columbia River is expected to have normal to above normal flows this summer, while much of the Snake River will be below normal, according to a May 7 water supply forecast by NOAA's Northwest River Forecast Center.
Throughout the Columbia Basin, streams in the north and east will generally have higher volumes, while the south and west will generally have lower volumes, Ryan Lucas, hydrometeorologist for the center said in a webinar.
Water supply at Grand Coulee Dam in the upper Columbia is predicted to be 106 percent of average from April through September, but is predicted to be 89 percent of average at Lower Granite Dam on the Snake River. These merge upriver of The Dalles, resulting in a water supply in the lower Columbia River that is 100 percent of normal, an increase of 2 percent since the center's April 2 briefing.
Lucas noted that there are some pretty significant decreases in the volume forecast for southeast Washington and southern Oregon. In the Willamette River basin—where most of the runoff occurs between November and May—water supply is expected to be about 65 percent of the 30-year average from October to September, he said.
Oregon has issued a drought declaration for a handful of counties in the southern part of the state, including Curry, Jackson and Klamath river basins, he said, and Idaho has issued drought declarations for Custer and Butte counties, The Olympic Mountains and southwestern Washington are also seeing below normal flows. In southwestern Oregon, the Applegate Reservoir is predicted to have just 44 percent of its average water supply.
Lucas noted that a lack of precipitation in April is contributing to some of the shortages. Nearly the entire basin—except for the upper Snake, western Montana and the Columbia's headwaters in Canada—saw less than 50 percent of normal precipitation in April. Precipitation has lagged throughout the season, registering below normal throughout the basin since Oct. 1, he said. But snowpack remains at or above normal throughout most of the basin, except in southern Oregon and Idaho.
However, May precipitation improved water conditions in the Willamette River basin, where the U.S. Army Corps of Engineers now predicts most of its 13 reservoirs will fill by summer.
The Corps' Willamette Valley Project includes nine hydropower dams with a total capacity of just over 400 MW.
The Willamette Valley is primarily a rain-driven system, and its reservoirs combined are currently 80 percent full, a May 15 news release from the agency said. That's due largely to precipitation so far in May. According to NOAA's Northwest River Forecast Center, the Willamette River basin upstream of Portland has received 4.2 inches of rain from May 1 through May 20, which is 165 percent of average for the month. In April, it received 3.12 inches of rain for the full month, which was 57 percent of average.
At the end of April, the Corps announced it was preparing for serious water shortages and agency officials were preparing for another drought year. Now, "The Corps anticipates most reservoirs to fill at the beginning of the conservation season, facilitating recreation through the summer. A few reservoirs may not fill completely, however," the news release said.
NOAA is forecasting below-normal water supply and higher-than-average temperatures throughout Oregon over the next month. The forecast means that, even with full to nearly full reservoirs, the general lack of rain so far this year will likely result in conditions that range from abnormally dry to moderate drought through the summer.
Snowpack in the Willamette Basin was slightly above average this spring. But warm temperatures caused an early snowmelt, which brought the snowpack to less than 50 percent of average.
For this water year, precipitation in the Willamette Basin upstream of Portland since Oct. 1 is currently 72 percent of normal, with 42.3 inches compared to the 30-year average of 59.1 inches, according to the Northwest River Forecast Center. The situation is similar, or drier, in much of the rest of Oregon. The Clackamas River basin is currently 71 percent of normal, and the Rogue-Illinois basins are 55 percent of normal.
Much of western Washington is now above the 30-year average, while the Columbia River above The Dalles has 81 percent of its normal precipitation, and above Grand Coulee has 87 percent of its normal precipitation. Snake River tributaries are ranging from 71 to 105 percent of normal since Oct. 1.