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NW Fishletter #381, May 7, 2018

[11] 'Fast and Furious' Runoff Expected In Parts Of Columbia River Basin

A good snowpack that's ripe for melting, combined with forecasts for warmer-than-average temperatures this month, will likely cause rapid snowmelt and the potential for flooding throughout portions of the Columbia River Basin, water supply forecasters say.

"It'll probably come off fairly fast and furious," Kevin Berghoff, senior hydrologist at NOAA's Northwest River Forecast Center said at a May 3 webinar. He predicted that flooding could occur in parts of the basin in the coming weeks.

That potential is not due to more snow over the winter or the possibility of significant rain, he said. Instead, it's the sudden warm weather and a saturated snowpack in many areas that could quickly melt snow that stayed frozen longer because of this year's cool spring weather.

Berghoff said several people in his office were hoping for a cooler May, which would provide for a more normal runoff, but they now know that's not likely. "It's going to be a fairly busy season," he said. "We'll probably be going 24/7 as we get into the flood situations."

Overall, water supply forecasts for April through September this year still show some parts of the basin--especially the northeast portions--significantly above the 30-year normal, and a few places that are significantly below.

In the upper Columbia, total water supply this summer will range from about 108 percent of normal at Mica Dam to 225 percent of normal on the Clark Fork River above Missoula. The Kootenai River at Libby Dam, Coeur d'Alene River at Coeur d'Alene Lake and Columbia River at Grand Coulee Dam are all hovering around 125 percent of normal, while the South Fork Flathead River at Hungry Horse Dam and the Bitterroot River above Missoula and Pend Oreille River at Albeni Falls Dam all fall at about 150 percent of normal.

Several Snake River and lower Columbia River dams will also see above-normal streamflows this summer, according to the forecasts. Jackson Lake and Palisades dams are forecast at 132 percent of normal, the Dworshak Dam should see about 120 percent of normal, and the Lower Granite Dam will get about 113 percent of normal flows. On the lower Columbia River, The Dalles Dam is forecast for about 122 percent of normal flows.

It's the middle Snake River tributaries that will see low flows this year, Berghoff said, including Lucky Peak Dam at 86 percent of normal, and Owyhee Dam at just 29 percent of normal. In Oregon, the Willamette River is expected to see about 87 percent of its normal flows at Salem.

In total, the September-through-April water year ended with above-normal precipitation in the upper Columbia and below-normal precipitation in Oregon and southern Idaho.

Basin-wide, however, total precipitation for the water year was about average, compared with the 30-year normal. The Columbia River Basin above Grand Coulee Dam ended the year at 108 percent of normal; the Columbia River above The Dalles ended at 101 percent of normal; and the Snake River Basin above Ice Harbor Dam ended at 95 percent of normal, Berghoff said.

It's significantly less precipitation compared with last year. But a cool spring has enabled many parts of the region to hold on to the snow that did fall. Several areas actually gained snowpack in April this year.

Berghoff said forecasts show very little chance of precipitation over the next 10 days, and below-normal precipitation is forecast for the next three months.

"So a lot of the runoff over the next one to two weeks is going to be due to snowmelt as opposed to precipitation," he said. And the snowmelt will be prompted by warmer-than-normal temperatures, especially over the next 10 days, but continuing through the summer. -K.C. Mehaffey

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NW Fishletter is produced by Energy NewsData.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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