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NW Fishletter #383, July 2, 2018

[10] Some Columbia Basin Flows Drop, Prompting Irrigation Curtailments, Drought Plans

May's rapid runoff combined with a lack of precipitation has resulted in forecasts for well-below-normal flows in some areas. Irrigators in the Yakima River basin are preparing for possible curtailment of water allocations, while the U.S. Bureau of Reclamation is releasing operations and drought plans to affected irrigators in the Klamath Project area.

Jeff Nettleton, Reclamation's Klamath Basin Area office manager, said in a news release that a lack of snowpack and legal obligations to mitigate for coho salmon will require water conservation and careful management of irrigation this year.

"On May 1, everybody's forecasts were very, very optimistic, especially for the mainstem Columbia," said Scott Pattee, water supply specialist for the Natural Resources Conservation Service. "Then we had a rapid runoff, so everybody kind of backed off their forecast, but it still wasn't outside the normal range of 80 to 100 percent" of normal, he said.

Pattee noted that rivers with dams were able to hold back some of the water. "But even then, in the Yakima, on June 1, they thought they were going to have plenty of water; and then we had a dry and warm June, so they had to start drafting off their reservoir. That's what put them into curtailment," Pattee said.

The significance of May's hot weather is seen in the U.S. Bureau of Reclamation's spill report to U.S. District Court, noting that May's runoff on the Columbia River at The Dalles Dam was 176 percent of the 30-year average, at a volume of 44.6 million acre-feet. On the Snake River at Lower Granite Dam, it was 149 percent of the 30-year average, at 10.3 million acre-feet.

On June 27, NOAA's Northwest River Forecast Center's 120-day forecast showed a wide range of predicted flows on the Columbia and Snake rivers and their tributaries. The Clearwater River at Spalding is predicted to flow at 157 percent of the 30-year average, while the Walla Walla River near Touchet had forecasted flows of just 28 percent of normal. Many other flow forecasts, however, are still relatively close to normal, including the Columbia River at Grand Coulee Dam, which is forecast to flow at 87 percent of normal, and at Bonneville Dam at 93 percent of normal; while the Snake River from Lower Granite to Lower Monumental dams is predicted to have flows at 105 percent of normal.

In early June, the Forecast Center predicted that river flows would be below average in June and July, because so much of the mountain snow melted off in May. "We had a really warm and a really dry May," Ryan Lucas, a hydrologist for the center said in a June 7 webinar. His maps showed that almost the entire basin recorded average temperatures between May 1 and May 31 that were at least 3 degrees above normal, and often more than 6 degrees above normal.

This means many locations that still held well above average snowpack on May 2 were reduced to close to average snow on June 1. Sites above the Flathead River in Montana dropped from 161 percent of normal on May 2 to 115 percent of normal on June 1, Lucas noted. The Snake River above Palisades dropped from 125 percent to 102 percent of normal during the same timeframe.

At a Technical Management Team meeting on June 5, the Bureau of Reclamation's Joel Fenolio reported that Hungry Horse Dam saw its highest inflow volume by about 200,000 acre feet in May, beating the last record set in 1928.

But flows may still return to normal in August and September because aquifers have been fully recharged, and because the snowpack that started out significantly above normal in some areas.

In addition, total water supply for this water year--from April through September--is still predicted to be above normal in most parts of the basin, with more than twice as much water coming down the Clark Fork River above Missoula, with forecasts at 218 percent of normal. -K.C. Mehaffey

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