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NW Fishletter #382, June 4, 2018

[9] Montana Flood Warnings Issued While Northern Snowpack Remains High

An above-average snowpack throughout much of western Montana, northern Idaho and parts of Canada is good news for this summer's water supply throughout the Columbia River Basin. But it also had meant flooding, especially in Montana, where flood warnings were issued in early May for the Clark Fork and Bitterroot rivers near Missoula, and residents were evacuating.

The runoff isn't close to slowing, Ron Abramovich, water supply specialist for the Natural Resources Conservation Service, told the Northwest Power and Conservation Council at its meeting in Boise on May 8. He noted 70 percent of the snowpack around Missoula remains in the mountains, and has yet to melt.

Rivers in Pend Oreille, Ferry and Okanogan counties in Washington; in Bonner, Boundary, and Benewah counties in northern Idaho; and in Mineral and Missoula counties in Montana were under flood warnings on May 10, according to the National Weather Service. "Major flooding is occurring," the NWS website said of the Clark Fork River.

"We're going to need some cool weather to slow down the melt," Abramovich told the Council two days earlier. "This is just low valley snow that's melting now."

He said forecasts for variable temperatures in the coming weeks could help prevent the mountain snowpack from melting suddenly, and instead allow for multiple peaks in this spring's runoff. The snowpack on May 1 ranged from about 130 to 180 percent of normal at many sites in Montana, and above 180 percent of normal in some, he said.

In any case, it's likely to mean an extended runoff season, with river levels remaining high for a longer period of time from Montana into Idaho.

The healthy winter precipitation came immediately after 2017's high water-supply year, which left reservoirs and aquifers recharged. "We could get by with a below-normal snowpack," Abramovich noted. Instead, some areas, like the Clark Fork, are pushing record-high snowpack for the 30-year period from 1981 through 2010.

He said those recharged reservoirs mean irrigators from the Snake River basin, the Salmon Creek basin, and even into the Owyhee River basin where snowpack was well below average, have a 90 percent chance of an adequate irrigation supply this summer.

Abramovich's Council presentation also noted that precipitation in the coming weeks and months has a big impact on runoff, but was not included in his streamflow forecasts. -K.C. Mehaffey

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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