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March Precipitation Gives Most of Basin a Boost

by Barry Espenson
Columbia Basin Bulletin - April 18, 2003

March precipitation that in some areas of the Columbia-Snake river basin totaled as much as 200 percent of average did not erase an overall moisture deficit, but it did provide some flexibility for those who will be doling out water in the coming months for irrigation, power generation and to help migrating fish.

A mid-month forecast developed by the federal Northwest River Forecast and released Thursday estimates the January-July runoff as measured at The Dalles Dam would total 87 million acre feet of water. That's still only 81 percent of the normal runoff amount, but it's huge leap from the NWRFC's March "early bird" forecast published Feb. 27 that predicted runoff at The Dalles would be 73 percent of normal for the period.

The Snake River got an infusion too in March. The March early bird forecast predicted t that with normal precipitation for the remainder of the season, the January-July runoff would be 73 percent of normal as measured at Lower Granite Dam on the lower Snake River. That forecast is now 80 percent of normal or would be 24 million acre feet.

"Really, the spring that we're having, the rains and cool weather, is great," said Hal Anderson, administrator for the Idaho Department of Water Management's planning and technical services Division.

The rains that most recently have been soaking up depleted soils across the state were preceded by plenty of March snow. The middle part of the state, in particular, benefited. The water content in the snowpack of the Clearwater and Salmon river drainages rose from 81 percent of average on March 1 to 96 percent of average on April 15.

"We were worried about Dworshak" filling, Anderson said of the central Idaho reservoir fed by the Clearwater. The reservoir is a summertime recreational haven, as well as a pool of water called on by fish managers to increase and cool flows for the migrations of salmon and steelhead to and from the Pacific Ocean.

The Snake River snowpack above Palisades grew slightly, from 83 percent of normal on March 1 to 86 percent on April 1.

The extreme southern part of the state was missed by the March storms, leaving drainages such as the Bruneau, Salmon Falls, Goose and Raft at only 55 percent of normal on April 15. The Owyhee and Malheur drainages of southern Idaho and Oregon were also deprived with 52 percent of their average snowpack water content as of this week. Idaho's Weisesr, Payette and Boise river drainages, on the other hand, have snowpacks at 95 percent of average now.

For most of March a more normal westerly flow pattern dominated the north Pacific, according to a NWRFC summary. The pattern produced much above average rainfall and snow accumulations across the Columbia-Snake basin.

"Of particular note, the northern areas of the basin, where two thirds of the flow for the Columbia River at The Dalles comes from, is the wettest and has the greatest snow accumulations," the summary said.

March precipitation was: 200 percent of normal (1971-2000) at Columbia above Coulee, 134 percent of normal at the Snake River above Ice Harbor, and 175 percent at Columbia above the Dalles, the summary says. The area above Grande Coulee Dam includes that part of Washington, northern Idaho, northwestern Montana and the British Columbia Part of the basin.

"It definitely helped us in an area we needed it," Nancy Stephan said of the March downpour above Coulee. Stephan is supervisory meteorologist for the Bonneville Power Administration, which markets power produced in the federal Columbia/Snake power system. That area above Coulee normally provides about 60 percent of the Columbia's down stream flows with 28 percent coming from the Snake above Lower Granite Dam and 12 percent from the Cascade Mountains' east slope.

"And we got a lot of it in snow," Stephan said of the precipitation March precipitation. The National Resource Conservation Service reported that the snowpack above The Dalles grew from 74 percent of average on March 1 to 86 percent of average on April 1. The Snake River snowpack above Ice Harbor Dam took the biggest leap, 14 percent to 94 percent of average.

Many the NRCS' automated "SNOTEL" sites reported precipitation of 200 percent of average in the Idaho panhandle and the Clearwater, Flathead, and Clark Fork basins. Sites in the Salmon, Boise/Payette, Upper Snake, Yakima, and northeastern Oregon basins reported over 150 percent of average precipitation.

As a result, the Kootenai River Basin snowpack increased 17 percent over the March 1 composite, the Pend Oreille increased 19 percent, the Spokane increased 12 percent, the Yakima increased 11 percent, the Salmon increased 13 percent, and the Clearwater River Basin increased a whopping 24 percent. Snowpack increases of about 4 to 8 percent were common in other watersheds of the Columbia Basin. Only the central Oregon and Kettle snowpacks did not increase from last month.

"It's spotty as you go around to the different basins," the NWRFC's Tom Fero of occasional holes in the "much improved" Columbia basin water forecast.

"It gets wetter as you go north," Fero said.

As of April 15, some of those northern snowpacks were in decent shape. The Bitterroot (106 percent of normal water content) and Upper Clark Fork River basin (100 percent) were the only two drainages in the Columbia basin at or above normal. Others were close -- the Kootenai in Montana at 94 percent and the Flathead at 89 percent. The Idaho panhandle region and lower Clark Fork basin each boasted 78 percent of their average snow pack water content this week, up 13 and 16 percent respectively since March 1.

In Washington snowpacks are also much improved with the Columbia tributaries above the Methow River at 98 percent of normal. The Chelan, Entiat, Wenatchee basins have snowpack water content of 78 percent of average, actually down 5 percent since March 1. The Yakima, Ahtanum snowpack is 85 percent of normal.

The Grande Ronde, Powder, Burnt and Imnaha drainages of northeast Oregon saw their snowpack's water content climb from only 64 percent March 1 to 80 percent this week. Among the poorest snowpacks on April 15 were the Umatilla, Walla Walla and Willow basin snowpacks (65 percent) and Deschutes, Crooked and John Day (63 percent).

Also in poor shape were the drainages that feed into the Hood River, and then the Columbia (56 percent) and the Willamette (57 percent). Both were much improved from March 1 snowpacks of 12 percent and 22 percent of average.

The March deluge brought the seasonal (October through March) precipitation up to 89 percent of normal on the Columbia above Coulee, 94 percent of normal at the Snake River above Ice Harbor, and 93 percent at Columbia above the Dalles.

Snow accumulations increased markedly at mid and upper elevation snow sites, while warmer temperatures and snowmelt has reduced low elevation snow accumulations.

Rainfall and snowmelt increased flows basinwide during March. Most northern areas had streamflows in the average to above average range, while southern Idaho basins fall in the 60 to 90 percent range. Lowest March streamflow was in Eastern Oregon and on the main stem of the Snake River from Milner to Weiser at 30 to 50 percent, according to the NWRFC summary.

Related Sites:
NWRFC streamflow forecasts:

Barry Espenson
March Precipitation Gives Most of Basin a Boost
Columbia Basin Bulletin, April 18, 2003

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