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Columbia Basin April-August Runoff Through Dalles Dam Likely To Be At 30-Year Average
Posted on Friday, March 11, 2011 (PST)

The snowpack in the mountains that ring the Columbia-Snake river basin has -- through a boisterous La Nina winter -- continued to store a spring-summer water supply that appears likely to be near the 30-year average.


And that’s good news for both fish and humans, since nine of the past 10 years have been below average, though 2002 and 2008 came close with 99 percent of the normal runoff flowing past The Dalles Dam on the lower Columbia River. Runoff for the April through August period in 2006 was 103 percent of average. All of the runoff from the mid- and upper Columbia and from the Snake passes through The Dalles Dam.


The latest “final” monthly forecast issued by the NOAA Weather Service’s Northwest River Forecast Center Monday predicts that the most likely scenario this year is that runoff will again be at 99 percent of the 1971-2000 average, 92.3 million acre feet. The Feb. 9 final was also a forecast of 99 percent of average runoff past The Dalles from April through August. That was up from a Jan. 7 final that forecast 97 percent of normal.


The latest forecast employed observed precipitation data through February and snow-water equivalent totals through March 1. It also assumed precipitation across the basin in March would be 110 percent of normal. The NWRFC updates its primary forecasts three times a month into July.


The water supply forecasts have held a relatively steady course so far this winter, but its strengths have shifted as storms continue to pound different areas within the basin. The snow-making storms often missed the upper Columbia, and particularly the area that is north of the Canadian border, during the early part of the winter and battered the southern reaches such as the Snake headwaters.


But all that changed after a relatively dry end to January. More snow dropped in the upper Columbia, and the Snake basin and much of the Cascades got shorted.


The end result is that much of the basin is expecting something close to average runoff, though many snowpacks are benefiting from a series of storms that have strafed the basin just before, and since, the forecast was completed.


There are a few sweet spots of particular interest to salmon managers and reservoir users. The in-flow to Libby Dam’s reservoir in northwest Montana is expected to be 6.55 MAF or 105 percent of normal for the April-August period, which is much improved from the January final forecast of 93 percent.


Libby’s water supply, in addition to generating electricity, is used to supplement flows for Kootenai River white sturgeon in north Idaho and for salmon and steelhead in the lower basin. The Kootenai eventually flows north into British Columbia, from whence it came, and joins the Columbia.


In west-central Idaho, the North Fork of the Clearwater River watershed is expected to produce 109 percent of its average runoff from April through September, according to the March 7 NWRFC forecast. That’s up from a 100 percent of average prediction made in early January.


The North Fork fills Dworshak Dam’s reservoir. In addition to providing recreation and power generation, Dworshak’s cool water is a valuable tool for cooling the tepid Snake in late summer to make it more hospitable for migrating salmon. The North Fork flows into the Clearwater, then the Snake and finally the Columbia.


A U.S. Army Corps of Engineers forecast released early this month for Libby predicts April-August runoff of 7.105 MAF, which would be 112 percent of the 1929-1999 average. The Corps forecast for Dworshak is 3.329 or 124 percent of average. The Corps operates both dams.


The upper Columbia forecast overall for the April through September period as measured at central Washington’s Grand Coulee Dam is 66.1 MAF or 103 percent of normal, according to the new NWRFC forecast. That’s up from a 94 percent of normal prediction made in January.


February precipitation was 115 percent of normal (1971-2000) in the portion of the Columbia basin above Grand Coulee, 75 percent in the Snake basin above Ice Harbor Dam and 98 percent of normal above The Dalles, according to the NWRFC. That brought season (October through February) precipitation totals to 111 percent of normal above Grand Coulee, 115 percent of normal above Ice Harbor and 111 percent of normal above The Dalles.


Snow-water equivalent accumulations were near normal in February, with the exception of western Montana and the north and central Idaho, where SWEs increased by 10 to 30 percent.


The overall SWEs in the Kootenai River basin in Montana were at 117 percent of average through March. 9, which is a big improvement from a snowpack at 105 percent of average Feb. 15. Elsewhere the lower Clark Fork River basin SWE jumped from 105 to 114 percent of average, according to the National Resources Conservation Service’s Columbia River Basin SNOTEL snow/precipitation Update Report. The Idaho Panhandle snowpack increased from 87 percent of average Feb. 15 to 100 percent through March 9.


Many of the lowest, as a percentage of average, snowpacks have also seen improvement in that timeframe. The Chelan-Entiat-Methow SWE rose from 72 percent to 84 percent of normal and the Yakima-Ahtanum increased from 72 to 86 percent. Both are in central Washington fed by the Cascades.


Snake River reporting sites in southern Idaho show a range of SWE’s from 111 percent of normal (Snake above Palisades) to 81 percent (Big and Little Wood basins). The Owyhee-Malheur SWE rose from 84 to 98 percent of normal and the Grand Ronde-Powder-Burnt-Imnaha climbed from 75 to 88 percent of normal over the past month. All feed into the lower Snake.


Central Oregon’s Deschutes-Crooked-John Day SWE increased from 81 percent of normal Feb. 15 to 94 percent of normal this week.


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