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Wet, Wet Weather Finally Moves Snake River Basin Water Supply Forecast Above Average
Posted on Friday, March 30, 2012 (PST)

A recent surge of precipitation, and a forecast of more to come, the Snake River basin’s water supply forecast has become rosier, with the predicted April-August outflow pushing above 100 percent of average for the first time this season.


The Northwest River Forecast Center updates its Columbia-Snake river basin water supply forecasts at least once each week and has, pretty much since the start of the New Year, predicted that runoff from the upper Columbia would be well above the 30-year average (1971-2000) for the spring and summer months. That water, largely from accumulated snowpack, is vital during the dry months for salmon migrations, navigation, hydro power generation, irrigation, municipal water supplies and other uses.


But many of the earlier winter storms have skirted the south Idaho region that feeds and consumes the upper Snake River. Just two weeks ago, the forecast issued by the NWRFC pegged likely runoff down past the lower Snake River’s Lower Granite Dam would be at about 93 percent of normal for the April-August period.


The latest forecast, issued Tuesday and based on data through Monday, says that, in the most likely scenario, 101 percent – 23.15 million acre feet -- of the average runoff can be expected to flow through Lower Granite.


The forecasts take into account snow-water equivalents in snowpacks, overall precipitation, soil moisture, runoff to-date and other factors. And, particularly in the case of Lower Granite, other agencies and entities are consulted regarding potential irrigation needs and the state of the water storage.


In southern Idaho’s case, the snowpack SWE is generally subpar despite recent improvements. Most of the National Resources Conservation Service’s automated SNOTEL measuring sites in southern Idaho are in the 80-90 percent of average ranges. But most precipitation totals are nearer 100 percent.


“They’ve certainly been getting a lot of rain” in recent weeks in the middle Snake region, according to the NWRFC’s Harold Opitz. And water storage, primarily in Bureau of Reclamation reservoirs, is in great shape coming off a very wet 2010-2011 season.


As an example the Bureau and the U.S. Army Corps of Engineers announced Thursday that flows from Lucky Peak Dam on the Boise River will be increased today to help reduce the risk of flooding this spring which can occur with precipitation and rapidly melting snow. The increased flows are due to near normal snow pack and above normal reservoir carryover.


Current water storage in the Boise River reservoirs is about 76 percent of capacity. Flows will increase by 500 cubic-feet-per-second on March 30-31, and April 3; reaching approximately 6,500 cubic feet per second at the Glenwood Bridge gauging station.


Flows could potentially increase to higher levels in coming months as system inflows increase. A flow rate of 7,000 cfs is considered flood-stage level at the Glenwood Bridge gauge.


Precipitation in the Snake River basin above Ice Harbor Dam was at 155 percent of average March 1-26, and right at 100 percent for the season (Oct. 1-March 26). Ice Harbor is the lowermost dam on the river, located in southeast Washington just above the Snake’s confluence with the Columbia River. The Snake River plain in south Idaho’s midsection has had 126 percent of its average precipitation for March 1-26, according to data posted online by the NWRFC. The upper Snake has 108 percent of its average precipitation so far in March.


The Columbia-Snake water supply forecast as updated early this week is well above average overall. The March 27 forecast says that, in the most likely scenario, runoff past The Dalles Dam on the lower Columbia will be 109 percent of average – 101.39 MAF during the April-August period.


The forecast for the mid-Columbia’s Grand Coulee Dam is 111 percent of average, about 67 MAF.


The Kootenai River forecast, as measured at Northwest Montana’s Libby Dam, is 114 percent of average, a total of 7.1 MAF. The Kootenai flows north into British Columbia where it joins the Columbia.


The Dworshak Dam reservoir is expected to be provided 106 percent of its average water supply from April through September. West central Idaho’s Dworshak Dam blocks the North Fork of the Clearwater, which flows into the Clearwater and then the Snake.


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