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March 1-19 Precip Above Dalles Dam 181 Percent Of Average; Moves Water Supply Forecast To Normal
Posted on Friday, March 23, 2012 (PST)

After a relatively slow start to the 2011-2012 wintertime snow-water collection period, Mother Nature has served up enough precipitation over the past few weeks to prompt evacuations of water behind hydro projects to assure there’s space to handle the spring meltdown that’s ahead.


A steady stream of March storms have lifted forecasts of annual spring/summer runoff to average or higher for the much of the Columbia-Snake river region.


The latest estimation from NOAA’s Northwest River Forecast Center says that in the most likely scenario, a total of 97.1 million acre feet of water will flow down from the upper Columbia and Snake rivers and flow past the lower Columbia’s The Dalles Dam from April through August.


That would be 104 percent of the 30-year average for that period. The annual average is 93.1 MAF.


The forecast issued Thursday is based on data (run-off to-date, snowpack snow-water levels, soil moisture, etc.) collected through Wednesday and incorporates the latest 10-day forecast for temperature and precipitation.


The water supply forecast has risen from a forecast issued Feb. 21 that anticipated, in the most likely scenario, that runoff past The Dalles would be 93 percent of the 1971-2000 annual average for April through August. That time period is used to calculate flood control storage requirements at dams such as Grand Coulee.


The snowpack in the mountains that ring the Columbia River basin in the United States and Canada’s British Columbia have been fortified in recent weeks by above average precipitation.


From March 1 through March 19 the area of the Columbia basin above The Dalles Dam has been 181 percent of the 30-year average, according to the NWRFC. Precipitation totals so far for the water year (Oct. 1 through March 19) have been 102 percent of average overall in the area above The Dalles in Washington, Idaho, Montana and British Columbia.


The March 1-19 precipitation in the upper Columbia above central Washington’s Grand Coulee Dam has been 221 percent of average. The Snake River basin above southeast Washington’s Ice Harbor Dam has received 181 percent of its normal precipitation in March.


“The trend is that it is going up,” the U.S. Bureau of Reclamation’s John Roache said of water volume forecasts. He told the Technical Management Team Wednesday that the Bureau would start drawing down Lake Roosevelt – the reservoir behind Grand Coulee – by at least a foot a day in order to get nearer what is expected to soon be a much lower flood control elevation target.


Based on a water supply forecast issued in early March (the so-called official final monthly forecast) the flood control goal for the end of April at the dam was set at an elevation of 1,237 feet. But given precipitation totals in recent weeks, and the latest forecasts, the flood control elevation may have to be dropped as low as 1,220 to 1,230 feet to allow space for the anticipated runoff. The elevation as of midday Wednesday was about 1,267 feet.


“We expect that level to drop significantly,” Roache said of the expected flood control elevation recalculation that will be based on the early April water volume forecast from the NWRFC.


Also to be established by the Bureau and U.S. Army Corps of Engineers is an April 10 elevation target that will seek to assure flood control downstream and allow the storage of as much water as possible in Lake Roosevelt to supplement flows in spring and summer for salmon and steelhead.


“That will be hard to achieve unless we start drafting now,” Roache told the TMT, whose membership includes representatives of state and federal agencies and tribes. The team mulls Columbia-Snake hydro operation that can be manipulated to ease migrations of salmon and steelhead that are protected under the Endangered Species Act.


The drafts at Grand Coulee and elsewhere are designed to pass enough water to reach flood control levels without resorting to spilling water, which stir up total dissolved gas levels that can be harmful to fish. Once juvenile salmon outmigrations begin in earnest, spill is prescribed at most mainstem Snake-Columbia dams to provide an alternative to mechanical and turbine passage. Salmon do not, however, pass up and downriver at Grand Coulee. They are blocked downstream at the mid-Columbia’s Chief Joseph Dam.


The increased flow through Grand Coulee turbines will be accommodated “either by cutting back [hydro generation] elsewhere or finding a market for it,” Roache said of the extra power.


Drafts are also planned at northwest Montana’s Libby and Hungry Horse dams, two other facilities that provide storage for the system. They back up the Kootenai and South Fork of the Flathead rivers respectively.


According to the most recent NWRFC forecast, flows at Libby will be 109 percent of average, 5.8 MAF, from April-August. That’s up from a Feb. 21 forecast of 93 percent of normal.


The runoff forecasts are buoyed, “particularly in the northern tier” where snowpacks in Montana and British Columbia are well above average, Roache said.


The upper Snake River region is “still behind” in terms of water volume projects though recent weather patterns have helped ease the situation in eastern Oregon and southern Idaho, according to the NWRFC’s Harold Opitz.


The recent forecast is for 100 percent of normal runoff, about 24 MAF, from April-August past Lower Granite Dam on the lower Snake in southeast Washington. That’s up from a forecast made a month earlier of 87 percent of average.


Snowpack snow-water equivalents – a key element in the water supply forecasts, have been on the rise. Kootenai SNOTEL measuring stations in Montana, as an example, reported SWE’s at 122 percent of normal through Wednesday, which was an increase from 95 percent of average reported Feb. 22. The automated snowpack measurement system is operated by the National Resources Conservation Service.


Central Idaho’s Clearwater and Salmon river systems’ snowpack was at 103 percent of average Wednesday as compared to 84 percent of average a month earlier. At Dworshak Dam on the North Fork of the Clearwater, drawdown operations will begin this week to make more space for the spring runoff, according to the Corps’ Steve Hall.


Snowpack SWEs ranged across the Columbia-Snake basin ranged from 82 percent of average in south central Idaho to 131 percent at the Lower Columbia, Hood River area, with the exception of the Owyhee-Malheur, which had 60 percent through Thursday. The Owyhee and Malheur rivers flow into the lower Snake at the Idaho-Oregon border upstream of the Hells Canyon Complex of hydro projects.


Most of the sub-100 percent readings are in southern Idaho and eastern Oregon.


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