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Big Water Moving Through Hydro System: Involuntary Spill, Reservoirs Drafted To Prepare For Melt
Posted on Friday, April 13, 2012 (PST)

The surf’s up as Columbia and lower Snake rivers and tributaries flow with rains and runoff from bountiful snowpacks -- water that is pouring down through the system earlier and at a higher level than normal.


So-called “involuntary spill” has been called on at times in recent weeks to move water along in anticipation of the annual spring meltdown that is highlighted by the flow freshet yet to come.


“There are times when they do involuntarily spill basically due to lack of load,” the U.S. Army Corps of Engineers’ Karl Kanbergs said of nighttime and weekend periods of lower power demand. The Corps operates four lower Columbia River and four lower Snake River mainstem hydro projects.


The release of specified amounts of water through spill gates was triggered on the lower Snake April 3 and the lower Columbia April 10 as required by NOAA Fisheries’ Federal Columbia River Power System biological opinion. The goal is to provide an additional bypass route, aside from turbine and mechanical bypass, for outmigrating juvenile salmon and steelhead that are listed under the Endangered Species Act.


In high runoff times, “involuntary” spill beyond ESA requirements is often needed to pass water.


Flows are already up as storage reservoirs such as east central Idaho’s Dworkshak Dam in the Snake system and central Washington’s Grand Coulee Dam on the mid-Columbia shed water to carve reservoir space for the larger runoff ahead. That space is needed to provide flood control in the coming weeks as snowmelt increases.


The NOAA National Weather Service’s Northwest River Forecast Center in a forecast released Tuesday predicts that runoff past The Dalles Dam on the lower Columbia will total 110 million acre feet from April-September, which would be 112 percent of the 1971-2000 average and the 13th highest amount in the past 52 years.


The Dalles prediction is fortified by strong forecasts from the upper Columbia in the United States and British Columbia. Most of the water supply forecasts for British Columbia are well above average. Inflows to Mica Dam’s reservoir in the very upper part of the Columbia basin are expected to be at 116 percent of average for the April through July period when juvenile salmon and steelhead are headed to the sea.


Inflows to Libby Dam’s reservoir in northwest Montana are forecast to be 112 percent of average for April-July and the Clark Fork River above Missoula, Mont., is expected to pass 130 percent of its average annual flow. Forecasts, in the most likely scenario, for the Flathead River basin, as measured at Columbia Falls, Mont., are 109 percent of normal and 111 percent over that 30-year average on the Pend Oreille River as projected at Albeni Falls Dam in the Idaho panhandle. The Flathead and Pend Oreille waters funnel into the Columbia.


The snowpack through Wednesday in the Montana portion of the Kootenai River drainage was at 128 percent of average and had received 113 percent of its average precipitation for the season, according to data from National Resources Conservation Service automated SNOTEL measuring sites. Overall, mountain snow accumulations generally peak around the first of April.


In anticipation of heavy runoff later in the season, Lake Roosevelt is being drafted to a level of 1,220.2 feet elevation. Bureau of Reclamation dam operators are amidst a “heavy draft” of the reservoir behind Grand Coulee Dam.


“We’re drafting as hard as we can right now” to reach the April 30 flood control elevation prompted by the early April runoff forecast, said the Bureau’s John Roache. As of midnight Wednesday the reservoir was at 1,246 feet elevation.


The reservoir elevation will likely to cross the 1,228 foot elevation by about April 23. That low level prevents use of the Inchelium ferry, which hauls automobiles across the reservoir about 16 miles above Grand Coulee. Roache said the ferry would be out of operation “until probably sometime in May” when the Bureau can start refilling the reservoir.


The reservoir behind Dworshak Dam, with a forebay elevation of 1,515 at midday Thursday, is headed for an April 30 flood control elevation of 1,499.6 feet.


The April 10 forecast, based on snowpack, precipitation, runoff, soil moisture and other data through April 9, pegs runoff past Grand Coulee to be 114 percent of the 30-year average or 61.145 MAF during the April-July period. All of the British Columbia, northeast Washington, north Idaho and northwest Montana parts of the Columbia basin flow past Grand Coulee.


The runoff from the Snake River basin is expected to be less robust. The recent NWRFC forecast projects that runoff past Lower Granite Dam on the lower Snake will be 102 percent of average, about 22 MAF, for the April-July period. Low snowpacks for the Owyhee and Malheur basins bog that forecast down with anticipated flows at 53 percent of normal. Snowpack “snow-water equivalents” were at 51 percent of normal at midweek, even those basins had received 85 percent of average precipitation for the season (Oct. 1 through April 11).


The Owyhee and Malheur rivers flow into the Snake along the Idaho-south Oregon border. The Snake feeds into the Columbia in southeast Washington.


Precipitation for the season has largely been above average. In the Columbia-Snake basin above The Dalles precipitation has been 107 percent of normal Oct. 1-April 9 and 112 percent of normal above the mid-Columbia’s Grand Coulee. The Snake River basin above Ice Harbor Dam has had 101 percent of its average precipitation for the period. Ice Harbor is in southeast Washington just above the Snake’s confluence with the Columbia.


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