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Heading Into Summer Water Supply Forecasts Across Columbia Basin Above Normal; Once Of Wettest Years
Posted on Friday, May 12, 2017 (PST)

No matter the location in the Columbia and Snake river basins, as the region heads into summer, forecasts for water supply are all above normal, driven by higher than normal precipitation and snowfall during the 2016-17 water year.

 

Overall precipitation across the basin since October 2016 has put the water year as the eighth wettest in a 57-year record.

 

And the January to July water supply forecast at The Dalles Dam on the mainstem Columbia River was the fifth wettest on record at 139 percent of normal, although that has dropped slightly in the April to August forecast to 127 percent of normal with a forecast of 111.123 million acre feet.

 

“The take-home message is that this year we’re ranking in the highest number of wet years, ever,” Doug Baus of the U.S. Army Corps of Engineers told the interagency Technical Management Team at its first meeting in three weeks Wednesday, May 10.

 

Even if below record levels, all of the Columbia River basin’s storage dams are forecasted to have above normal water supplies this year, according to the May water supply forecasts posted to the Corps’ website May 3 (http://www.nwd-wc.usace.army.mil/tmt/documents/WSF/WSF_WY17_05.pdf).

 

The April to July water volume forecast for Hungry Horse Dam, according to Mary Mellema of the U.S. Bureau of Reclamation, which operates the dam, is 1.878 million acre feet, 119 percent of normal. The dam is on the South Fork of the Flathead River in Montana.

 

On the Snake River at Lower Granite Dam, the upstream dam of the four lower Snake dams, the water supply forecast, April to July, is 29.118 MAF, 147 percent of normal.

 

Libby Dam is 8.190 MAF, April to August, 139 percent of normal. Libby is located on the Kootenai River in Montana.

 

A water supply of 2.941 MAF, 121 percent of normal, April to July, is expected at Dworshak Dam on the North Fork of the Clearwater River in Idaho.

 

Grand Coulee Dam on the upper Columbia River in Washington is expected to have a water supply, April to August, of 68.159 MAF, 120 percent of normal.

 

The water supply forecast, April to August, at Albeni Falls Dam on the Pend Oreille River in Idaho is 16 MAF, 130 percent of normal.

 

Precipitation throughout the basin was far above normal (average of annual precipitation from 1981 to 2010), especially in the Snake River basin where annual precipitation in the upper Snake River basin upstream of Hells Canyon Dam was 140 percent of normal and 145 percent in the basin above American Falls. The Payette River basin was 147 percent of normal, the middle Snake River tributaries were at 143 percent of normal.

 

In the Upper Columbia River basin, the Flathead River precipitation was 138 percent of normal and Kootenai River basin was 133 percent of normal. In Canada, the Columbia River upstream of Arrow Dam was 113 percent of normal.

 

Precipitation in the middle Columbia River upper tributaries was 118 percent of normal; the Yakima River was 112 percent of normal; and the lower tributaries in the middle Columbia River was 118 percent of normal.

 

The Columbia River mainstem precipitation, as measured both above Grand Coulee Dam and above The Dalles Dam was 126 percent of normal. Precipitation at The Dalles Dam in March was 176 percent of normal and in April it was 129 percent of normal, according to Baus.

 

Precipitation in the Willamette River basin upstream of Portland was 138 percent of normal.

 

Water Year Precipitation Tables for water year October 1, 2016 through May 9, 2017 are at https://www.nwrfc.noaa.gov/water_supply/wy_summary/wy_summary.php?tab=4.

 

The water in the basin, both rain and the melting of lower level and some higher level snow, is resulting in significant flows and spill that is causing high total dissolved gas in the tailraces of Northwest dams (http://www.nwd-wc.usace.army.mil/ftppub/water_quality/spill/201705.html).

 

Spill is generally capped when total dissolved gas exceeds 115 percent to 120 percent, but with the high flows, spill over the gas cap is unavoidable this year.

 

The 24-hour average flow at Little Goose Dam on the lower Snake River was 165,000 cubic feet per second this week on Tuesday, May 9. Some 79.7 percent of the flow, or 131.4 kcfs, was from spill, which created a TDG in the dam’s tailwater of around 128 percent.

 

Of the 429.9 kcfs of flow at McNary Dam, 300.7 kcfs or 70 percent was from spill, which on May 9 created a TDG in the dam’s tailwater of 129 to 130 percent. The Dalles Dam flow, spill and TDG were similar, but spill at the John Day Dam was a lower 194.9 kcfs or 44.4 percent spill of the total flow of 433.4 kcfs. TDG in the John Day tailwater, however, was high, between 133 and 136 percent.

 

The 6 to 10 day forecast is for continued below average temperatures, but with near normal precipitation, Baus said. Looking out over 30 days, however, there is an equal chance of normal or above normal temperature and rainfall (https://www.nwrfc.noaa.gov/climate/climate_fcst.cgi).

 

Also see:

 

--CBB, April 14, 2017, “Big Water Mainstem: Runoff Supply Forecasts Continue To Rise At Columbia, Snake River Dams,” http://www.cbbulletin.com/438713.aspx

 

--CBB, April 7, 2017, “2017 Runoff: Central Idaho’s Deadwood Summit gets 147 Inches Snow; Sees Five Times Above Normal,” http://www.cbbulletin.com/438672.aspx

 

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