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Above Normal Precip, Lower Temps Has Water Supply Forecasts Far Above Normal For Much Of Basin
Posted on Friday, April 13, 2018 (PST)

Higher than normal precipitation and lower than normal temperatures in the lower Snake and upper Columbia river basins are bringing more snow and a deeper snowpack.


The most recent April to August water supply forecast at The Dalles Dam is 103.258 million acre feet, 118 percent of the 30-year average, Doug Baus of the U.S. Army Corps of Engineers told the interagency Technical Management Team this week, and the forecast at all stations in the north and eastern Columbia River basin is far above normal.


Forecasts are made monthly by the Northwest River Forecast Center, as well as by the Corps. The latest forecasts that were completed April 4 include:


--Hungry Horse on the Southfork Flathead River in Montana is forecasted to be 2.636 MAF, 136 percent of average, April through August.

--Lower Granite on the lower Snake River in Washington is forecasted at 23.202 MAF, April through July, 117 percent of average.

--Libby on the Kootenai River in Montana is 7.189 MAF, April through August, 122 percent of average.

--Dworshak on the North Fork of the Clearwater River in Idaho is 3.040 MAF, April through July, 125 percent of average.

--Grand Coulee on the upper Columbia River in Washington is 68.482 MAF, April through August, 121 percent of average.

--Albeni Falls on the Pend Oreille River in Washington is 18 MAF, April through August, 145 percent of average.


Baus said there have been several significant rain events April 1 through April 10. Some 2.63 inches of rain has fallen in the lower Snake River so far this month, a rainfall amount that is 247 percent of normal. Flathead Lake received 1.88 inches, 212 percent of normal. The Columbia River at The Dalles received 1.15 inches, 171 percent of normal. In the lower Columbia River, rain at the Cowlitz River in Washington totaled 3.54 inches, 200 percent of normal and rain in the Willamette River upstream of Portland, Oregon totaled 3.3 inches, 202 percent of normal.


Couple the high precipitation with lower than normal temperatures and the snowpack continues to grow. For example, in the Clearwater River basin, the daily average temperature has averaged 4.7 degrees below average, Baus said.


Also at the TMT meeting on Wednesday, April 11, Steve Hall of the Corps said outflows at Dworshak Dam dropped from 10,000 cubic feet per second to 7.5 kcfs, Tuesday morning, April 10. That, he said, would drop total dissolved gas in the river to about 112 percent and at the hatcheries downstream of the dam to about 101 percent.


The Corps works with regional water managers, other agencies and tribes, and fish managers and hatcheries as the Corps reduces flood risk. That includes joint efforts to keep total dissolved gasses below the Idaho State maximum threshold of 110 percent TDG, when possible. When water is released from the dam, gasses can be absorbed into the water. High TDG levels can be unhealthy for fish.


The water surface elevation of the mainstem Clearwater River is expected to lower by about 3-4 inches during this outflow decrease, as measured at the U.S.G.S. stream gage located near Peck, Idaho, about 4 miles downstream of Ahsahka, the Corps said in a news release. Additional changes in water releases may be necessary as weather and inflow conditions change.


The Corps’ highest priority is public safety, so we operate to provide flood risk reduction benefits, while continuing our efforts working with other agencies to reduce the impact of flood operations on fish, according to Corps reservoir managers. The Corps will continue to monitor river basin conditions, weather forecasts and impacts on fish.


Also see:


--CBB, April 6, 2018, “Basin Snowpack Showing A Line Dividing ‘Haves’ To The North, ‘Have-Nots’ To The South,’”


--CBB, April 6, 2018, “Dworshak Dam Outflows Rise To Meet Flood Control Management Requirements,”

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