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As Flooding Hits Parts Of Basin, Water Supply Forecasts For Spring, Summer Dropping
Posted on Tuesday, April 09, 2019 (PST)

As of April 3, water supply forecasts at Columbia, Snake and Willamette river dams continue at a below normal levels, but, with rains and warmer temperatures, flooding is now an issue in some Willamette river tributaries.

 

Even with the recent rain, water supply forecasts at Columbia basin dams continue at below normal levels (1981 – 2010), as they have for most of this year.

 

In Montana, the April forecast at Hungry Horse Dam on the Flathead River in Montana is 1.560 million acre feet, April to August, just 81 percent of normal, a drop from 90 percent of normal as of March 6. The forecast at Libby Dam on the Kootenai River, April to August, is 4.752 MAF, also 81 percent of normal, a drop from 93 percent of normal in March. Both are storage dams.

 

The April water supply forecast at Dworshak Dam on the North Fork Clearwater River, April to July, is 1.964 MAF, 81 percent of normal. The storage dam was at 88 percent of normal in March.

 

Also in the Snake River system, the water supply forecast at Lower Granite Dam, April to July, is 18.783 MAF, 95 percent of normal. The forecast in March was 99 percent of normal.

 

The April forecast in the upper Columbia River at Grand Coulee Dam, April to August, is 47.853 MAF, 84 percent of normal compared to 87 percent in March.

 

Nearby on the Pend Oreille River, Albeni Falls Dam forecast, April to August, is 10.234 MAF, 83 percent of normal. March’s forecast was 84 percent of normal.

 

The resulting balance for the Columbia River at The Dalles Dam, April to August, is 75.577 MAF, 86 percent of normal. As of March 6 the forecast was 88 percent of normal.

 

The Portland District U.S. Army Corps of Engineers is reporting this week that reservoirs at its Willamette Valley dams are nearly full and that is forcing controlled water releases at most of the dams.

 

The flows include water release at historic rates from Dorena Reservoir, and the Corps has worked in partnership with Lane County Emergency Management, which has issued evacuation notices for those in the Row River Floodplain and the Coast Fork Floodplain of the Willamette River, a Corps news release says.

 

"We are managing flows in a way to protect the downstream public from flood to the extent we are able, and to maintain space in the reservoirs to avoid uncontrollable releases," said Ross Hiner, Portland District Dam Safety Program Manager. “However, inflows are so high that we are having to increase our releases to minimize the overall flooding risk."

 

These releases mean that the Corps is maximizing reservoir storage within safe limits, the Corps said

 

Prior to this rain event, below-average precipitation in the Willamette Valley contributed to low reservoir levels throughout the system of dams, while snow levels in the basin were above average.

 

"We are currently experiencing an atmospheric river coupled with melting snow," said Salina Hart, Portland District Chief of Reservoir Regulation and Water Quality.

 

In the Willamette River, the Corps said its reservoirs are able to capture a portion of the flood waters in order to reduce downstream flows, but will not be able to eliminate all of its impacts.

 

Water managers expect levels in the Willamette Valley to be at flood stage now through the middle of the week at several gauges, including Goshen and on the main stem of the river at Harrisburg and Albany. Additionally, forecasts show Salem near flood stage Wednesday.

 

A flood stage will result in water inundating areas that are not normally covered by water, and teams at the Portland District are running forecasting models multiple times a day in conjunction with the Northwest River Forecast Center, the Corps said. The models are used to assess forecasted inflows to Willamette Valley reservoirs and flows from unregulated tributaries to plan reservoir outflows for downstream flood risk management. The models are rerun with updates to the forecast and the latest information on projected operations is shared with local emergency managers.

 

Residents in affected areas should follow the instructions of local emergency managers as unregulated flows from tributaries continue to contribute to water levels.

 

For the most up to date official river flow information, refer to the Northwest River Forecast Center athttps://www.nwrfc.noaa.gov/rfc/

 

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The Columbia Basin Bulletin, Bend, Oregon. For information or comments call 541-312-8860.
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