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Warm Temperatures Bring Spring Flooding In Parts Of Upper Columbia Basin; 15 Rivers At Flood Stage
Posted on Friday, May 11, 2018 (PST)

Warm temperatures have triggered spring flooding in parts of the Columbia Basin, particularly in watersheds below mountains with heavy snowpack.


The U.S. Army Corps of Engineers issued press releases this week related to flooding hot spots including Montana’s Clark Fork River Basin, Idaho’s Pend Oreille River below Albeni Falls Dam, and on the Okanogan River in Washington.


The Northwest River Forecast Center in Portland reports that 22 rivers are reaching their banks, 15 are at flood stage, and four are at major flood stage. Those include the Clark Fork River above Missoula and at St. Regis, Mont.; the Okanogan River near Tonasket, Wash., the Chehalis River in Washington; and the Pend Oreille River exceeding flood stage below Albeni Falls in coming days.


Warm temperatures combined with above-average snowpack have created a situation where Pend Oreille River flows downstream of Albeni Falls Dam are likely to exceed flood stage this week.


The U.S. Army Corps of Engineers is releasing the full capacity of Albeni Falls Dam between now and when Lake Pend Oreille reaches the peak elevation for the year.  The normal maximum elevation for Lake Pend Oreille during the summer is between 2,062 and 2,062.5 feet above sea level. The lake could rise higher than normal summer elevation.


“Corps modeling using National Weather Service River Forecast Center hydrologic forecasts suggest we could see peak Lake Pend Oreille elevations ranging between 2,062.5 and 2,066 feet sometime between May 19-29, and outflows exceeding 100,000 cubic feet per second by May 11,” said Logan Osgood-Zimmerman, Upper Columbia Senior Water Manager for the Army Corps of Engineers.


Lake Pend Oreille was lowered to the bottom of the one foot band at 2,051 feet above sea level before spring inflows began to rise. Albeni Falls Dam transitioned to free-flow April 29 and will remain on free-flow until further notice.


Albeni Falls Dam outflows are now completely controlled by the natural constriction between Lake Pend Oreille and the dam; outflows will vary gradually as the lake level varies. This free flow operation will allow Albeni Falls Dam to release the maximum flow possible, based on the natural constriction. The operation will reduce the eventual peak lake level, to the extent possible. It will also conserve space in Lake Pend Oreille, reducing the likelihood of releasing even higher flows downstream during the peak of the snowmelt event. Inflow to Lake Pend Oreille has been on a continued upward trend, as the warming spring weather increases snowmelt.


Meanwhile, flooding is well underway on the Clark Fork River, and some of its major tributaries, including the Blackfoot River.


A Corps of Engineers team is assisting the City of Plains and Sanders County, Montana, with implementing emergency protective measures to reduce the risk of scour on the 1948 levee at Plains. Work is estimated to conclude May 14. The team is providing technical assistance in Missoula at the request of Missoula County, Montana, with some of those efforts directed at the upper Blackfoot Basin near Lincoln, Mont.


The Corps’ Seattle District initially provided 25,000 sandbags to Missoula, with an additional 75,000 sandbags delivered later this week.


About 20,000 sandbags were sent to Okanogan County, 5,000 to Tonasket, Wash., and about 15,000 sent to the Colville Confederated Tribes.


Flood fight teams are coordinating with the States of Washington, Idaho and Montana. Agency representatives will continue to coordinate with the counties and cities along the rivers, and the Colville Confederated Tribes.


The Northwest River Forecast Center shows rivers currently at or exceeding flood stage within the next few days to include:


The Bitterroot River near Darby, Victor and Missoula, Mont.; the Blackfoot River near Bonner; the Chehalis River near Grand Mound, Wash.; the Clark Fork River above Missoula and downriver at St. Regis, Drummond and Plains; the Flathead River at Columbia Falls; the Methow River and the Similkameen River near Nighthawk in Okanogan County, Wash.; the Stehekin River in Chelan County, Wash.; the Moyie River in Idaho; the Okanogan near Tonasket; the Pend Oreille River below Albeni Falls Dam and on Lake Pend Oreille near Hope, Idaho; the St. Joe River at St. Maries, Idaho; the Swan River near Bigfork, Mont.; and the Yaak River near Troy, Mont.


Private citizens seeking sandbags should contact their local government offices. The National Weather Service issues flood watches and warnings and should be consulted for that information. 


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