SIOUX CITY -- Runoff into the Missouri River's upper basin continues at levels well above average. Water releases from Gavins Point Dam remain above average, too, and will continue to be so for much of November.
Even winter releases from the dam will be higher than normal.
By keeping those releases high, the Missouri River Mainstem System of six dams and reservoirs should be able to evacuate all of 2018's runoff and begin the 2019 spring runoff season on March 1 with all of its flood storage capacity available, U.S. Army Corps of Engineers officials said at a public meeting Wednesday afternoon.
The corps has been challenged to meet that goal because of runoff from abnormally high mountain snowpack this spring and rainfall totals throughout the spring, summer and fall that have reached record levels in some locations in Montana and the Dakotas. Runoff totals have been above average each month this year, said Kevin Grode, a member of the corps' Missouri River Basin Water Management Office.
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"This has been a very high runoff year," Grode said at Sioux City's Stoney Creek Hotel & Conference Center, where corps representatives discussed 2018 runoff and the Annual Operating Plan for the upcoming year with about three dozen people.
The corps has forecast 2018 runoff into the upper river basin above Sioux City at 41.4 million acre feet, 163 percent of average and the third highest runoff total in 120 years of record keeping. The section of river beneath Gavins Point Dam from Yankton, South Dakota, to Sioux City saw the highest-ever recorded runoff for the months of September and October, Grode said.
Data presented Wednesday showed snowpack in the northern Rocky Mountains was 137 percent to 141 percent of normal this year and peaked later than the April 15 average. That snow melts and eventually makes its way into the Missouri River in Montana and North Dakota.
As of Monday, 59.3 MAF of water was being stored in the system's reservoirs, corps data showed. That water occupied 3.2 MAF of the system's 16.3 MAF flood control zone. To get the system storage down to 56.1 MAF by March 1, the corps will continue releases of 58,000 cubic feet per second from Gavins Point Dam near Yankton through much of November.
By late November, Grode said, only 1 MAF of water should remain in the flood control zone. The corps will reduce Gavins Point releases to 20,000 cfs by Dec. 11 and keep them at that level throughout the winter to evacuate the rest of the water from the flood control zone. Average winter releases are 17,000 cfs.
A weak El Nino pattern in the Pacific Ocean could lead to a warmer, drier winter in Montana, Wyoming and the Dakotas, said Mike Gillespie, a National Weather Service hydrologist from Sioux Falls. Closer to Sioux City, winter temperature and precipitation outlooks call for an equal chance of above average or below average conditions, Gillespie said.
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