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Runoff in Missouri River basin on pace for second-highest mark ever

Releases from Missouri River mainstem dams expected to remain high through November

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The United States flag and the U.S. Army Corps of Engineers flag fly at Gavins Point Dam in March 2019. (Republic file photo)

The U.S. Army Corps of Engineers says widespread and heavy rainfall in the upper Missouri River basin north of Sioux City, Iowa, resulted in another month of above-average runoff, as areas of Montana, Wyoming, South Dakota, and Nebraska received two to three times normal precipitation during July.

July runoff in the upper basin was 7 million acre feet, which is 213 percent of average. The average July runoff is 3.3 million acre feet. Runoff remained particularly high in the reaches from Garrison Dam in North Dakota to Sioux City, Iowa, which ranged between 3 to 7 times average.

The 2019 upper basin runoff forecast is 52.9 million acre feet, and if realized, this runoff total will be the second-highest runoff in 121 years of record-keeping, only surpassed by 2011 (61 MAF) and exceeding 49 MAF observed in 1997. Total upper basin runoff through July 31 was 45.3 MAF, exceeding the total upper basin runoff in all of 2018 of 42.1 MAF.

System releases from Gavins Point Dam in Yankton are currently 70,000 cubic feet per second, which is nearly twice the average release for this time of the year.

“We will maintain Gavins Point releases at this rate to continue evacuating water from the Missouri River mainstem reservoir system (System),” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division, in a statement.

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System storage was 68.0 million acre feet as of Aug. 1, occupying 11.9 MAF of the 16.3 MAF flood control zone. System storage, which normally peaks in early July, peaked at 68.5 MAF on July 20. Garrison is currently in its exclusive flood control zone while Fort Peck and Oahe are slightly below their respective exclusive flood control zones. As a result of the high reservoir levels and the forecast above-average runoff during the summer and fall, releases from all mainstem system projects will be above average for the next several months, and possibly as late as November, to ensure evacuation of all stored flood waters.

From a power standpoint, the six mainstem power plants generated 1490 million kilowatt hours of electricity in July. Typical energy generation for July is 951 million kilowatt hours. The power plants are projected to generate 13.1 billion kilowatt hours of electricity this year, compared to the long-term average of 9.4 billion kilowatt hours.

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