Missouri River runoff higher than usual last month
OMAHA — Higher-than-average releases from all Missouri River Mainstem System projects, including Gavins Point, will continue through the fall.
“Due to this year’s high runoff and the water currently being stored in the reservoirs, Gavins Point releases will remain near 58,000 cubic feet per second (cfs) for the remainder of the navigation season to ensure evacuation of all stored flood waters prior to the 2019 runoff season with much of that occurring before the river freezes over in the northern reaches,” said John Remus, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division.
The 2018 runoff forecast in the Missouri River Basin above Sioux City, Iowa, is 40.6 million acre feet, 160 percent of average.
The September runoff summation above Sioux City was 1.7 MAF, 151 percent of average. The September runoff above Gavins Point Dam was about 80 percent of average.
Due to heavy September rainfall, the runoff in the unregulated reach from Gavins Point Dam to Sioux City was .8 MAF, which is about eight times the average September runoff, and the highest September runoff in 120 years of record-keeping (1898-2017).
The Missouri River Mainstem reservoir system storage was 62.2 MAF as of Oct. 1, occupying 6.1 MAF of the 16.3 MAF flood control zone.
“System storage declined 2.5 MAF in September and will continue to gradually decline in October,” Remus said.
System storage peaked on July 8 at 68.4 MAF, occupying 12.3 MAF of the designated 16.3 MAF of flood control storage.
“Reservoir releases will be adjusted as necessary to provide downstream flood risk reduction and continue evacuation of stored flood water,” Remus said.
It is important to note that the ability to significantly reduce flood risk along the lower Missouri River diminishes at locations further downstream due to the large uncontrolled drainage area and the travel time from Gavins Point Dam.
Based on the Sept. 1 system storage, winter releases from Gavins Point will be at least 17,000 cfs. Based on the latest reservoir studies, there will be a 10-day extension to navigation flow support and a Gavins Point winter release ranging from 17,000 to 22,000 cfs depending on runoff.
Navigation flow support at the mouth of the Missouri River will end on Dec. 11.
The corps will continue to monitor basin and river conditions and will adjust system regulation based on the most up-to-date information.