Widespread and heavy rainfall in the upper Missouri River basin (above Sioux City) resulted in another month of above average runoff. Areas of Montana, Wyoming, South Dakota, and Nebraska received two to three times normal precipitation during July.
July runoff in the upper basin was 7.0 million acre feet (MAF), which is 213% of average. The average July runoff is 3.3 MAF. Runoff remained particularly high in the reaches from Garrison Dam in North Dakota to Sioux City, Iowa, which ranged between three to seven times average.
The 2019 upper basin runoff forecast is 52.9 MAF. If realized, this runoff total will be the second highest runoff in 121 years of record-keeping, only surpassed by 2011 (61.0 MAF) and exceeding 49.0 MAF observed in 1997. Total upper basin runoff through July 31 was 45.3 MAF, exceeding the total upper basin runoff in 2018 of 42.1 MAF.
System releases from Gavins Point Dam are currently 70,000 cubic feet per second (CFS), 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.
System storage was 68.0 MAF as of August 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 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.
Updates on basin conditions, reservoir levels and other topics of interest can be viewed here: http://www.nwd-mr.usace.army.mil/rcc/reports/pdfs/weeklyupdate.pdf.