Missouri River Outlook Easing, Full Navigation Season Expected
The outlook for major flooding on the Missouri River system has eased significantly since early spring, even though recent localized heavy rainfalls have resulted in minor to moderate flooding in some parts of the system. That was the take-away from a Missouri River Basin Water Management Conference call June 4.
Doug Kluck, central region director – climate services for the National Oceanic and Atmospheric Administration, said flows were near or slightly above normal for this time of year. National Weather Service hydrologist Kevin Low said snowmelt is currently peaking, resulting in flows that were higher than normal in some tributaries that could result in minor to moderate flooding in the lower basin, which he said is “not atypical” at this time of year.
The March 15 system storage check indicated flow support for Missouri River navigation will be at least full service for the first half of the 2020 season, which began on April 1 at the mouth. Full-service flow support is generally sufficient to provide a 9-foot-deep by 300-foot-wide channel. Flow support for the second half of the navigation season, as well as navigation season length, will be based on the July 1 system storage.
Water releases from Gavins Point Dam will remain at 33,000 cubic feet per second (cfs.) in June, which the Corps said is about average. May runoff in the upper basin was about 130 percent of average, but the summer climate outlook indicates a return to warmer and drier conditions in the upper basin. The 2020 calendar year upper basin runoff forecast is 32.3 million acre-feet (maf.), 125 percent of average.
“The upper basin runoff forecast remains above average in 2020. The upper basin runoff for the remainder of the year will depend on the mountain snowmelt, which will enter the Fort Peck and Garrison reservoirs in early June, and summer rain events,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “With nearly 80 percent of the system flood control storage available, the reservoirs are well positioned to capture and manage runoff from the mountain snowmelt and precipitation in the upper Missouri River basin. This also provides the Corps with some added flexibility to respond to rainfall events below the system,” Remus said.
The June 1 forecast is in the top 25 percent of the 122 years of runoff record. Average annual runoff for the upper basin is 25.8 maf. The runoff forecast is updated monthly, more often if basin conditions warrant. As of June 4, the total volume of water stored in the system was 59.9 maf, up 1.2 maf since May 1, occupying 3.8 maf of the system’s 16.3 maf. flood control zone.
Soils are drying out in the upper Missouri River Basin, following much wetter-than-normal conditions in 2018 and 2019. The potential for flooding remains in the Missouri River Basin, particularly in the lower river, due to the potential for locally heavy rain on the many uncontrolled tributaries downstream of the Missouri River Mainstem Reservoir System.
Remus urged interested parties to remain aware of the current and forecast conditions by checking the Corps’ Missouri River Basin Water Management and the National Weather Service websites.
As of June 1, the mountain snowpack was 84 percent of average in the reach above Fort Peck and 83 percent of average in the reach from Fort Peck to Garrison. The mountain snowpack peaked in both reaches on April 16 in the Fort Peck reach at 109 percent of average and on April 19 in the Fort Peck to Garrison reach at 112 percent of average.
Weekly updates on basin conditions, reservoir levels and other topics of interest can be viewed at: https://go.usa.gov/xE6wa.