Missouri River Runoff Declines In April, Navigation Effects Expected In Fall

Very dry conditions in April resulted in much lower runoff in the upper Missouri River Basin and could lead to navigation restrictions in the second half of the year, the Corps of Engineers reported. The upper Basin runoff was 44 percent of average, which was the ninth driest April in 123 years of record.

The updated 2021 upper basin runoff forecast is 17.8 million acre-feet (maf.), 69 percent of average, which, if realized, would rank as the 22nd lowest calendar year runoff volume.

“The extremely dry April, current drought conditions and below-normal mountain snowpack has led our office to significantly lower the 2021 calendar year runoff forecast,” said John Remus, chief of the Corps of Engineers’ Missouri River Basin Water Management Division.

“Based on this forecast, the May reservoir monthly studies indicate reduced flow support for navigation during the second half of the navigation season and a 12,000 cfs. Gavins Point winter release rate,” he said. “I urge all water users, particularly intake owners, to begin preparing for the possibility of lower river levels later this summer and during the fall and winter.”

System storage is currently 55.3 maf., 0.8 maf. below the base of the Annual Flood Control and Multiple Use Zone. System storage is expected to remain in the Carryover Multiple Use Zone during 2021. 

Mountain snowpack in the upper Basin has peaked, and melting is underway. The mountain snowpack peaked above Fort Peck in late March at 86 percent of the normal peak, while the mountain snowpack in the Fort Peck to Garrison reach peaked in late April at 96 percent of average. Mountain snowpack normally peaks near April 15.

Navigation

Gavins Point Dam releases will provide full-service navigation flow support at all four target locations (Sioux City, Iowa; Omaha, Neb.; Nebraska City, Neb.; and Kansas City, Mo.) through July 1. Full-service flow support, in combination with the Bank Stabilization and Navigation Project, is generally sufficient to provide a 9-foot-deep by 300-foot-wide channel, the Corps said.

Flow support for the second half of the navigation season, as well as the navigation season length, will be based on the actual July 1 system storage. If the May 1 runoff forecast is realized, navigation flow support would be lowered below the full-service levels, to an intermediate-service level, for the second half of the season, and the season length would be a full eight-month season.