Western Drought Reflected In Reduced Missouri River Runoff
The Corps of Engineers reported June 3 that below-average precipitation and dry soil conditions continue to persist in the upper Missouri River Basin above Sioux City, Iowa, and said the lower runoffs would continue through the summer and into the fall.
The entire western half of the United States has been affected by dry conditions. The March 18 Spring Outlook of the National Oceanic and Aeronautic Administration, which runs through June 30, had forecast drier conditions in the southwest U.S. NOAA said the dry conditions, associated with a La Niña weather oscillation and the failed 2020 summer monsoon, have contributed to “the most significant U.S. spring drought since 2013, which will impact approximately 74 million people.”
The updated 2021 upper basin runoff forecast is 17.9 million acre-feet (maf.), or 69 percent of average. If realized, this runoff amount would be the 22nd driest year in the upper basin since 1898. The May upper basin runoff was 64 percent of average. May runoff in the Fort Peck and Garrison reaches, where much of the upper basin runoff from mountain snowmelt originates, was 60 percent and 68 percent of average, respectively.
“Per our June 1 upper basin forecast, we expect runoff to continue to be well-below average through the summer and fall,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division.
“The June 1 reservoir studies indicate the navigation service level, based on the July 1 system storage check, will be reduced by approximately 1,000 cubic feet per second (cfs.) for the second half of the navigation season. The studies also indicate that the winter release from Gavins Point will be at minimum levels, which is 12,000 cfs.”
System storage is currently 55.2 maf., 0.9 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 was below the June 1 average and is melting rapidly. The mountain snowpack peaked above Fort Peck in late March at 86 percent of average, while the snowpack in the Fort Peck to Garrison reach peaked in late April at 96 percent of average. Mountain snowpack normally peaks near April 15.Gavins Point Dam releases will provide full-service navigation flow support at all four target locations (Sioux City, Omaha, Nebraska City, and Kansas City) through July 1, the Corps said. 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.
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 June 1 runoff forecast is realized, navigation flow support would be lowered about 1,000 cfs. below the full-service levels for the second half of the season. The season length will be a full eight-month season.