Below-average precipitation and dry soil conditions persist in the upper Missouri River Basin above Sioux City, Iowa (upper Basin).
The updated 2021 upper Basin runoff forecast is 17.9 million acre-feet (MAF), 69% of average. If realized, this runoff amount would be in the 22nd driest year in the upper Basin since 1898. The May upper Basin runoff was 64% of average. May runoff in the Fort Peck and Garrison reaches, where much of the upper Basin runoff from mountain snowmelt originates, was 60% and 68% 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 U.S. Army Corps of Engineers’ 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:
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% of average, while the mountain snowpack in the Fort Peck to Garrison reach peaked in late April at 96% of average. Mountain snowpack normally peaks near April 15. The mountain snowpack graphics can be viewed at: http://go.usa.gov/xARQC.
System storage is 55.2 MAF, 0.2 MAF more than last week. Upper Basin runoff in May was about 60% of average Mountain snowpack has peaked in both reaches and is starting to melt (lower right quadrant). Runoff is expected to increase slightly during June. The latest drought outlook (upper right quadrant) indicates that drought conditions are expected to persist in the upper Basin in June. As per the Master Manual, Gavins Point releases are being made to meet full-service navigation flow targets. The runoff forecast of 17.9 MAF is 69% of average. The monthly studies indicate that drought conservation measures of reduced navigation flow support will be enacted on July 1, 2021.
Image credit US Army Corps of Engineers.
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