Dry August Reduces Runoff In Upper Missouri River Basin
The Corps of Engineers reports that an unusually dry August has left Missouri River monthly flow levels below normal. During the month, the Missouri River basin, especially the western and far northern portions, received less than 25 percent of normal precipitation, offsetting higher-than-normal flows earlier in the season. If the dry conditions continue through the fall, the system storage may fall below the base of the annual flood control zone by the start of the 2021 runoff season, the Corps said.
The lack of precipitation and dry soil conditions resulted in 74 percent of average August runoff in the Missouri River Basin above Sioux City, Iowa (upper basin). The 2020 calendar year forecast for the upper basin, updated on September 1, is 30.6 million acre-feet (maf.), or 119 percent of average. Average annual runoff for the upper basin is 25.8 maf. Runoff in the upper basin during the remainder of 2020 is forecast to be below average.
“Reservoir inflows in August, particularly into Fort Peck and Garrison, were much lower than average due to ongoing warmer-than-normal and drier-than-normal conditions,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “For the remainder of the year, we expect below-average inflows in the upper reservoir reaches due to the warmer-than-normal and drier-than-normal climate outlook and the intensifying drought conditions. We will begin reducing releases from Fort Peck and Garrison in mid-September.”As of September 3, the total volume of water stored in the system was 60.1 maf., occupying 4.0 maf. of the system’s 16.3-maf. flood control zone. System storage peaked at 61.8 maf. on July 16 and is forecast to decline during the fall. All 16.3 maf. of flood control storage is expected to be available prior to start of the 2021 runoff season. If fall runoff continues to be below average, system storage may fall below the base of the annual flood control zone by the start of the 2021 runoff season.
According to the National Drought Mitigation Center (NDMC), drought degradation is occurring across much of the upper basin. Widespread areas of drought classified as “extreme” and “severe” are evident in Colorado and portions of Wyoming and Iowa. Abnormally dry conditions are affecting most of the upper basin and areas of the lower basin.
The July 1 system storage check indicated that flow support for the second half of the navigation season would be at least at the full-service level for a full-length season. Full-service flow support is designed to provide a 9-foot-deep by 300-foot-wide navigation channel from Sioux City to the mouth of the river near St. Louis, Mo. Full-service flow support will be provided for an eight-month season from April 1 through December 1 at the mouth. During a full-length season, Gavins Point releases are usually reduced beginning around November 22.
“We plan to adjust Gavins Point Dam releases throughout the fall in order to meet full-service navigation flow targets through the end of the navigation season,” Remus said. The winter release rate from Gavins Point Dam this year will be at least 17,000 cfs., based on the September 1 storage.