Missouri River Flows Are Below Average For September
The Corps of Engineers’ Northwestern Division reported October 5 that September precipitation was well below normal in the Missouri River Basin. As a result, September runoff in the upper basin above Sioux City, Iowa, was 69 percent of average.
Since January 1, precipitation in the upper basin has been well below normal. The National Weather Service’s Climate Prediction Center is forecasting that below-normal precipitation will continue in October. The 2020 calendar year runoff forecast for the upper basin, updated on October 1, is 30.2 million acre-feet (maf.), 117 percent of average. Average annual runoff for the upper Basin is 25.8 maf.
“Upper basin runoff was below average in September. We expect runoff to be below average during the remainder of the calendar year. Lower basin runoff has been below average as well,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division. “Releases from Gavins Point Dam are being made to meet full-service Missouri River navigation flow targets,” Remus said. The navigation flow support season ends on December 1 at the mouth of the Missouri River.
As of October 5, the total volume of water stored in the system was 58.6 maf., occupying 2.5 maf. of the system’s 16.3-maf. flood control zone. System storage peaked at 61.8 maf. on July 16 and will decline during the fall. All 16.3 maf. of flood control storage is expected to be available prior to the start of the 2021 runoff season. If fall and winter runoff continues to be below average as forecasted, system storage will be about 0.8 maf. below the base of the annual flood control zone by the start of the 2021 runoff season.
According to the National Drought Mitigation Center, drought conditions continue to worsen across much of the upper basin. Widespread areas of drought classified as “extreme” are evident in Colorado and Wyoming. “Moderate to severe” drought conditions are present in large areas of Montana, North Dakota, South Dakota, Nebraska and Iowa.
The July 1 system storage check indicated flow support for the second half of the navigation season would be at least at the full-service level for a full-length eight-month flow support season. Full-service flow support is designed to work in tandem with the Missouri River Bank Stabilization and Navigation Project 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. Gavins Point releases will be reduced to winter levels beginning around November 22. The winter release rate is determined based on the September 1 system storage. Per this year’s September 1 system storage, winter releases from Gavins Point Dam will be at least 17,000 cfs.
A recent study of historical data in the Proceedings of the National Academy of Sciences has confirmed that that drought in the Missouri River Basin from 2000 to 2010 was the worst experienced by the river in 1,200 years. That report said the river has been slowly drying out since the 1950s due to steady declines in Rocky Mountain snowmelts.
