Overall Runoff For September Above Average
Although overall runoff for September 2025 in the upper Missouri River Basin was above average, runoff in the Fort Peck and Garrison reaches continues to be well-below average.
“Beneficial rainfall occurred over central South Dakota and North Dakota in September resulting in wellabove average runoff into Oahe, Big Bend, Fort Randall and Gavins Point,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division.
“The above-average runoff combined with the lower releases from Gavins Point slightly improves the System storage outlook for the beginning of the 2026 runoff season,” Remus added.
September runoff was 1.3 million acre-feet, 109 percent of average above Sioux City. Upper Basin contributions from Garrison Dam to Sioux City, Iowa, made up 72 percent of the September runoff, while the reaches above Garrison Dam contributed 28 percent of the runoff.
By comparison, the Fort Peck and Garrison reaches normally contribute about 65 percent of September’s upper Basin runoff. Upper Basin runoff for October is forecast to be about 75 percent of average, due to ongoing below-average forecasts in the Fort Peck and Garrison reaches. The updated 2025 calendar year forecast for the upper Basin is 19.6 MAF, 76 percent of average. Average annual runoff for the upper Basin is 25.7 MAF.
As of Oct. 1, the total volume of water stored in the system was 50.4 MAF, which is 5.7 MAF below the base of the System’s flood control zone. System storage will continue to decline through the fall. The updated reservoir studies indicate that the system storage is expected to be 48.5 MAF at the start of the 2026 runoff season, approximately 7.6 MAF below the base of flood control.
