Below Normal Runoff Trend Persists In Upper Missouri Basin
July runoff in the Missouri River Basin above Sioux City, Iowa was 2.0 million acre-feet, 61 percent of average. Runoff was below-normal to well-below-normal in the Fort Peck, Garrison, and Oahe reaches due to lack of precipitation, dry soils, and below-average mountain snowpack. Runoff was above normal in the Fort Randall and Gavins Point reaches and near normal in the Sioux City reach.
“July brought warmer and drier weather to the Missouri River Basin,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “The dry soil moisture, below-normal rainfall in the mountain regions, and below-normal mountain snowpack runoff have led to well below- average inflows into the reservoir system this summer.”
The 2026 calendar year runoff forecast for the Missouri River Basin above Sioux City, Iowa, is 15.4 MAF, 60 percent of average and nearly the same as last month’s forecast. If realized, this forecast would be the 11th lowest runoff in the period of record.
The U.S. Drought Monitor indicated 80 percent of the basin is still considered abnormally dry or in drought conditions. Drought conditions are expected to improve for much of Montana, Wyoming and Colorado by the end of October, but are likely to persist or expand in the Dakotas, Nebraska, and Iowa. System storage on Aug. 1 was 48.5 MAF, down 0.6 MAF since July 1 and 7.6 MAF below the base of the Annual Flood Control and Multiple Use zone. The monthly reservoir studies indicate System storage will continue to decline in the Carryover Multiple Use Zone during 2026. “The winter release from Gavins Point, which is based on the Sept. 1 System storage check, will likely be at the minimum rate of 12,000 cfs (cubic feet per second),” added Remus. Releases from Garrison Dam were reduced from 18,000 cfs to 17,000 cfs on Aug. 9 to better balance storage in the upper three reservoirs. Fort Peck releases will remain at 7,000 cfs through September. Basin and river conditions continue to be monitored, and System regulation will be adjusted based on the most up-todate information.
