Below Average Runoff Forecasts For Upper Missouri River Basin

Runoff continues to be below average in the Missouri River Basin above Sioux City, Iowa. Below-normal precipitation, dry soil conditions in the western portions of the basin, and an overall lack of mountain snowpack resulted in a May runoff of 1.6 million acrefeet, 46 percent of average.

“The dry weather conditions persisted across the upper basin during May,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “The absence of snow in the plains and limited precipitation has caused the soils to dry out leading to abnormally dry or drought conditions in 80 percent of the basin. Mountain snowpack peaked below normal and is melting earlier than usual. As a result, the runoff forecast later this summer and fall has been reduced.”

The 2026 calendar year runoff forecast above Sioux City is 15.5 MAF, 60 percent of average, and 1.6 MAF lower than last month’s forecast. If realized, the annual runoff will be the 11th lowest runoff in the period of record from 1898. The runoff forecast is based on current soil moisture conditions, mountain snowpack, and long-term precipitation and temperature outlooks.

The volume of water stored in the Missouri River Mainstem Reservoir System is currently 48.7 MAF, 7.4 MAF below the top of the carryover multiple use zone.

“Given current reach runoff and basin conditions, we expect runoff in June to be below normal, but it will depend on rainfall,” said Remus. Basin and river conditions continue to be monitored, including soil moisture and drought conditions, and System regulation will be adjusted based on the most up-todate information.

Navigation: Gavins Point Dam releases will continue to provide navigation flow support at 5,500 cfs below full service for the first half of the navigation season, which began April 1 at the mouth of the river near St. Louis, Missouri. The service level was based on the total volume of water stored within the System on March 15, in accordance with the guidelines in the Master Manual. Flow support for the second half of the navigation season, as well as navigation season length, will be based on the storage in the System on July 1. The current runoff forecast suggests if conditions persist, navigation flow support will be reduced to the minimum service level, and the season length will be shortened.

Mountain Snowpack: The mountain snowpack peaked on March 16 for both reaches, earlier than the normal peak which occurs around April 17. The Fort Peck reach peaked at 77 percent of average and, as of June 1, 11 percent of the annual peak remained. The Garrison reach peaked at 79 percent of average and, as of June 1, 24 percent of the annual peak remained. On average, the mountain snowpack melts out in both reaches around July 1. Mountain snowpack graphics can be viewed at https:// go.mil/mr-mtn-snowpack.