Below average runoff forecasts for upper Missouri River Basin continue
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 acre-feet (MAF), 46% 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% 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% 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-to-date information.
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% of average, and as of June 1, 11% of the annual peak remained. The Garrison reach peaked at 79% of average, and as of June 1, 24% of the annual peak remained. On average, the mountain snowpack melts out in both reaches around July 1. The mountain snowpack graphics can be viewed at: https://go.mil/mr-mtn-snowpack.
Reservoir Forecasts:
- Fort Peck Dam
- Average releases past month – 5,800 cfs
- Current release rate – 7,000 cfs
- Forecast average release rate – 7,000 cfs
- End-of-May reservoir level – 2223.2 feet (up 0.3 feet from April 30)
- Forecast end-of-June reservoir level – 2223.4 feet
- Notes: Releases will be maintained at 7,000 cfs through September.
The forecast reservoir releases and elevations discussed above are not definitive. Additional precipitation, lack of precipitation or other circumstances could cause adjustments to the reservoir release rates.
Hydropower:
The six mainstem power plants generated 696 million kWh of electricity in May. Typical energy generation for May is 790 million kWh. The power plants are expected to generate 7.2 billion kWh this year, compared to the long-term average of 9.3 billion kWh.
To view the detailed three-week release forecast for the mainstem dams, go to https://go.mil/mr-3wk-fcast.
