The updated 2025 calendar year runoff forecast for the Missouri River Basin above Sioux City, Iowa, continues to be below average. May runoff in the Missouri River Basin above Sioux City was 3.4 million acre-feet, 100% of average.

“Heavy rainfall in western and central North Dakota and South Dakota during May supplemented the mountain snowmelt runoff,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “May runoff in the upper basin above Sioux City was average; however, mountain snowpack is melting more rapidly than normal. As a result, the runoff forecast later this summer and fall has been reduced slightly.” 

The 2025 calendar year runoff forecast above Sioux City is 19.7 MAF, 77% of average, and slightly lower than last month’s forecast. 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 51.5 MAF, 4.6 MAF below the top of the carryover multiple use zone.

Fort Peck releases will be decreased to 9,000 cfs on June 8 and Garrison releases will be decreased to 23,000 cfs due to the lower runoff forecast.  Releases are expected to stay at those rates into September.

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.

Navigation:

Gavins Point Dam releases will continue to provide navigation flow support at 4,000 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.

Mountain Snowpack:

The mountain snowpack peaked earlier than normal on April 5 for both reaches, earlier than the normal peak which occurs around April 17. The Fort Peck reach peaked at 92% of average, and as of June 1, 16% of the annual peak remained. The Garrison reach peaked at 95% of average, and as of June 1, 22% 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: http://go.usa.gov/xARQC. 

Monthly Water Management Conference Calls for 2025:

The June 2025 monthly conference call will be held Thursday, June 5, to inform basin stakeholders of current weather and runoff forecasts and the planned operation of the reservoir system in the coming months. Presentation materials will be available via webinar. The call is intended for Congressional delegations; Tribes; state, county and local government officials; and the media. It will be recorded in its entirety and made available to the public on our website at https://go.usa.gov/xARQv.

Reservoir Forecasts:

• Gavins Point Dam
  • Average releases past month – 27,600 cfs
  • Current release rate – 30,000 cfs
  • Forecast release rate – 30,000 cfs
  • End-of-May reservoir level – 1206.6 feet
  • Forecast end-of-June reservoir level – 1206.0 feet
  • Notes: The Gavins Point release will be adjusted to provide navigation flow support on the lower Missouri River.
     
• Fort Randall Dam
  • Average releases past month – 25,400 cfs
  • End-of-May reservoir level – 1355.2 feet
  • Forecast end-of-June reservoir level – 1355.0 feet
  • Notes: Releases will be adjusted as necessary to maintain the desired reservoir elevation at Gavins Point and to back up Gavins Point releases.
     
• Big Bend Dam
  • Average releases past month – 24,500 cfs
  • Forecast average release rate – 26,800 cfs
  • Forecast reservoir level – 1420.9 feet
     
• Oahe Dam
  • Average releases past month – 24,200 cfs
  • Forecast average release rate – 26,600 cfs
  • End-of-May reservoir level – 1598.8 feet (up 1.4 feet from April 30)
  • Forecast end-of-June reservoir level – 1598.8 feet
     
• Garrison Dam
  • Average releases past month – 22,000 cfs
  • Current release rate – 24,000 cfs
  • Forecast release rate – 23,000 cfs
  • End-of-May reservoir level – 1832.6 feet (up 1.8 feet from April 30)
  • Forecast end-of-June reservoir level – 1835.4 feet
  • Notes – Releases will be reduced to 23,000 cfs on June 8 and maintained at rate through mid-September.
     
• Fort Peck Dam
  • Average releases past month – 9,100 cfs
  • Current release rate – 10,000 cfs
  • Forecast average release rate – 9,000 cfs
  • End-of-May reservoir level – 2227.7 feet (up 0.1 feet from April 30)
  • Forecast end-of-June reservoir level – 2228.4 feet
  • Notes: Releases will be reduced to 9,000 cfs on June 8 and maintained at that rate 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 829 million kWh of electricity in May. Typical energy generation for May is 789 million kWh. The power plants are expected to generate 8.4 billion kWh this year, compared to the long-term average of 9.4 billion kWh.

To view the detailed three-week release forecast for the mainstem dams, go to https://go.usa.gov/xARQB.

The Missouri Basin Web App provides links to these reports and others that are updated more frequently. http://go.usa.gov/xE6fC