Missouri River Runoff Forecast Below Normal

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 percent 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 percent 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.

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.

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 percent of average, and as of June 1, 16 percent of the annual peak remained. The Garrison reach peaked at 95 percent 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.

Reservoir Forecasts:

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.