June Runoff Much Below Average
June runoff in the Missouri River Basin above Sioux City, Iowa was 3.3 million acrefeet, 60 percent of average. The updated 2025 calendar year runoff forecast for the Missouri River Basin above Sioux City, Iowa, continues to be below average.
“Mountain snowpack melted more rapidly than normal, and all reaches except the Sioux City reach experienced below normal precipitation during June,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division. “As a result, June runoff was less than forecast. Dry conditions are expected to continue in July.”
The 2025 calendar year runoff forecast above Sioux City is 19.1 MAF, 74 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.
System storage on July 1 was 52.1 MAF, 4.0 MAF below the top of the carryover multiple use zone. “Based on the July 1 System storage, flow support for navigation decreased from 4,000 cfs below full-service to 4,500 cfs below full-service,” said Remus. “Per the guidelines in the Master Manual, this will also result in a full navigation flow support season ending Dec. 1 at the mouth of the Missouri River. Gavins Point releases were reduced to 25,000 cfs in late June due to recent rainfall and to account for the decrease in navigation flow support.”
“The monthly study indicates that 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,” added 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 be set to provide navigation flow support at 4,500 cfs below full service at the four target locations (Sioux City, Omaha, Nebraska City, and Kansas City). The flow support season length will be a full 8-month season, ending Dec.1 at the mouth of the Missouri River.
Mountain Snowpack: The mountain snowpack melted quickly due to warmer- than-normal temperatures during the spring and early summer. By June 25, all the snow had melted in both the reach above Fort Peck and the Fort Peck to Garrison reach. The mountain snowpack peaked in the Fort Peck reach on April 5 at 92% of average, while the Garrison reach peaked on April 5 at 95% of average. Mountain snowpack normally peaks near April 17 and melts out in both reaches around July 1. The mountain snowpack graphics can be viewed at: http://go.usa.gov/ xARQC.
Reservoir Forecasts: Garrison Dam: Average releases past month – 23,100 cfs. Current release rate – 21,500 cfs. Forecast release rate – 21,500 cfs. End-of-June reservoir level – 1,835.2 feet (up 2.6 feet from May 31). Forecast end-of-July reservoir level – 1835.4 feet. Notes – Releases were reduced to 21,500 cfs on July 7 and will be maintained at that rate through mid-September.
Fort Peck Dam: Average releases past month – 9,400 cfs. Current release rate – 9,000 cfs. Forecast average release rate – 9,000 cfs. End-of-June reservoir level – 2227.1 feet (down 0.6 feet from May 31). Forecast end-of-July reservoir level – 2225.9 feet. Notes: Releases will be maintained at 9,000 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 810 million kWh of electricity in June. Typical energy generation for June is 842 million kWh. The power plants are expected to generate 8.1 billion kWh this year, compared to the long-term average of 9.4 billion kWh.
To view the detailed threeweek release forecast for the mainstem dams, go to https:// go.usa.gov/xARQB.
