Upper Basin runoff forecast continues to be below average
Below-average precipitation and dry soil conditions persist in the upper Missouri River Basin above Sioux City, Iowa (upper Basin).
The updated 2021 upper Basin runoff forecast is 17.9 million acre-feet (MAF), 69 percent of average.
If realized, this runoff amount would be in the 22nd driest year in the upper Basin since 1898.
The May upper Basin runoff was 64 percent of average.
May runoff in the Fort Peck and Garrison reaches, where much of the upper Basin runoff from mountain snowmelt originates, was 60 percent and 68 percent of average, respectively.
“Per our June 1 upper Basin forecast, we expect runoff to continue to be well-below average through the summer and fall,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division.
“The June 1 reservoir studies indicate the navigation service level, based on the July 1 System storage check, will be reduced by approximately 1,000 cubic feet per second (cfs) for the second half of the navigation season,” he said. “The studies also indicate that the winter release from Gavins Point will be at minimum levels, which is 12,000 cfs.”
System storage is currently 55.2 MAF, 0.9 MAF below the base of the Annual Flood Control and Multiple Use Zone. System storage is expected to remain in the Carryover Multiple Use Zone during 2021.
Mountain snowpack in the upper Basin was below the June 1 average and is melting rapidly.
The mountain snowpack peaked above Fort Peck in late March at 86 percent of average, while the mountain snowpack in the Fort Peck to Garrison reach peaked in late April at 96 percent of average.
Mountain snowpack normally peaks near April 15.
The mountain snowpack graphics can be viewed at: http://go.usa. gov/xARQC.
Gavins Point Dam releases will provide full-service navigation flow support at all four target locations (Sioux City, Omaha, Nebraska City, and Kansas City) through July 1.
Full-service flow support, in combination with the Bank Stabilization and Navigation Project, is generally sufficient to provide a 9-foot-deep by 300-foot-wide channel.
Flow support for the second half of the navigation season, as well as the navigation season length, will be based on the actual July 1 System storage.
If the June 1 runoff forecast is realized, navigation flow support would be lowered about 1000 cfs below the full-service levels for the second half of the season.
The season length will be a full 8-month season.
Winter Release Rate
The winter release rate is determined based on the Sept. 1 System storage.
Per the reservoir studies, if the June runoff forecast and reservoir studies are realized, the winter releases from Gavins Point Dam will be at the minimum rate of 12,000 cfs.
Gavins Point Dam
Average releases past month: 29,400 cfs
Current release rate: 29,500 cfs
Forecast release rate: 30,000 cfs
End-of-May reservoir level: 1206.5 feet
Forecast end-of-June reservoir level: 1206.0 feet
Notes: The Gavins Point release will be adjusted to provide full-service navigation flow support on the lower Missouri River through July 1, the first half of the navigation season.
Fort Randall Dam
Average releases past month: 25,800 cfs
End-of-May reservoir level: 1355.2 feet (up 0.3 foot from April 30)
Forecast end-of-June reservoir level: 1355.2 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: 25,300 cfs
Forecast average release rate: 25,100 cfs
Forecast reservoir level: 1420.6 feet
Average releases past month: 26,000 cfs
Forecast average release rate: 25,000 cfs
End-of-May reservoir level: 1605.9 feet (down 0.5 foot from April 30)
Forecast end-of-June reservoir level: 1606.2 feet
Average releases past month: 21,900 cfs
Current release rate: 22,000 cfs
Forecast release rate: 22,000 cfs
End-of-May reservoir level: 1835.2 feet
Forecast end-of-June reservoir level: 1837.0 feet
Notes: Releases will be maintained at 22,000 cfs through August.
Fort Peck Dam
Average releases past month: 8,400 cfs
Current release rate: 9,500 cfs
Forecast average release rate: 9,500 cfs
End-of-May reservoir level: 2233.0 feet
Forecast end-of-June reservoir level: 2233.1 feet
Notes: Releases will be maintained at 9,500 cfs through August.
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.
The six mainstem power plants generated 832 million kWh of electricity in May.
Typical energy generation for May is 797 million kWh.
The power plants are expected to generate 8.8 billion kWh this year, compared to the long-term average of 9.5 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.
Visit http://go.usa. gov/xE6fC for more information.