Drought conditions — particularly in the Missouri River Basin above Sioux City, Iowa (upper basin) – have led the U.S. Army Corps of Engineers to set winter releases from 132.3-MW Gavins Point Dam at 12,000 cubic feet per second, as part of overall water conservation measures.
“Reservoir inflows in August were much lower than average. We expect below-average inflows into the system through the rest of 2021,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division. “Several significant rainfall events occurred in August. In response, the Corps reduced Gavins Point releases 4,000 cfs as tributary flows downstream of the system increased. As the tributary runoff subsides, the Gavins Point releases will be adjusted to meet navigation flow targets.”
Despite localized heavy rainfall events throughout the basin during August, drought conditions expanded across the basin due to exceptionally dry soils. August runoff in the upper basin was 54% of average. The 2021 calendar year forecast for the upper basin, updated on Sept. 1, is 14.7 million acre-feet (MAF), 57% of average. Average annual runoff for the upper basin is 25.8 MAF.
As of Sept. 2, the total volume of water stored in the system was 52.1 MAF, which is 4.0 MAF below the base of the system’s flood control zone. System storage is expected to continue to decline through the fall. Updated reservoir studies indicate that system storage is expected to be more than 8.0 MAF below the base of flood control at the start of the 2022 runoff season.
According to the National Drought Mitigation Center, drought degradation is occurring across much of the upper basin. As of Sept. 2, about 83% of the Missouri River basin is experiencing some form of abnormally dry conditions or drought. The seasonal drought outlook, which extends through the end of November, shows drought conditions persisting across most of the upper basin.
The July 1 system storage check indicated navigation flow support for the second half of the season (July 1 to Dec. 1) would be at 1,500 cfs below the full-service level for a full eight-month-long season. Gavins Point releases will be reduced in a stair-step manner to winter release rates around Nov. 22.
In addition, releases from 185.3-MW and 517.8-MW Garrison dams will be reduced to their fall rates in mid-September. Beginning Sept. 16, Fort Peck release will be stepped down from 9,500 cfs to 5,000 cfs. The release reduction will result in a Missouri River stage reduction of about 2.5 ft at Wolf Point and Culbertson, Mt. The Garrison release will be reduced from 21,000 cfs to 20,000 cfs on Sept. 6 and then stepped down to 13,500 cfs in mid-September. The release reduction from 21,000 cfs to 13,500 cfs will result in a Missouri River stage reduction of about 2 ft at Bismarck, N.D.
The six mainstem power plants (adding in Oahe, Big Bend and Fort Randall) generated 965 million kWh of electricity in August. Typical energy generation for August is 1,012 million kWh. The power plants are projected to generate 8.7 billion kWh of electricity this year, compared to the long-term average of 9.5 billion kWh.