August Rainfall Was Above Average In Missouri Basin
Rainfall in the Missouri River Basin above Sioux City, Iowa, was above normal for the month of August, resulting in above-average runoff; however, due to drier-than-normal conditions in previous years, the total storage in the Missouri River mainstem reservoir system remains lower than normal, the Corps of Engineers reports.
Based on the September 1 system storage, winter releases from Gavins Point Dam will be 13,000 cubic feet per second (cfs.), per the criteria in the Master Manual. This release rate is slightly above the minimum rate of 12,000 cfs.
“Rainfall in the month of August provided some welcome inflow into the upper Missouri Basin Reservoir System,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division. “Several precipitation events occurred during the month, with some areas receiving 400 percent of normal rainfall. Despite this fact, drought conditions continue, and are predicted to persist and expand throughout the basin in the coming months.”
August runoff was 1.7 million acre-feet (maf.), 121 percent of average above Sioux City. The updated 2023 calendar year forecast for the upper basin is 29.1 maf., 113 percent of average.
Average annual runoff for the upper basin is 25.7 maf.
As of September 1, the total volume of water stored in the system was 55.5 maf., which is 0.6 maf. below the base of the system’s flood control zone. System storage is expected to continue to decline through the fall. The updated reservoir studies indicate that the system storage is expected to be approximately 3.7 maf. below the base of flood control at the start of the 2022 runoff season.
The Corps said it will set releases from Gavins Point Dam to provide navigation flow support at a level 1,500 cfs. below full service at all four target locations (Sioux City, Omaha, Nebraska City and Kansas City). Flow targets may be missed to conserve water if there is no commercial navigation in a given reach.
Navigation season support will end on December 1 at the mouth of the Missouri River.