Missouri River Basin Runoff Set Record In March

March runoff in the upper Missouri River Basin above Sioux City, Iowa, was a record 11 million acre feet (maf.), surpassing the previous record of 7.3 maf. set in 1952, the Corps of Engineers reported April 3.

The average March upper basin runoff is 2.9 maf..

“The record March runoff significantly altered our 2019 upper basin runoff forecast,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division.

“Our runoff forecast increased to 38.2 maf., which is 151 percent of normal. The March runoff was nearly four times the average. Runoff in the Fort Randall Dam to Gavins Point Dam reach was nearly twice the record highest March runoff. March runoff in the Gavins Point to Sioux City reach was more than that reach typically sees during a year,” said Remus.

The record high runoff in March was caused by 2-4 inches of rain falling on heavy plains snowpack, causing the snowpack to rapidly melt on frozen, saturated soils. Pool levels in the four system projects that have significant flood control storage—Fort Peck, Garrison, Oahe and Fort Randall—have all increased significantly to capture much of the runoff. The Corps planned to increase Gavins Point releases to 55,000 cubic feet per second (cfs.) by early the week of April 8. Gavins Point releases will be above average for the next several months, and possibly as late as November.

“We need to reduce pool levels in Oahe and Fort Randall over the next several weeks so that those reservoirs are in position to reduce flood risk during the spring and summer. The Fort Peck and Garrison reservoirs are well positioned to receive the runoff from the mountain snowmelt, which typically begins in May,” added Remus.

As of April 1, the mountain snowpack was 97 percent of average in the reach above Fort Peck and 93 percent of average in the reach from Fort Peck to Garrison. Normally the mountain snowpack peaks in mid-April.

Much of the plains snowpack in central North Dakota and eastern South Dakota has melted. The heaviest snowpack remains in isolated areas of north central South Dakota and south central North Dakota, where the snowpack’s liquid content, or snow water equivalency, ranges from 1 to 5 inches. Frost depth is still deep in much of the upper basin, including the areas with the remaining snowpack. The Corps is cooperating with other agencies to acquire plains snow measurements in the upper basin.

Based on the March 15 system storage check, flow support for the first half of the navigation season is full service. However, due to above-average runoff into Oahe and Fort Randall reservoirs, releases from Gavins Point have been above full-service levels to lessen the time that those projects have water stored in their exclusive flood control zones. Flow support for the second half of the navigation season, as well as navigation season length, will be based on the actual July 1 system storage. Flow support is generally sufficient to provide a 9-foot-deep by 300-foot-wide channel.

The six mainstem power plants generated 473 million kilowatt hours (kwh.) of electricity in March. Typical energy generation for March is 639 million kwh. The power plants are projected to generate 11.9 billion kwh of electricity this year, compared to the long-term average of 9.4 billion kwh.