Missouri Basin Runoff Forecast Increased

The Corps of Engineers has increased its Missouri Basin runoff forecast for 2018 to 30.2 million acre feet (maf.).

The forecast has been increasing each month this year; in March, the projection was for 29 maf. The average annual runoff is 25.3 maf.

“The updated forecast increased slightly from last month due to the continued accumulation of mountain and plains snowpack in the upper basin,” said John Remus, chief of the Missouri River Basin Water Management Division. “Based on the current plains and mountain snowpack and precipitation outlooks, runoff into the Fort Peck and Garrison reservoirs is expected to be above average from March through August.”

For comparison, the 2017 runoff was 29.6 maf., 117 percent of average.

As of April 1, the mountain snowpack was 127 percent of average in the reach above Fort Peck and in the reach from Fort Peck to Garrison. Normally, the mountain snowpack peaks in mid-April, the Corps said.

Navigation Season

In mid-March, Gavins Point releases were increased from 20,000 to 24,000 cubic feet per second (cfs.) in support of the Missouri River navigation season, which began April 1 near St. Louis, Mo.

“Because of the higher-than-average runoff forecast, the service level was increased 10,000 cfs. above full service to provide beneficial use of the excess runoff while reducing flood risk,” said Remus.

Although the service level was increased, releases from Gavins Point ranged from 22,000 to 24,000 cfs. during late March to reduce flood risk along the lower Missouri River while flows in tributaries downstream of Gavins Point remained high.

As tributary flows recede, releases from Gavins Point will be increased until they reach the expanded navigation support levels. Gavins Point releases will be adjusted, if needed, in response to basin conditions, the Corps said. When necessary, the Corps will reduce releases from the system projects and utilize the available flood control space in the reservoirs, in order to lessen downstream river levels; however, the ability to significantly reduce flood risk along the lower Missouri River diminishes at locations further downstream due to the large uncontrolled drainage area and the travel time from Gavins Point Dam.

The Missouri River mainstem reservoir system began the 2018 runoff season at the base of the annual flood control pool, providing the full 16.3 maf. of flood control storage to capture runoff. System storage was 58.3 maf. as of April 1, occupying 2.2 maf. of the 16.3 maf. flood control zone. “More than 85 percent of the system’s flood storage remains available to capture runoff from the remaining plains snowpack and spring rainfall as well as the runoff from the mountain snowmelt,” Remus said.

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