High Runoff Continues In Upper Missouri Basin

Widespread and heavy rainfall in the upper Missouri River basin (above Sioux City, Iowa) resulted in another month of above-average runoff, the Corps of Engineers reported September 5. Precipitation during August was more than 150 percent of normal in eastern Montana, portions of North Dakota, much of South Dakota and Nebraska.

As a result, the Fort Randall and Gavins Point reaches of the Missouri River experienced their wettest Augusts on record, while the Sioux City and Oahe reaches were second and third, respectively.

The 2019 upper basin runoff forecast is 54.6 million acre-feet (maf.). If realized, this runoff total would be the second-highest runoff in 121 years of record-keeping, only surpassed by 2011 (61.0 maf.) and exceeding the 49.0 maf. observed in 1997. Accumulated runoff in the lower four reaches (Oahe, Fort Randall, Gavins Point, and Sioux City) was 27.3 maf. In each of the lower four reaches, the observed runoff exceeded the maximum annual runoff with four months of the year remaining to accumulate additional runoff. By the end of 2019, the forecasted runoff in these four reaches is roughly 30.0 maf., which exceeds the average annual runoff for the entire upper basin.

The Missouri River mainstem reservoir system storage was 65.6 maf. as of September 1, occupying 9.5 maf. of the 16.3 maf. flood control zone. All of the upper three reservoirs (Fort Peck, Garrison, and Oahe) have fallen out of their exclusive flood control zones but remain high, the Corps said.

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“As a result of the high reservoir levels and the forecasted above-average runoff for the remainder of the summer and fall, releases from all system projects will be much above average for the next several months, and possibly as late as November, to ensure evacuation of all stored flood waters prior to the start of the 2020 runoff season,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division. “System releases from Gavins Point Dam are currently 70,000 cfs., which is nearly twice the average release for this time of the year.”

Seven public meetings will be conducted throughout the basin October 22–25. The purpose of these meetings is to update the region on current hydrologic conditions and the planned operation of the mainstem reservoir system during the remaining fall months as well as present the draft plans for operating the system during 2020. Meeting times and locations will be announced when additional details become available.

The six mainstem power plants generated a record 1,540 million killowatt hours of electricity in August. Typical energy generation for August is 1003 million kwh. The power plants are projected to generate 13.4 billion kwh. of electricity this year, compared to the long-term average of 9.4 billion kwh.