Heavy, widespread rains in Eastern Montana in September — more than 200 percent of normal — have helped boost runoff into the Missouri River Basin to near record levels.

The Yellowstone River at Sidney spiked up to 16,000 cubic feet per second at the end of September because of the precipitation. The flow on the river at this time of the year is normally around 7,300 cfs. Other rivers showed similar increases at a time of year when water flows are typically dropping or stable. 

What's more, poised in the mountains along the Rocky Mountain Front is 4 to 5 feet of heavy, wet snow that has the potential to melt and run off into the Milk River this fall if temperatures heat up, a scenario that would add to the Army Corps of Engineers' woes.

September runoff into the upper basin above Sioux City, Iowa, was nearly twice the record runoff, which was recorded in 1986. Rainfall in much of North Dakota, portions of South Dakota and northern Nebraska added to the torrent that has destroyed levies and flooded homes and towns along the Missouri River since spring.

The 2019 upper basin runoff forecast is 61 million acre-feet. If realized, the runoff total would equal the highest runoff in 121 years of record-keeping, the high water year of 2011 that turned Eastern Montana into one large lake and required Fort Peck Dam to crank open its large spillway gates.

Runoff in the Gavins Point to Sioux City reach in September was more than 16 times the long-term average and more than twice the previous record. Runoff in the Fort Randall to Gavins Point reach was over four times average and almost twice the previous record. Runoff between Oahe and Fort Randall was over 12 times average and set a new record. Runoff between Garrison and Oahe was over four times average. Finally, runoff between Fort Peck and Garrison was over two times average and is the second highest runoff of record, runoff above Fort Peck was 1.5 times average.

The January-September observed runoff (53.6 MAF) has already exceeded the second highest runoff in 121 years of record-keeping, 49 MAF observed in 1997, with three months still remaining.

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“In response to the increased upstream runoff, releases from Gavins Point Dam have been increased to 80,000 cfs,” said John Remus, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division, in a press release. "This release rate is more than twice the average release for this time of the year."

Storage in the reservoirs along the upper Missouri River was 64 MAF as of Oct. 1, occupying 7.9 MAF of the 16.3 MAF flood control zone. All three of the upper three reservoirs (Fort Peck, Garrison, and Oahe) have fallen out of their exclusive flood control zones but remain high.

“As a result of the high reservoir levels and the forecast above-average runoff for the remainder of the fall, releases from all system projects will be much above average through November, to evacuate all stored flood waters prior to the start of the 2020 runoff season," Remus said. “We are monitoring the situation very closely and will make any necessary adjustments. Failure to evacuate the stored flood water will lead to increased risk of flooding in 2020.”

Fort Peck Reservoir dropped a little more than a foot in September to an elevation of 2,242.8 feet. To reach the base of the annual flood control zone, a target the Corps likes to reach before runoff, the lake must drop another 9 feet.

Seven public meetings will be conducted throughout the basin Oct. 22-25. The purpose of the 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 are available at the following link: https://www.nwd.usace.army.mil/MRWM/Public-Meetings/One upside to pushing all of that water through the Missouri River's dams is a surplus of power being generated. The six mainstem power plants generated a record 1393 million kilowatt hours of electricity in August. Typical energy generation for September is 895 million kWh. The power plants are projected to generate 13.3 billion kWh of electricity this year, compared to the long-term average of 9.4 billion kWh.

Updates on basin conditions, reservoir levels and other topics of interest can be viewed at  https://www.nwd.usace.army.mil/MRWM/MRWMApp/.

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