Widespread and heavy rainfall in the upper Missouri River basin resulted in another month of above average runoff.
The US Army Corps of Engineers says 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 experienced their wettest Augusts on record, while the Sioux City and Oahe reaches were second and third, respectively.
All three of the upper reservoirs (Fort Peck, Garrison and Oahe) have fallen out of their exclusive flood control zones but remain high.
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.
◾Big Bend Dam
◦Average releases past month – 54,100 cfs
◦Forecast average release rate – 56,400 cfs
◦Forecast reservoir level – 1420.0 feet
◾Oahe Dam
◦Average releases past month – 57,000 cfs
◦Forecast average release rate – 56,500 cfs
◦End-of-August reservoir level – 1615.7 feet (down 1.7 feet during August)
◦Forecast end-of-September reservoir level – 1613.4 feet
The forecast reservoir releases and elevations discussed above are not definitive. Additional precipitation, lack of precipitation or other circumstances could cause adjustments to the reservoir release rates.
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.
Updates on basin conditions, reservoir levels and other topics of interest can be viewed here: http://www.nwd-mr.usace.army.mil/rcc/reports/pdfs/weeklyupdate.pdf.
To view the detailed three-week release forecast for the mainstem dams, go to http://www.nwd-mr.usace.army.mil/rcc/reports/twregfcast.pdf.
