OMAHA, Neb. — Higher-than-average releases from all Missouri River Mainstem System projects, including Gavins Point, will continue through the fall.
"Due to this year’s high runoff and the water currently being stored in the reservoirs, Gavins Point releases will remain near 58,000 cubic feet per second (cfs) for the remainder of the navigation season to ensure evacuation of all stored flood waters prior to the 2019 runoff season with much of that occurring before the river freezes over in the northern reaches," said John Remus, Chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division.
The 2018 runoff forecast in the Missouri River Basin above Sioux City, Iowa is 40.6 million acre feet, 160 percent of average. The September runoff summation above Sioux City was 1.7 MAF, 151 percent of average. The September runoff above Gavins Point Dam was about 80 percent of average. Due to heavy September rainfall, the runoff in the unregulated reach from Gavins Point Dam to Sioux City was 0.8 MAF, which is about eight times the average September runoff, and the highest September runoff in 120 years of record-keeping (1898-2017).
The Missouri River Mainstem reservoir system storage was 62.2 MAF as of October 1, occupying 6.1 MAF of the 16.3 MAF flood control zone. "System storage declined 2.5 MAF in September and will continue to gradually decline in October," said Remus.
System storage peaked on July 8 at 68.4 MAF, occupying 12.3 MAF of the designated 16.3 MAF of flood control storage.
"Reservoir releases will be adjusted as necessary to provide downstream flood risk reduction and continue evacuation of stored flood water," said Remus. It is important to note that 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.
Based on the Sept. 1 System storage, winter releases from Gavins Point will be at least 17,000 cfs. Based on the latest reservoir studies, there will be a 10-day extension to navigation flow support and a Gavins Point winter release ranging from 17,000 to 22,000 cfs depending on runoff. Navigation flow support at the mouth of the Missouri River will end on Dec. 11.
Weekly 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.
The Corps will continue to monitor basin and river conditions and will adjust System regulation based on the most up-to-date information.
The 2018-2019 draft Annual Operating Plan for the Missouri River Mainstem System was posted in September at: http://www.nwd.usace.army.mil/MRWM/Public-Meetings/.
Fall public meetings will be held in six cities throughout the basin in early November. These meetings will include a presentation regarding 2018 operations and plans for regulating the reservoir system in 2019, followed by a question and answer session. The public meetings will be conducted at the times and locations listed below.
• Wednesday, Nov. 7 — Pierre:
Start time: 10 a.m. (CST)
Casey Tibbs Conference Center
210 Verendrye Drive, Ft. Pierre, SD
• Wednesday, Nov. 7 — Sioux City, Iowa
Start time: 4 p.m. (CST)
Stoney Creek Hotel & Conference Center
300 3rd St.
Gavins Point Dam releases averaged 56,100 cfs during September. Releases will remain near 58,000 cfs during October, downstream conditions permitting. The Gavins Point reservoir ended September at elevation 1207.6 feet. The reservoir will end October near 1207.5 feet.
Fort Randall Dam releases averaged 52,600 cfs in September. Releases will be adjusted as necessary to maintain the desired reservoir elevation at Gavins Point. Releases are being made from both the powerhouse and outlet tunnels. The reservoir ended September at elevation 1352.6 feet, falling 4.6 feet during the month. The reservoir will gradually fall to near 1345.0 feet during October. The reservoir is normally drawn down to 1337.5 feet in the fall to provide space for winter hydropower generation at Oahe and Big Bend. The annual drawdown will continue through November.
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.
The six mainstem power plants generated 1,222 million kWh of electricity in September. Typical energy generation for September is 895 million kWh. The power plants are projected to generate 12.6 billion kWh of electricity this year, compared to the long-term average of 9.3 billion kWh.
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.