The U.S. Army Corps of Engineers (Corps) Missouri River Basin Water Management Division plans to maintain higher-than-average releases from all System projects, including Gavins Point, through the summer and fall. “Due to the water currently being stored in the reservoirs and the higher-than-average runoff being forecast in the Missouri River Basin above Sioux City, Iowa, the service level was increased 25,000 cubic feet per second (cfs) above full service in late June to facilitate the evacuation of stored flood waters. The increased service level means that Gavins Point releases will be increased to approximately 60,000 cfs, as downstream tributary flows recede. 

Gavins Point releases will remain near 60,000 cfs for the remainder of the navigation season to ensure evacuation of all stored flood waters,” said John Remus, Chief of the Missouri River Basin Water Management Division. Gavins Point releases will be adjusted, when needed, in response to basin conditions. 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 flooding downstream of all the projects. 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.

The 2018 runoff forecast in the Missouri River Basin above Sioux City, Iowa, is 39.8 million acre feet (MAF), 157 percent of average according to the Corps. “The updated runoff forecast reflects the melting of the above-average mountain snowpack and moderate to heavy plains snowpack, as well as above-average rainfall that fell throughout the basin over the last four months,” said Remus. The June runoff was 10.0 MAF, which is the third highest monthly runoff in 120 years of record.

As of July 2, the mountain snowpack was nearly all melted in the reach above Fort Peck and in the reach from Fort Peck to Garrison. The reach above Fort Peck has less than 0.1 inches of snow water equivalent remaining and the reach from Fort Peck to Garrison has 0.1 inches of snow water equivalent remaining. View the mountain snowpack graphic here: https://www.nwd-mr.usace.army.mil/rcc/reports/snow.pdf.

The Missouri River mainstem reservoir system (System) storage was 67.8 MAF as of July 1, occupying 11.7 MAF of the 16.3 MAF flood control zone. “Approximately 25 percent of the System’s flood storage remains available to capture runoff from the remaining mountain snowmelt and summer rainfall events. By comparison, on July 1, 2011, 16.0 of the 16.3 MAF flood control zone was occupied. The current amount of vacant flood control storage provides flexibility to lessen downstream flooding should suddenly-developing large rainfall events occur anywhere in the basin,” said Remus. System storage is expected to peak around mid-July.

Weekly updates on basin conditions, reservoir levels and other topics of interest can be viewed here: https://www.nwd-mr.usace.army.mil/rcc/reports/pdfs/weeklyupdate.pdf.

The Corps will continue to monitor basin and river conditions, including rainfall and mountain snowmelt, and will adjust the regulation of the System based on the most up-to-date information.

Reservoir Forecasts

• Gavins Point Dam releases averaged 37,000 cfs during June. Releases, which are currently 38,000 cfs, will continue to increase as downstream conditions permit until they reach approximately 60,000 cfs. The Gavins Point reservoir ended June at elevation 1207.6 feet and will fall to near 1206.0 feet during July.
• Fort Randall Dam releases averaged 33,500 cfs in June. 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 June at elevation 1361.8 feet, rising 4.0 feet during the month. The reservoir will gradually fall to near 1359.0 feet during July.
• Big Bend Dam releases averaged 31,600 cfs in June. Releases are expected to average 43,500 cfs during July. The reservoir will remain near its normal elevation of 1420.0 feet during July.
• Oahe Dam releases averaged 35,600 cfs during June. Releases are expected to average 43,200 cfs in July. The reservoir ended June at elevation 1614.3 feet, rising 3.2 feet during the month. The reservoir level is expected to rise 2.7 feet, peaking near the base of its 3-foot Exclusive Flood Control Zone, during July.
• Garrison Dam releases averaged 48,700 cfs during June, ranging from 40,000 cfs to 60,000 cfs during the month. Releases will be remain near 60,000 cfs during July. Releases are above the maximum powerhouse release so releases are being made from both the powerhouse and the outlet tunnels. Garrison reservoir is forecast to peak in July near elevation 1853.2 feet, 3.2 feet into the 4-foot Exclusive Flood Control Zone, before falling to near elevation 1852.0 feet by the end of the July.
• Fort Peck Dam releases averaged 19,400 cfs during June. Releases were increased from 18,000 cfs to 20,000 cfs in early June. Releases were decreased to 16,000 cfs in late June to help reduce inflows into Garrison reservoir. Releases were increased to 18,000 cfs in early July and will be held near that rate during the month of July. Releases are greater than the maximum powerhouse release, so releases are being made from both the powerhouse and spillway. The reservoir ended June at elevation 2247.5 feet, rising 3.1 feet during the month. The reservoir is expected to peak in July near elevation 2247.9 feet, 1.9 feet into the 4-foot Exclusive Flood Control Zone, before falling to near elevation 2247.1 feet by the end of the July.

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 1,153 million kWh of electricity in June. Typical energy generation for June is 834 million kWh. The power plants are projected to generate 13.7 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 https://www.nwd-mr.usace.army.mil/rcc/reports/twregfcast.pdf.