Much-above average runoff in the upper Missouri River basin (above Sioux City, Iowa) extended into May following widespread and heavy rainfall in South Dakota and Nebraska. Additionally, widespread and heavy rainfall in the lower basin, particularly in Kansas, has led to large tributary and Missouri River flows downstream of the six main stem reservoirs on the Missouri River.
May runoff in the upper basin was 8.9 million acre feet (MAF), which is 267 percent of average. The average May runoff is 3.3 MAF. May runoff was the second highest on record, only surpassed by 2011’s 9.2 MAF. Runoff in the Fort Randall Dam to Gavins Point Dam reach was 1.4 MAF, which is more than the average annual runoff for that reach.
The high May runoff increased the 2019 upper basin runoff forecast to 50.0 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. Runoff in 2018 was 42.1 MAF, which is currently third highest.
“System releases from Gavins Point Dam are currently 75,000 cfs, which is more than twice the average release for this time of the year. We will maintain Gavins Point releases at this rate to continue evacuating water from Oahe and Fort Randall, which have used much of their respective flood storage,” said John Remus, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division.
Oahe and Fort Randall are currently in their respective exclusive flood control zones. Fort Peck and Garrison are expected to enter their exclusive flood control zones during June as the remaining mountain snowpack melts. As a result of the high reservoir levels, the Corps expects that releases from all System projects will be above average for the next several months, and possibly as late as November.
The Corps has been coordinating with the U.S. Bureau of Reclamation (USBR) regarding the USBR projects in Montana and Wyoming with designated flood control storage. Releases from several of the USBR projects are being adjusted, and flood control storage is being used. This measure provides additional ability to manage the pools at all the mainstem reservoirs during June and July.
The mountain snowpack peaked in both reaches: on April 18 in the Fort Peck reach at 105%of average and on April 17 in the Fort Peck to Garrison reach at 104% of average. Normally the mountain snowpack peaks in mid-April. Cooler temperatures and additional snow in the mountains has slowed the melt, especially in the Fort Peck to Garrison reach. As of June 4, 34% of this year’s peak remains in the Fort Peck reach and 56% of this year’s peak remains in the Fort Peck to Garrison reach.
“Fort Peck and Garrison reservoir levels are positioned to ensure that there is adequate flood control space to capture and manage the mountain snowmelt runoff,” added Remus. View the mountain snowpack graphic here: http://www.nwd-mr.usace.army.mil/rcc/reports/snow.pdf.
Based on the March 15 System storage check, flow support for the first half of the navigation season is full service. However, due to above average runoff in the upper basin, releases from Gavins Point have been above full service levels to reduce the occupied flood storage at Oahe and Fort Randall. Based on the June 1 service level check, the service level was increased from 55,000 to 70,000 cfs, which is 35,000 cfs above the full service level of 35,000 cfs.
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 Basin Update as well as forecast information from partner agencies is available through the Missouri Basin Web app at http://go.usa.gov/xE6fC.
Monthly Water Management Conference Calls
Due to the ongoing flooding in the lower basin, the monthly water management calls are being reformatted. The calls will be held weekly, and will include a briefing from the National Weather Service’s Missouri Basin River Forecast Center, an updated on the Missouri River mainstem reservoir system operations, and updates on the ongoing and planned flood recovery efforts in both the Omaha and Kansas City Districts. The first of these calls will be held Thursday, June 6. The call is intended for Congressional delegations; Tribes; state, county and local government officials, levee and drainage districts; and the media. It will be recorded in its entirety and made available to the public on the Defense Video and Imagery Distribution System at www.dvidshub.net/unit/usace-nwd.
Reservoir ForecastsGavins Point Dam Average releases past month – 57,400 cfs Current release rate – 75,000 cfs Forecast release rate – 75,000 cfs End-of-May reservoir level – 1207.6 feet Forecast end-of-June reservoir level – 1206.0 feet Fort Randall Dam Average releases past month – 47,800 cfs End-of-May reservoir level – 1370.2 feet (up 6.7 feet from April) Forecast end-of-May reservoir level – 1370.2 feet Notes: Releases will be adjusted as necessary to maintain the desired reservoir elevation at Gavins Point. Big Bend Dam Average releases past month – 41,300 cfs Forecast average release rate – 47,600 cfs Forecast reservoir level – 1420.0 feet Oahe Dam Average releases past month – 35,900 cfs Forecast average release rate – 47,000 cfs End-of-May reservoir level – 1618.8 feet (rising 2.4 feet during May) Forecast end-of-June reservoir level – 1617.0 feet Garrison Dam Average releases past month – 23,500 cfs Current release rate – 15,000 cfs Forecast average release rate – 46,000 cfs (late June) End-of-May reservoir level – 1847.8 feet (rising 1.4 feet during May) Forecast end-of-June reservoir level – 1851.8 feet Notes: Releases will be stepped up from 15,000 cfs to 46,000 cfs during June. Fort Peck Dam Average releases past month – 8,200 cfs Current release rate – 9,000 cfs Forecast average release rate – 15,000 cfs (late June) End-of-May reservoir level – 2243.8 feet (up 3.2 feet from April) Forecast end-of-June reservoir level – 2247.4 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.
The six mainstem power plants generated 1025 million kWh of electricity in May. Typical energy generation for May is 790 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.
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.