The U.S. Army Corps of Engineers  Missouri River Basin Water Management Division reports April runoff in the Missouri River Basin above Sioux City, Iowa, was 1.5 million acre feet (MAF), 52 percent of normal. The 2015 runoff forecast in the Missouri River Basin above Sioux City, Iowa, has decreased 1 MAF from last month to 19.3 MAF, 76 percent of normal. Average annual runoff is 25.2 MAF.

“April inflows were well below normal, a result of a lack of plains snowpack and below normal precipitation in the Dakotas,” said Jody Farhat, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division. “The Corps will continue to conserve water while still serving all authorized purposes. Most notably, we are currently only providing full-service navigation flow support in reaches where commercial navigation is occurring.” 

The mountain snowpack accumulation peaked in the reach above Fort Peck Dam and the reach between Fort Peck and Garrison dams earlier than usual. “Mountain snowpack typically peaks in mid-April. This year’s mountain snowpack was tracking near normal through late February, but leveled off in mid-March, and is now declining,” said Farhat. Mountain snowpack in the reach above Fort Peck Dam peaked on March 9 at 72 percent of the normal peak. In the reach between Fort Peck and Garrison dams, the mountain snowpack peaked on March 14 at 78 percent of the normal peak.

View mountain snowpack graphic here: https://www.nwd-mr.usace.army.mil/rcc/reports/snow.pdf.

The potential still exists for minor to moderate flooding along the lower Missouri River and its tributaries due to normal thunderstorm activity. The Corps will utilize the available flood control space in the reservoirs, if necessary, to limit downstream river levels. “Typical summer thunderstorms can cause localized flooding downstream of the reservoir system. Reducing releases from Gavins Point in response to downstream rainfall is part of our normal flood risk management measures,” said Farhat.

USACE will continue to monitor the mountain snowpack, basin soil conditions, and rainfall events to fine tune the regulation of the reservoir system based on the most up-to-date information.

Reservoir Forecasts

Gavins Point Dam releases averaged 24,900 cubic feet per second (cfs) during April. Releases, which are currently at 24,000 cfs, are expected to reach 28,000 to 30,000 cfs around mid-May to meet the downstream flow targets and to prevent endangered least terns and threatened piping plovers from nesting on low sandbars. These sandbars could be inundated later in the summer when higher releases are needed to meet downstream navigation flow targets. The nesting season runs from May to late August. The reservoir behind Gavins Point Dam ended April at elevation 1206.3 feet. The reservoir will remain near elevation 1206 feet during May.

Fort Randall Dam releases averaged 22,900 cfs during April. Fort Randall releases are currently at 23,000 cfs and will be adjusted, as necessary, to maintain the desired pool elevation at Gavins Point. The reservoir ended April at elevation 1355.7 feet, up 3 feet during the month. The pool is expected to remain near 1355.2 feet through the end of May.

Big Bend Dam releases averaged 24,700 cfs during April. They are expected to average 24,700 cfs this month. The reservoir will remain near its normal elevation of 1420 feet in May.

Oahe Dam releases averaged 26,800 cfs during April. Releases are expected to average 24,200 cfs this month. In early April, it appeared unlikely that the Corps would be able to keep Oahe steady to rising during the forage fish spawn despite plans to favor the reservoir this year. However, an increase in releases from Garrison combined with rain in the lower basin, which reduced release requirements, made it possible to halt the decline of the reservoir during the peak of the fish spawn season. Oahe reservoir declined just over a foot during the first three weeks of April but has remained steady to rising since that time. The reservoir is expected to remain steady through early May, then fall less than 1 foot by the end of the month.

Garrison Dam releases were increased from 18,000 cfs to 23,000 cfs during April to help provide a rising reservoir during the forage fish spawn at Oahe. Next year Garrison will be favored during the forage fish spawn if inflows are not sufficient to keep all three of the upper reservoirs steady to rising. Releases will remain near 23,000 cfs in May. Garrison ended April at elevation 1838.5 feet, down nearly 1 foot from the previous month. It is expected to fall less than 1 foot during May.

Fort Peck Dam releases averaged 6,800 cfs in April. Releases will be increased to 8,000 cfs in early May. The reservoir ended April at elevation 2235.8 feet, up fractionally over the month. The reservoir is forecast to fall less than 1 foot during May.

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 804 million kilowatt hours (kWh) of electricity in April. Typical energy generation for April is 687 million kWh. The power plants are projected to generate 9.2 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: https://www.nwd-mr.usace.army.mil/rcc/reports/twregfcast.pdf.

MISSOURI RIVER MAINSTEM RESERVOIR DATA 

WATER RELEASES AND ENERGY GENERATION FOR APRIL