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Corps decreases runoff forecast due to lower than normal mountain snowpack

Published April 6, 2015
The Missouri River Water Management office releases a report at the beginning of each month to the public documenting the monthly river forecast and release schedule. The Missouri River Water Management Division is part of the Northwestern Division of the U.S. Army Corps of Engineers and is located in Omaha, Nebraska.

The Missouri River Water Management office releases a report at the beginning of each month to the public documenting the monthly river forecast and release schedule. The Missouri River Water Management Division is part of the Northwestern Division of the U.S. Army Corps of Engineers and is located in Omaha, Nebraska.

OMAHA, NE — The U.S. Army Corps of Engineers  Missouri River Basin Water Management Division is decreasing the annual runoff forecast for the Missouri River Basin above Sioux City, Iowa, to 20.3 million acre feet (MAF), which is 80 percent of normal and 4.9 MAF less than average. The decreased forecast is due to below normal mountain snowpack and the lack of plains snow in the basin.

“While below normal runoff is expected, the reservoirs are well positioned to meet all of the authorized purposes this year,” says Jody Farhat, chief of the Missouri River Basin Water Management Division. “The Missouri River Mainstem Reservoir System is designed and operated to provide the Corps with the necessary flexibility to adjust for varying conditions.” The total volume of water stored in the reservoir system is 57.6 MAF, 1.5 MAF into the 16.3-MAF annual flood control and multiple use zone. 

Above normal temperatures coupled with below normal precipitation patterns have stalled mountain snowpack accumulation and melted the plains snow. As of April 1, mountain snowpack was 68 percent of normal in the reach above Fort Peck Dam and 74 percent of normal in the reach between Fort Peck and Garrison dams. Mountain snowpack appears to have peaked nearly a month earlier than normal this year in the reaches above the Fort Peck and Garrison dams. The mountain snowpack typically peaks in mid-April, and runoff from the melting snow enters the reservoir system from May through July. 

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

In mid-March, Gavins Point Dam releases were increased from 17,000 cubic feet per second (cfs) to approximately 28,000 cfs in support of the navigation season, which began April 1 near St. Louis, Mo. “Flow support for the first half of the navigation season will be full service,” said Farhat. Full service navigation flow support is generally sufficient to provide a 9-foot-deep by 300-foot-wide channel. Flow support for the second half of the navigation season as well as the season length will be determined following the system storage check on July 1.

Steady-to-rising reservoir levels during the forage fish spawn at the three, large upper reservoirs (Fort Peck, Garrison and Oahe) are preferred but may be difficult to accomplish without significant rainfall in the Missouri River Basin during the coming weeks. If the runoff distribution allows, the Corps will set releases to result in steady to rising pools at Fort Peck and Oahe dams. The forage fish spawn generally occurs from early April through mid-June. The Corps will continue to monitor the plains and 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 21,900 cfs during March. Releases are currently at 28,000 cfs and will be adjusted in April to meet downstream navigation flow targets. In the event of heavy rains downstream of the reservoir system, releases will be reduced to lesson flood risk along the lower river. The reservoir behind Gavins Point Dam ended March at elevation 1206.3 feet. The reservoir will remain near elevation 1206 feet during April. 

Fort Randall Dam releases averaged 19,800 cfs during March and will be adjusted as necessary to maintain the desired pool elevation at Gavins Point. The reservoir ended March at elevation 1352.7 feet, up 3 feet during the month. The pool is expected to gradually rise to near elevation 1355 feet during April. 

Big Bend Dam releases averaged 21,600 cfs during March. They are expected to average 30,000 cfs during the month. The reservoir will remain near its normal elevation of 1420 feet in April. 

Oahe Dam releases averaged 23,500 cfs during March and are expected to average 29,500 cfs this month. The reservoir ended March at elevation 1608.1 feet, down 0.1 feet during the month. The reservoir is expected to fall approximately 1 foot during April.

Garrison Dam releases were stepped down from 23,000 cfs to 18,000 cfs in early March averaging 19,400 cfs during the month. Releases will be increased to 21,000 in April to help raise the Oahe reservoir during the forage fish spawn. Garrison ended the month at elevation 1839.4 feet, up 0.6 feet from the previous month. It is expected to rise less than 1 foot during April. 

Fort Peck Dam releases averaged 6,900 cfs in March. Releases will be increased from 7,000 cfs to 8,000 cfs around mid-April. The reservoir ended March at elevation 2235.7 feet, up 0.5 feet from the previous month. The reservoir is forecast to rise less than 1 foot during April. 

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. 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

The six mainstem power plants generated 744 million kilowatt hours (kWh) of electricity in March. Typical energy generation for March is 638 million kWh. The power plants are projected to generate 9.5 billion kWh of electricity this year, compared to the normal rate of 10 billion kWh.

MISSOURI RIVER MAINSTEM RESERVOIR DATA

 

Pool Elevation (feet above mean sea level)

Water in Storage - 1,000 acre-feet

 

On March 31

Change in March

On March 31

% of 1967–2014 Average

Change in March

Fort Peck

2235.7

+0.5

15,155

108

+108

Garrison

1839.4

+0.6

18,349

109

+203

Oahe

1608.1

-0.1

18,869

106

-24

Big Bend

1420.4

+0.0

1,657

97

+4

Fort Randall

1352.7

+3.0

3,217

86

+239

Gavins Point

1206.3

-0.6

335

91

-12

 

 

 Total

57,582

106

+518

WATER RELEASES AND ENERGY GENERATION FOR MARCH 

 

Average Release in 1,000 cfs

Releases in 1,000 acre-feet

Generation in Million kWh

Fort Peck

6.9

424

68

Garrison

19.4

1,195

186

Oahe

23.5

1,443

218

Big Bend

21.6

1,330

82

Fort Randall

19.8

1,215

128

Gavins Point

21.9

1,346

62

 

 

Total

744



Contact
Michael A. Coffey, U.S. Army Corps of Engineers, Northwestern Division
503-808-3722
michael.a.coffey@usace.army.mil
or
Serena Baker, U.S. Army Corps of Engineers, Northwestern Division
503-808-3710
serena.baker@usace.army.mil

Release no. 20150406-001