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Gavins Point Releases Reduced to Lessen Downstream Flooding

Published May 5, 2016
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, Neb. – Runoff in the Missouri River Basin above Sioux City, Iowa, was 2.6 million acre feet (MAF) during April, 89 percent of average, according to the U.S Army Corps of Engineers (Corps). “Conditions in most of the basin during the first half of April were dry and warm. However, heavy rains during the last half of the month increased runoff into the reservoir system,” said Jody Farhat, chief of the Corps’ Missouri River Basin Water Management Division. The widespread rain also increased flows on many of the tributaries that join the Missouri River below the reservoir system, causing river levels to rise above flood stage. “Rainfall events like we’ve seen over the past couple of weeks can cause localized flooding downstream of the reservoir system. Gavins Point releases were reduced from 21,000 cfs to 14,000 cfs over several days because many locations downstream of Nebraska City rose above flood stage,” said Farhat.

When possible, the Corps will utilize the available flood control space in the reservoirs to limit downstream river levels; however, 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 the Gavins Point Dam. “Flood risk reduction remains a primary consideration as we progress through 2016. While the risk of widespread flooding from upper basin runoff is low this year, floods can and will occur as a result of spring and summer thunderstorms, particularly along the lower Missouri River where soil moisture is much wetter than normal,” Farhat added.

Gavins Point releases will be increased as the downstream stages fall, which will slow the decline of the river, but will not contribute to peak stages.

Fort Randall releases were cut to as low as 1,500 cfs to limit the rise of the Gavins Point reservoir level, which was impacted by high runoff from localized rain and reduced releases. At Fort Randall, the reductions in releases combined with high inflows, caused the Fort Randall reservoir level to climb rapidly over the past several days. The reservoir is currently near elevation 1361 feet. “Releases have been adjusted at the upstream dams, Oahe and Big Bend, to minimize the rise of the Fort Randall reservoir. Oahe and Big Bend releases will be held at lower rates for several days to allow the Fort Randall reservoir to be gradually lowered to the normal operating level,” said Farhat. The reservoir is currently about 5 feet above its normal elevation at this time of year.

The Corps will continue to monitor basin conditions and adjust the regulation of the reservoir system based on the most up-to-date information.

The mountain snowpack peaked in early April, which is about two weeks earlier than normal. The snowpack peaked at 95 percent of average above Fort Peck and 89 percent of average in the reach between Garrison and Fort Peck. Much of the low elevation snowpack has melted; however, the middle to high elevation snowpack is declining slowly. As of May 1, the mountain snowpack was 75 percent of average in the reach above Fort Peck and 87 percent of average in the reach between Fort Peck and Garrison. View the mountain snowpack graphic at http://www.nwd-mr.usace.army.mil/rcc/reports/snow.pdf

The 2016 runoff forecast is 22.5 MAF, 89 percent of average. The total volume of water currently stored in the Missouri River Mainstem Reservoir System is 57.9 MAF, occupying 1.8 MAF of the 16.3 MAF flood control zone. “While slightly below normal runoff is expected, the reservoirs are well positioned to meet all authorized purposes this year,” said Farhat. Based on the current runoff and reservoir regulation forecasts, the reservoir system is expected to start next year’s runoff season below the base of the annual flood control zone, which is the desired operating zone of the system. Reductions in releases in response to high flows on the lower river reduces flood risk below the system while also conserving water in the reservoirs for future use.

The Corps will continue to monitor basin conditions, including rainfall and mountain snow accumulation, and will adjust the regulation of the reservoir system based on the most up-to-date information.

Reservoir Forecasts

Gavins Point Dam releases averaged 19,200 cfs in April. Releases are expected to be stepped up from 14,000 cfs to 25,000 cfs by mid-May to prevent the endangered least tern and threatened piping plover from nesting on low sandbars that would be flooded later this summer when higher releases are needed to support navigation. The nesting season runs from May to late August. The reservoir behind Gavins Point Dam ended April at elevation 1206.8 feet and will decline to elevation 1206.0 feet during May.

Fort Randall Dam releases averaged 15,600 cfs in April. Releases will be adjusted as necessary to maintain the desired elevation at Gavins Point Dam. The reservoir ended April at elevation 1359.6 feet, rising 5.5 feet during the month. The reservoir is expected to gradually be lowered, ending May near elevation 1357.5 feet.

Big Bend Dam releases averaged 17,200 cfs during in April. Releases are expected to average 12,900 cfs this month. The reservoir will remain near its normal elevation of 1420.0 feet during May.

Oahe Dam releases averaged 17,700 cfs during the month of April. Releases are expected to average 12,200 cfs this month. The reservoir ended April at elevation 1609.2 feet, rising 0.2 feet during the month. The reservoir level is expected to rise 2 feet in May.

Garrison Dam releases averaged 14,900 cfs in April. Releases are expected to be increased from 14,000 cfs to 21,000 cfs around mid-May when the terns and plovers begin nesting on the river reach below the reservoir. Garrison ended April at elevation 1837.6 feet, nearly steady during the month. The reservoir is expected to rise less than 1 foot during May.

Fort Peck Dam releases averaged 6,500 cfs in April. Releases will be increased from 7,000 cfs to 9,000 cfs during May. The reservoir ended April at elevation 2234.2 feet, up 0.3 feet during the month. The reservoir level is forecast to rise slightly 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 574 million kilowatt hours of electricity in April. Typical energy generation for the month of April is 690 million kWh. The power plants are projected to generate 8.4 billion kWh of electricity this year, compared to the normal of 10 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    

 

Missouri River Mainstem Reservoir Data

 

 

Pool Elevation

(feet above mean sea level)

Water in Storage - 1,000 acre-feet

 

On April 30

Change in April

On April 30

% of 1967-2015 Average

Change in April

Fort Peck

2234.2

+0.3

14,883

105

+57

Garrison

1837.6

+0.0

17,771

104

+4

Oahe

1609.2

+0.2

19,219

105

+153

Big Bend

1420.7

+0.5

1,672

97

+30

Fort Randall

1359.6

+5.5

3,811

99

+478

Gavins Point

1206.8

+0.5

346

94

+12

 

 

 Total

57,652

104

+734

 

 

Average Release in 1,000 cfs

Releases in 1,000 acre-feet

Generation in Million kWh

Fort Peck

6.5

389

63

Garrison

14.9

886

137

Oahe

17.7

1,053

165

Big Bend

17.2

1,025

62

Fort Randall

15.6

929

100

Gavins Point

19.2

1,142

48

 

 

Total

574

 


Contact
Michael Coffey
503-808-3722
michael.a.coffey@usace.army.mil

Release no. 20160505-001