Omaha, Neb. — The U.S. Army Corps of Engineers (Corps) Missouri River Basin Water Management Division reports runoff in the Missouri River Basin above Sioux City, Iowa during May was 4.3 million acre feet (MAF), 130 percent of normal. However, based on the current soil moisture and snowpack conditions, the 2014 runoff forecast has decreased slightly from 31.7 MAF last month to 31.1 MAF, 123 percent of normal. Average annual runoff is 25.2 MAF.
“As expected, May runoff was above normal due to the melt of the above average mountain snowpack, which began to enter the reservoirs in May,” said Jody Farhat, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division. “We are also forecasting above normal runoff in June and July as the snowmelt continues. However, based on the amount of snow that has already melted, it appears that the total volume of runoff from the mountain snowmelt will be somewhat less than previously forecasted.”
The total volume of water stored in the Missouri River Mainstem reservoir system on June 1 was 56.3 MAF. “System storage gained 2.1 MAF in May. The reservoirs continue to be well positioned to capture the expected above normal runoff,” explained Farhat. “System storage is just above the base of the annual flood control and multiple use zone, which begins at 56.1 MAF.” The annual flood control zone is the portion of the reservoirs that the Corps prefers to use for normal operations. Ideally, the system of reservoirs is at the base of the zone at the start of the spring runoff season. Runoff is collected through the spring and early summer, peaking about July 1. The zone is then evacuated during the rest of the year, back to the base on March 1 of the following year.
According to information from the Natural Resources Conservation Service, the mountain snowpack accumulation peaked in the reach above Fort Peck Dam at 132 percent of the normal peak, and in the reach from Fort Peck to Garrison dams, it peaked at 140 percent of the normal peak. As of June 9, less than 25 percent of this year’s peak snowpack remains.
View mountain snowpack graphic here: http://www.nwd-mr.usace.army.mil/rcc/reports/snow.pdf.
Heavy rain fell across much of the lower basin in early June resulting in increased flows on many of the tributaries below the system of reservoirs. “Rainfall events like this can cause localized flooding downstream of the reservoir system. Gavins Point Dam releases were reduced from 30,000 cubic feet per second (cfs) to 27,000 cfs for a short time, as part of our normal flood risk reduction measures,” stated Farhat. The Corps will utilize the available flood control space in the reservoirs, if necessary, to limit downstream river levels.
The Corps 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 29,100 cfs during May. Releases, which are currently at 30,000 cfs, are expected to remain near that level during June to prevent endangered least terns and threatened piping plovers from nesting on low sandbars. These sandbars would be flooded 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 June at elevation 1206.4 feet. The reservoir will remain near elevation 1206 feet during June.
Fort Randall Dam releases averaged 26,700 cfs during June. Fort Randall releases are currently at 27,000 cfs, and will be adjusted, as necessary, to maintain the desired pool elevation at Gavins Point Dam. The reservoir ended May at elevation 1355.3 feet, up 1.1 feet during the month. The pool will remain near 1355 feet during June.
Big Bend Dam releases averaged 24,100 cfs during May. They are expected to average 24,000 cfs this month. The reservoir will remain near its normal elevation of 1420 feet in June.
Oahe Dam releases averaged 25,000 cfs during May. Releases are expected to average 23,200 cfs this month. The reservoir ended May at elevation 1607.9 feet, up 2.1 feet during the month and 0.4 feet into the annual flood control zone. The reservoir is expected to rise less than 3 feet during June.
Garrison Dam releases were increased from 24,000 cfs to 30,000 cfs during May, averaging 28,300 cfs during the month. Releases will remain at 30,000 cfs during June. Garrison ended May at elevation 1839.8 feet, up 2.5 feet from the previous month and 2.3 feet into the annual flood control zone. It is expected to rise nearly 5 feet during June.
Fort Peck Dam releases were increased from 8,000 cfs to 9,000 cfs during May. Releases will remain at 9,000 cfs during June. The reservoir ended May at elevation 2228.3 feet, up 2.5 feet from the previous month, but still 5.7 feet below the base of the annual flood control zone. The reservoir is forecast to rise 6 feet during June.
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 880 million kilowatt hours (kWh) of electricity in May. Typical energy generation for May is 784 million kWh. The power plants are projected to generate 9.5 billion kWh of electricity this year, compared to the normal 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 (ft msl)
|
Water in Storage - 1,000 acre-feet
|
|
On May 31
|
Change in May
|
On May 31
|
% of 1967-2013 Average
|
Change in May
|
Fort Peck
|
2228.3
|
+2.5
|
13,634
|
94
|
+483
|
Garrison
|
1839.8
|
+2.5
|
18,455
|
105
|
+766
|
Oahe
|
1607.9
|
+2.1
|
18,777
|
101
|
+634
|
Big Bend
|
1420.7
|
+0.1
|
1,674
|
98
|
+5
|
Fort Randall
|
1355.3
|
+1.1
|
3,432
|
89
|
+95
|
Gavins Point
|
1206.4
|
-0.0
|
336
|
89
|
-1
|
|
|
Total
|
56,308
|
100
|
+1,982
|
WATER RELEASES AND ENERGY GENERATION FOR MAY
|
Average Release in 1,000 cfs
|
Releases in 1,000 acre-feet
|
Generation in million kWh
|
Fort Peck
|
8.7
|
534
|
83
|
Garrison
|
28.3
|
1,742
|
266
|
Oahe
|
25.0
|
1,539
|
235
|
Big Bend
|
24.1
|
1,481
|
89
|
Fort Randall
|
26.7
|
1,642
|
176
|
Gavins Point
|
29.1
|
1,789
|
30
|
|
|
Total
|
880
|
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