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
|