In a delicate balancing act, the U.S. Army Corps of Engineers is increasing releases from Gavins Point Dam on the South Dakota-Nebraska border following heavy rains upstream.
The decision to increase outflows to 60,000 cubic feet per second comes after 2 to 5 inches of rain fell over central Nebraska and much of South Dakota.
"The ground is very wet, and weather patterns have been very active, just about any significant rainfall in north central Nebraska, and central and or western South Dakota will likely require an increase in releases from Gavins Point Dam in order to manage pool levels at Oahe and Fort Randall dams," said John Remus, chief of the Missouri River Basin Water Management Division.
Because of the rain, inflows into Oahe, Big Bend, Fort Randall, and Gavins Point dams have been increasing and causing pool levels to rise.
The forecast rise at Oahe Dam and Fort Randall Dam includes storing runoff in their exclusive flood control pools. Releases from Gavins Point Dam will allow more water to pass through the system and slow the rise in the pool levels at these projects.
Because of the rain, the Corps updated its three-week forecast and changed Fort Peck Dam releases from 9,500 cfs to 10,000 cfs. The lower Missouri River was pumping in 17,500 cfs to the reservoir while the Musselshell River was adding another 2,870 cfs.
Although river flows have dropped in much of eastern Montana because of cooler weather, the rivers are trending higher than normal for this time of year.
The Missouri and Yellowstone rivers combine before flowing into Sakakawea. The Yellowstone was flowing at 28,100 cfs at Sidney, dropping from almost 40,000 cfs earlier in the week. The Bighorn River, a tributary to the Yellowstone, has been rising this week and should hit 4,500 cfs on Sunday as water releases from Yellowtail Dam have been stepped up.
Upstream in Wyoming, the Bighorn River flowing into Bighorn Reservoir had dropped to 4,070 cfs after hitting a peak of about 4,700 cfs late last week.
While Fort Peck will release more water, downstream at Garrison Dam, which creates Lake Sakakawea in North Dakota, outflows will be cut from 30,000 cfs to 27,000 cfs.
Even though releases from Fort Peck will increase, the reservoir is expected to hit an elevation of 2,244.5 by mid-June. The exclusive flood control zone starts at an elevation of 2,246 feet.
Once the lake reaches the exclusive flood control zone water may be released from the dam's spillway as quickly as downstream conditions permit.
“Although the mountain snowpack is normal, we still have a little more than 8 inches of snow water equivalent coming from the mountain snowmelt and want to ensure we have storage available to capture and manage that runoff,” Remus said.
The travel time from Gavins Point Dam to the lower Missouri River and the large uncontrolled drainage area diminishes the effects of releases at locations further downstream.
“We are monitoring conditions along the length of the Missouri River and this increase at Gavins Point will not reach Kansas City until after Monday, May 27. By then, the peak flooding from the recent rain should be declining,” Remus said.
“There is no estimate on how long releases will remain at 60,000 from Gavins Point,” he added.
Releases will remain higher than average into the fall because the reservoirs have to be at the base of the annual flood control pool by the beginning of the 2020 runoff season.
“We will continue to monitor conditions and make adjustments as necessary,” Remus said.