The increased rains this week over large parts of South Dakota led the U.S. Army Corps of Engineers to adjust the dam outflows, including reducing Oahe Dam releases, to help downriver dams deal with increased runoff and inflows.
The Army Corps reduced the releases from Oahe Dam on Thursday, Sept. 12, to 49,000 cubic-feet-per-second (cfs), down from 57,000 cfs. It also cranked down releases from Big Bend Dam and Fort Randall Dam downriver, while keeping Gavins Point Dam’s releases at the relatively high 70,000 cfs.
“These reductions are to offset some of the inflows we are seeing and to reduce the need for increasing releases from Gavins Point Dam,’ said John Remus, chief of the Army Corps’ Missouri River Water Management Division based in Omaha.
In a news release on Wednesday, Remus said the updated three-week forecast “reflects the increased rainfall over parts of South Dakota that have fallen in the past few days. We have increased inflows primarily at Fort Randall and coming into Gavins Point from the Niobrara River.”
Remus said it appears the release levels from Oahe, Big Bend and Fort Randall would return to the previous release rates by late next week.
But he said on Wednesday — before the heavy rains hit much of eastern South Dakota, “conditions on the ground,” may require further adjustments.
“We are continuing to monitor conditions and will make adjustments as necessary,” Remus said.
Releases from Oahe Dam went down to 49,000 cfs on Thursday, Sept. 12, according to Army Corps figures. That is a “daily average release,” with actual release rates varying during the day, in part to accomodate what the electrical needs from the seven power plants.
The Corps plans to keep releases from Gavins Point Dam — the “bottom” of the six mainstem dams — at 70,000 cfs through September, about twice the normal release rate during September.
That’s because the Upper Missouri River Basin has seen the second-highest runoff in history, keeping the six reservoirs fuller than usual.
As of Sept. 3, the forecast for the calendar year runoff by Dec. 31 above Sioux City in the Basin is 54.6 million acre-feet (MAF), which is 215 percent of average.
The storage in the six-dam system is at 64.9 MAF, after peaking at 68.5 MAF on July 20.
About 46 percent of the system’s flood control storage remains available to store runoff.
Lake Oahe was at 1615.2 fee above sea level on Wednesday, Sept. 11, 1.8 feet below the “exclusive flood control zone,” seen as the top of the reservoir.
The Corps aims to have the reservoirs lowered enough by winter freeze-up — when releases can’t be very high due to ice — to be ready to accommodate next spring’s melting snow and rain.
The Army Corps has the releases from Oahe slated to increase back up to 57,000 cfs on Friday, Sept. 20; but that could be adjusted after Wednesday’s heavy rains across much of South Dakota.
As of Wednesday, the Army Corps has projected that releases from Oahe — once they are raised from 49,000 cfs to 57,000 cfs on Sept. 20, would remain at 57,000 cfs until at least Oc. 4.
The Corps reported on Wednesday that up to 6 inches of accumulated precipitation had fallen over areas in Montana and North Dakota the previous four days, with more forecast through Friday night, Sept. 13.