The heavy rains across much of South Dakota in recent days prompted the U.S. Army Corps of Engineers to quickly decide on Friday, Sept. 13, to lower releases from Gavins Point Dam near Yankton to ease flooding below the dam between Sioux City and Omaha. Gavins Point is the “lowest” of the six big dams on the Upper Missouri.
The recent rains also caused the Army Corps to schedule an “ad hoc,” conference call on Monday, Sept. 16 to brief and take questions from stakeholders such as national and state lawmakers and tribal and local officials as well as news reporters.
It sounds like crisis mode as the Army Corps must deal with the latest bad news of more rain while having to keep in sight the longer-range problem of getting the Upper Missouri’s system of six big dammed reservoirs low enough by winter’s freeze to be ready for runoff next year. It looks like it will be a close call.
As much as 11 inches of rain fell over parts of eastern South Dakota during two days late last week, with many areas from Brookings to Madison getting 5 inches to 8 inches of rain. The rain washed out small dams and roads and isolated towns, forcing evacuations.
But it’s a wider problem.
The National Weather Service said the first two weeks of September brought 200 to 600 percent of normal rainfall over the entire Upper Missouri River Basin, the drainage from Sioux City up to the river’s origins in Montana, involving parts of several states.
For weeks the emphasis has been on keeping Gavins Point Dam’s releases high, at 70,000 cubic-feet-per-second (cfs), about twice normal rates, to facilitate the lowering of reservoir levels upstream at the other five mainstem Missouri dams, from Fort Randall, including Oahe, and up to Fort Peck, Montana.
But the recent heavy rains saw a swift change by the Army Corps to deal with a new reality.
On Saturday, Sept. 14, the Army Corps reduced the Gavins Point releases to 65,000 cfs and planned to reduce it to 60,000 on Sunday morning.
“Current models suggest lowering releases form Gavins Point Dam may lower the peak flood stage forecast on the Missouri River between Sioux City and Omaha,” Army Corps officials said in a news release late Friday afternoon. “After a brief reduction to 60,000 cfs, releases from Gavins Point will then be incrementally increased by 5,000 cfs a day going up to 80,000 cfs.”
More water must be released as soon as possible to ensure the Upper Basin dam/reservoir system will have the 16.3 million acre-feet (MAF) of “designated flood control storage” available before the 2020 runoff season begins, according to the Army Corps.
“We have already seen four times the normal precipitation for September over the entire Upper Missouri River Basin,” said John Remus, chief of the Corps’ Missouri River Basin Water Management Division based in Omaha. “(The Division) continues to monitor the conditions on the ground and we will make adjustments as necessary.”
Already it’s a near record runoff before the big rains came late last week.
By Sept. 13, runoff in the Upper Basin the the Big Mo — above Sioux City — totaled 49.9 MAF, surpassing the full year’s runoff in 1997 of 49 MAF, which had been the second highest in 121 years of recorded history. Only the 61 MAF of runoff seen in the big flood year of 2011 exceeds the runoff in 2019.
And that record is within reach.
On Sept. 1, the Army Corps was forecasting that 2019’s total runoff would amount to 54.6 MAF. On Friday, Sept. 13, the Corps jacked up its projection to 58.8 MAF of runoff by Dec. 31.
Since Sept. 1, the runoff area in South Dakota, not only into the Missouri but into the James River and Vermillion River, has seen 3 inches to 8 inches of rain; all of North Dakota has seen 2 inches to 8 inches during the same time, the Army Corps said.
The release rate from Oahe Dam was reduced a few days ago well below the 57,000 cfs level — as a daily average — and was at 43,500 cfs on Saturday, Sept. 14, according to the Corps. Oahe Reservoir has been slightly lowering for weeks. On Saturday, Sept. 14, it was at 1615.3 feet above sea level, a slight rise from 1615.2 on Sept. 10; that’s 1.7 foot below the beginning of the “exclusive flood control” zone, the “top” three feet from 1617 to 1620 feet, according to the Corps.