Federal managers of the Missouri River system are running a gantlet of sorts between reducing river flows to help out flooding areas south of Sioux City while remaining poised to keep water flowing as fast as possible out of the six mainstem dams north of Nebraska to Montana before winter so that next year’s runoff can be handled.
In a quickly scheduled teleconference Monday, officials of the U.S. Army Corps of Engineers’ Missouri River Management Division based in Omaha explained their rapidly changing plans sparked by the huge rains last week over eastern South Dakota and other parts of the Upper Basin.
This has been a record rainy start to September in the region.
In just two days -- Sept. 11-12 -- in eastern South Dakota, Sherman received 13.5 inches, Ward 11.5 inches, Madison about 11 inches and Brookings 9.5 inches, said Kevin Grode during the conference call that had about 104 listeners and included representatives of Congressional delegations from several states, local and tribal officials and news reporters.
Grode is team leader of the river regulation division at the Army Corps office in Omaha.
He said Mitchell has received 7.6 inches of rain during the first two weeks of September; it normally receives less than 1 inch in the whole month of September.
Williston, North Dakota received more than 6 inches in the first half of September; average rainfall for the whole month is about 1 inch, Grode said.
Culbertson, Montana, also near the Missouri River, received 4.8 inches in the first half of this month when normal rainfall for the whole month is less than a half-inch, he said.
And the rain hasn’t been soaking into dry, dusty, brown pastures and fields that are usual this time of year.
“All this rain is falling on wet, saturated soils caused by the above-average rains received in July, and August has resulted in most of that rain becoming direct runoff, filling up the tributaries when they drain into the Missouri River upstream of the mainstream (dams and reservoirs, such as Lake Oahe)” Grode said. “And in the case of eastern South Dakota with the James, Vermillion and Big Sioux rivers, (the runoff is going) downstream of Gavins Point.”
That’s the lowest of the six big mainstem dams that include Oahe and Garrison — the biggest two — up to Fort Peck.
“We will continue to closely monitor streamflow conditions as some of these tributaries have not yet peaked,” Grode said on Monday.
The big rains are showing up as runoff measured.
The new forecast for runoff into the Upper Missouri in September is nearly 5 million acre-feet (MAF), far outstripping the long-term average of 1.1 MAF in September and a full 50 percent above the previous record runoff for September of 3.3 MAF set in 2014, Grode said on Monday.
The effect of the late season record runoff also shows up in how much water is being held back by the six big dams on the Missouri. On Tuesday, the Army Corps released a new chart showing the “system storage,” moving up in recent days to 65.1 MAF from 64.9 MAF a week earlier. That may not seem like such a big jump, except that it rarely ever moves upward at all this time of year.
Normally at this time of year, the system storage figure is in steady decline from the peak which usually hits around July 1; but now there’s a contra-seasonal little uptick in this year’s line as of Sept. 17.
This means, among other things, that 9 MAF of the total 16.3 MAF of exclusive flood control storage space is occupied, leaving 45 percent of it open to handle future runoff. Perhaps more importantly, the 55 percent of the flood control space now occupied needs to be evacuated in a short time to be sure there is enough storage for next year’s spring rush of runoff.
The Army Corps cannot release much water from the six reservoirs during the winter because ice can cause local flooding.
Grode said the Army Corps’ forecast for total runoff in 2019 in the Upper Missouri Basin has been raised in recent days from 54.6 MAF to 58.8 MAF, which would be only about 2 MAF from the 130-year record set in 2011.
The long-term average annual runoff in the Basin is only 25 MAF or 42 percent of this year’s forecast.
Meanwhile, the National Weather Service’s Climate Prediction Center says the outlook for this fall is for a “slightly increased chance of above-normal precipitation” across most of the Upper Missouri River Basin, Grode said. So, he said “(O)ur runoff forecast for the last three months of the year is about twice average.”
But in Nebraska, Iowa, Kansas and Missouri, of course, the concern right now is about flooding when many of the levees ripped out by early summer flooding still aren’t fixed, said several people who took part in the teleconference Monday.
That’s why the Army Corps reduced the release rates from Gavins Point from 70 cubic feet per second (cfs) to 60,000 cfs over the weekend: to mitigate the high water coming down the James, Vermillion and Big Sioux rivers.
“This was an attempt to reduce the peak stage between Sioux City and Omaha,” said John Remus on Monday in the teleconference. He’s the chief of the Army Corps’ Missouri River Basin Water Management Division in Omaha. “Our models indicated this reduction may prevent the flood from overtopping Interstate 29 north of Omaha.”
But by Saturday, Sept. 21, the Army Corps says it will switch to its other main mission to evacuate the system's storage and jack up the Gavins Point releases to 80,000 cfs--well over twice the normal release rate this time of year--and will keep it there through October.
Releases from Oahe Dam were reduced from about 57,000 cfs to 49,000 cfs over the weekend to help with the recent floods downstream. But they are slated to go back up to 57,000 on Wednesday, Sept. 18 and up to 58,000 cfs on Sept. 23; then 61,000 on Sept. 29 through October, Army Corps officials said.
The level of Oahe Reservoir is at 1,615.3 feet above sea level, where it’s been for a week or more. That’s 1.7 feet below the “exclusive flood control zone” and 7.8 feet above the “annual flood control and multiple use zone.”
Now the longer-range challenge is to get the reservoir levels down enough by Dec. 1 to be ready for what 2020 brings, Remus said Monday.