The Army Corps of Engineers on Friday outlined its plans to return Missouri River flows to pre-flood levels by late summer.
The corps will reduce Garrison Dam releases gradually over the next two months, reaching 25,000 cubic feet per second by Sept. 26 from the peak flows of 150,000 cfs arrived at in June.
Releases from Garrison Dam are scheduled to fall to 60,000 cubic feet per second Sept. 1, which will mark the first time since May 24 that dam releases will be under the record peak that stood from 1975 until the 2011 flood, the largest seen since the dam’s construction.
Corps leadership announced its schedule in a conference call Friday morning.
“The bottom line is that we need to get water out of the system as soon as possible,” said Brig. Gen. John McMahon, commander of the corps’ Northwestern Division. “We have a very short time in which to do that. The 2012 evacuation season begins March 1.”
The corps’ priority is to regain flood storage capacity in its reservoirs, but McMahon said it would not lower reservoirs below their standard spring flood control levels. For Lake Sakakawea, that is a surface elevation of 1,837 feet.
“We also looked at the probability of this event occurring again in 2012,” said McMahon, who called this year’s runoff a “one in 500-year event.”
“We came to the conclusion that we would not need additional storage in the system,” he said.
Corps officials said that a priority in their plan was to return the river to normal levels as soon as it was practical to give people and municipalities along the river time to rebuild damaged infrastructure and other property and allow for inspections of dams and levees. Lowering reservoirs below their annual flood control levels would require the corps to keep the river running high and would not allow rebuilding and maintenance.
“We decided based on other factors including getting people back into their homes and businesses that it wasn’t prudent this year,” said Jody Farhat, chief of Missouri River basin management. That does not mean that the corps could not later decide to change flood control levels.
“We are certainly not precluding a longer range change to that level,” McMahon said, referring to Sakakawea’s 1,837 level.
The higher flow rates necessary to lower reservoirs below their normal flood control levels would also put more strain on flood protections along the river that have already been tested by the high summer flows.
“We have serious concerns about how those levees are performing,” McMahon said.
He also said that releases must come down gradually or the sudden loss of pressure against levees and riverbanks could cause them to collapse.
The surface elevation of Lake Sakakawea on Friday was 1,852.1 feet. The corps projects that it will be 1,842.7 on Sept. 30.
Releases from Garrison on Friday were 110,000 cfs. To get down to 25,000 cfs, the corps will lower outflows in increments of 5,000 cfs every few days. Its next reduction will be to 105,000 cfs on Aug. 4. The corps’ full release forecast through Sept. 30 is available here.
Todd Sando, state engineer with the Water Commission, said that the corps’ plans were “reasonable” and addressed the need to move the river out of flood stage and get the reservoirs ready for next year’s flood control needs.
“I think that’s the right approach. We can’t allow the flood to continue all fall and winter,” he said. “I think they found a pretty good middle ground for everyone.”
There are varying estimates of how far flows will have to fall before the river in Bismarck-Mandan goes below flood stage of 16 feet. As the release rates have fallen, the river level has not reacted in the way it did while rates were being ramped up. “I’ve got numbers ranging from 75,000 to 85,000 cfs,” according to Sando, who said that the river was high in mid-May when flow rates were 55,000 cfs. “We’re going to be above 100-year flows for the next three weeks.”
The Missouri was at 18.19 feet at 3 p.m. Friday, according to the National Weather Service, roughly a foot below its peak this this summer.
Although the corps will be entering next year’s flood season with Lake Sakakawea at the same elevation as last year, Sando said they had little choice if flows are to return to normal this year. However, he said the corps had to do a better job planning for the run-off of plains snowmelt, which along with mountain snowpack and heavy May rainfall contributed greatly to the flooding.
“I don’t think they have a good measure of how to handle the plains,” he said. “There’s got to be a better understanding of how much comes off of the plains.”
McMahon and Farhat said that the plan could be adjusted if weather makes it necessary.
“As with any plan, it’s just a plan,” McMahon said.
(Reach reporter Christopher Bjorke at 250-8261 or firstname.lastname@example.org.)