Her Missouri colleague in the Senate, Republican Roy Blunt, acknowledged "friction" between upper Missouri River interests that control the flow and those downstream on the lower Missouri and Mississippi rivers. He said the corps "needs to manage that balance."
During the years, parts of the river have occasionally been closed because of low water, barge accidents, dredging, ice and flooding. But this shutdown, if it happens, would affect a pivotal stretch that is used for two-way traffic -- shipments going south to the Gulf as well as transports from the Illinois and Ohio rivers headed north to Chicago and Minneapolis.
A two-month shutdown -- the length of time that some observers fear given current conditions -- would have an estimated impact of $7 billion, according to the river industry trade group American Waterways Operators.
Consider agricultural products. It costs 30 to 35 cents more per bushel to send grain to the Gulf by rail instead of barge -- a massive figure when calculating the millions of bushels shipped downriver.
"When you think of all we buy at the grocery store that has grain and corn, consumers could really see it hit them in the pocketbooks," said Ann McCulloch of the Waterways Operators group.
The Coast Guard controls navigation on the river and decides when to require restrictions or shut it down.
"It's really played by ear," said Coast Guard Lt. Colin Fogarty. "The Mississippi River is a dynamic environment."
River shippers are bracing for the worst, weighing train and truck alternatives to move a staggering volume of cargo, if necessary.
Seven million tons of farm products are shipped via barge in a typical December-January period, along with 3.8 million tons of coal, 1.7 tons of chemical products, 1.3 tons of petroleum products and 700,000 tons of crude oil, McCulloch said.
Trains already haul a vast volume of material, but switching from river to rail isn't that easy, especially on short notice. Cargill, for example, uses 1,300 of its own barges on inland waterways. Finding that much capacity elsewhere is no simple task.
"We'll look for other sources of transportation to the extent we can. But if you take away this important artery, you can't just snap your fingers and replace it with trains," Calhoun said. "There aren't just trains sitting around. They're already pretty busy with their business on their books."
Tractor-trailers can pick up some of the slack. But some cargo, such as coal, just isn't cost-effective to haul by truck over long distances, said Bob Costello, an economist with the American Trucking Associations.
Businesses operating directly on the river are bound to suffer, too.
George Foster founded JB Marine Service Inc. in St. Louis 36 years ago to make a living fixing and cleaning barges. An extended river closure may force layoffs, he said. And he figures many other companies will be forced to cut jobs, too.
"It's extremely dire," Foster said. "There's no way to sugarcoat it."