The drought has already created a low-water choke point south of St Louis, near the town of Thebes, where pinnacles of rock extend upwards from the river bottom making passage treacherous.

Tim McDonnell/Climate Desk

Cutting off the transport route would be a disaster that would resonate across the midwest and beyond.

"There are so many issues at stake here," said George Foster, owner of JB Marine Services. "There is so much that moves on the river, not just coal and grain products, but you've got cement, steel for construction, chemicals for manufacturing plants, petroleum plants, heating oil. All those things move on the waterways, so if it shuts down you've got a huge stop of commerce."

Local companies which depend on the river to ship their goods are already talking about lay-offs, if the Mississippi closes to navigation. Those were just the first casualties, Foster said. "It is going to affect the people at the grocery store, at the gas pump, with home construction and so forth."

And it's going to fall especially hard on farmers, who took a heavy hit during the drought and who rely on the Mississippi to ship their grain to export markets.

Farmers in the area typically lost up to three-quarters of their corn and soy bean crops to this year's drought. Old-timers say it was the worst year they can remember.

"We have been through some dry times. In 1954 when my dad and grandfather farmed here they pretty much had nothing because it was so dry," said Paul McCormick who farms with his son, Jack, in Ellis Grove, Illinois, south of St Louis. "But I think this was a topper for me this year."

Now, however, farmers are facing the prospect of not being able to sell their grain at all because they can't get it to market. The farmers may also struggle to find other bulk items, such as fertiliser, that are typically shipped by barge.

"Most of the grain produced on our farm ends up bound for export," said Jack McCormick, who raises beef cattle and grain with his father. "It ends up going down the river. That is a very good market for us, and if you can't move it that means a lower price, or you have to figure out a different way to move it. It all ends up as a lower price for the farmers."

The shipping industry in St Louis wants the White House to order the release of more water from the Missouri river, which flows into the Mississippi, to keep waters high enough for the long barges that float down the river to New Orleans.

Foster said the extra water would be for 60 days or so--time for the Army Corps of Engineers to blast and clear the series of rock pinnacles down river, near the town of Thebes, that threaten barges during this time of low water.

But sending out more water from the Missouri would doom states upstream, such as Montana, Nebraska, and South Dakota, which depend on water from the Missouri and are also caught in the drought.