The Mississippi River has receded to critically low levels after months of drought, severely affecting U.S. commerce.

Already, barges are being filled only to 50 or 60 percent capacity so they won't sit as low in the water, easing passage through shallower channels, the St. Louis Post-Dispatch reports. But that means more vessels are required to transport the same tonnage.

Upstream of St. Louis, a network of locks and dams makes the channel deep enough for navigation but below St. Louis there are no dams to control the depth of the channel.

This week the U.S. Army Corps of Engineers began removal of submerged rock formations, on the river at Thebes, Ill., south of St. Louis. The program, the corps said, is expected to add 6 inches of water depth by next week.

"Every inch counts right now. It's not a permanent fix. But we're using every tool at our disposal to keep commerce moving," Mike Peterson, a corps spokesman was quoted as saying by The Christian Science Monitor.

But the Mississippi River shipping sector says that action isn't enough.

"While any additional water is welcome, shippers and barge operators caution that this offers only a delay of the inevitable, an effective halting of barge transportation around the end of this month as Mississippi levels continue to fall to a level that cannot support most navigation," the American Waterways Operators and the Waterways Council, Inc., said in a release.

The groups have been calling for what they say are minimal flows from the Missouri River to be released to avert an effective shutdown of Mississippi River to barge transportation.

They warn that for the next one to two months there will be just one-way traffic for just eight hours a day "in the very best case scenario."

"The effects of this crisis are already being felt by industry workers, shippers, farmers and manufacturers up and down the river and they are going to get worse," said AWO President and Chief Executive Officer Tom Allegretti.

If navigation on the Mississippi isn't maintained, the groups warn, the potential supply-chain disruption could affect nearly 20,000 jobs and $130 million in wages in Mississippi River states as well as $7 billion in commodities in December and January.

More than one-third of those losses -- $2.3 billion -- represent farm exports.

Other expected losses include 1.3 million tons of petroleum products worth more than $1.3 billion; more than 700,000 tons of crude oil worth $534 million and 3.8 million tons of coal worth $192 million, the groups said.