For eight hours a day, shipping is allowed to move one way in the 180 miles of river between St. Louis, Mo., and Cairo, Ill., depending on the hour. For the other 16 hours, boats go nowhere, because the river is closed to traffic.
The mighty Mississippi, parched by the historic summer drought, is on the verge of reaching a new low. That could mean that tugboats pushing barges loaded with billions of dollars worth of cargo - enough to fill half a million 18 wheelers - would not be able to make their way up and down the river.
Through the night, contractors for the U.S. Army Corps of Engineers remove rocks from a stretch near Thebes, Ill., that threaten to cut boats to shreds. The corps has assured state officials, farmers and coal barons who rely on the shipping that it can maintain the nine-foot level it says makes navigation safe.
But those who rely on the river say they are worried nevertheless.
As of Friday, National Weather Service hydrologists forecast that the river near Thebes could drop below a point that would allow barges to safely navigate with heavy cargo, forcing the Coast Guard to restrict weight, and effectively shutting down commerce late this week, according to reports by the Associated Press.
But the Army Corps and Coast Guard assured state officials that the Mississippi will remain open. Recent rains and water releases from the corps' Carlyle Lake in Illinois improved water levels for the Middle Mississippi River, the corps said.
"There's nothing pretty about this," Coast Guard Lt. Colin Fogarty said Friday. "We are facing a historic drought. River levels are at record lows we haven't seen since 1941. Over six weeks the Army Corps has dredged record amounts of the river."
But, Fogarty said, reports that the Mississippi will close are as reliable as doomsday projections "based on the Mayan calendar." Tamara Nelson, senior director of commodities for the Illinois Farm Bureau, has faith in the corps, but is worried by the long Midwest dry spell.
"Not being able to move anything on that river will be critical, a big hit," she said. "It affects tax revenues for the federal government, it affects jobs. "I don't think you can really exaggerate the level it has fallen to," Nelson said. "I've been here 15 years in Illinois, worked in agriculture almost 30 . . . and to see what is typically a bank-to-bank Mississippi full of water . . . now become almost hillsides of sand is like watching a lake empty."
Iowa, Missouri and Louisiana are also heavily dependent on the Mississippi. In 2010, the Port of Metropolitan St. Louis shipped and received more than 30 million tons of cargo worth about $7.5 billion, making it the nation's third-busiest inland port, according to the Waterways Council.
During a typical January, $2.8 billion worth of goods flows between St. Louis and Cairo, 5 to 7 million tons of grain for cattle feed all over the world, coal for power plants across the globe and cement for construction, according to Illinois officials.
In that state alone, the commerce supports 6,500 to 8,000 jobs with $50 to $54 million in wages, said Adam Pollet, acting director of the state Department of Commerce.
Even if shipping continues on the shallow water, tugboat operators will order barges to lighten their loads as a safety precaution. Lightening loads is like money falling through a hole in a pocket.
"We can't say everything is being threatened, but there's some amount of risk to those jobs," Pollet said.
More than a quarter of all commodities that Illinois sells go down the river and 5percent of its imports go up, said Ann Schneider, secretary of the state's Department of Transportation.
On the one hand, Gov. Pat Quinn, D, and Sen. Dick Durbin, D-Ill., pressed the corps to do everything in its power to keep the water level above eight feet, prompting removal of rocks scheduled to begin at the end of this month to be moved up.
On the other, Quinn has order his administration to line up more rail cars and trucks in case the worst happens. A tugboat hauling 15 barges carries as much freight as more than 200 rail cars or 1,000 trucks.
The stage was set for record-low water levels in the summer, during the great drought. More than three-fifths of the contiguous United States were struck by moderate drought or worse, the most extensive dry spell since 1956 and comparable to the Dust Bowl era of the 1930s.
The National Weather Service in Louisiana forecast then that the lack of rain would persist and predicted low flows on the Mississippi River.
David Gay, an Illinois family farmer with 1,500 acres in the Mississippi Valley, 90 miles north of St. Louis, lost half his corn crop in the drought. He was happy that the Army Corps kept the river open so that his grain could move to the purchaser, a Japanese importer.
But now Gay is worried that the river will be too low for boats to bring the fertilizer he needs to prepare his land in March for planting. "I think there was more fertilizer applied in fall than usual because some people are worried about that. I know we did," Gay said.
The shallow Mississippi "is a concern that could turn into a crisis down the road," said Gay, whose farm is on its banks.
"If you don't live near the river system and see how important it is as far as commerce, it's easy to forget about it, and what it means to the middle part of the country," he said.