ST. LOUIS — After months of drought, companies that ship grain and other goods down the Mississippi River are being haunted by a potential nightmare: If water levels fall too low, the nation’s main inland waterway could become impassable to barges just as the harvest heads to market.

Any closure of the river would upend the transport system that has carried American grain since before steamboats and Mark Twain. So shipping companies are scrambling to find alternative ways to move tons of corn, wheat and other crops to the Gulf Coast for shipment overseas.

‘You can’t just wait until it shuts down and suddenly say, ‘There’s a problem,” said Rick Calhoun, head of marine operations for Chicago-based Cargill Inc. ‘We’re always looking at Plan B.’

The mighty Mississippi is approaching the point where it may become too shallow for barges that carry food, fuel and other commodities. If the river is closed for a lengthy period, experts say, economic losses could climb into the billions of dollars.

It isn’t just the shipping and grain industries that will feel the pinch. Grocery prices and utility bills could rise. And deliveries of everything from road-clearing rock salt for winter and fertilizer for the spring planting season […]