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 crops to the Gulf Coast.

The focus of greatest concern is a 180-mile stretch of the river between St. Louis and Cairo, Ill. That's where lack of rain has squeezed the channel from its normal width of 1,000 feet or more to a just a few hundred feet. And it's shallow.