The Coast Guard has interrupted shipping along the major artery for moving grain from farms in the Midwest to the Gulf of Mexico over fears that the bulging Mississippi River could strain levees that protect hundreds of thousands from flooding. Already, thousands have sought refuge from floodwaters up and down the river.

The Coast Guard said it closed the Mississippi River at the port in Natchez, Miss., because barge traffic could increase pressure on the levees. Heavy flooding from Mississippi tributaries has displaced more than 4,000 in the state, about half of them upstream from Natchez in the Vicksburg area.

Several barges were idled at Natchez at the time of the closure, and many more could back up along the lower Mississippi. It wasn't clear when the river would reopen, but port officials said the interruption could cost the U.S. economy hundreds of millions of dollars per day.