NEW ORLEANS - The Coast Guard has interrupted shipping along the country's busiest inland waterway 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 and because of fears that barges couldn't operate safely in the flooded river. 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 major artery for moving grain from farms in the Midwest to the Gulf of Mexico. 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.

The closure is the latest high-stakes decision made to protect homes and businesses that sit behind levees and floodwalls along the river. To take pressure off levees surrounding heavily populated New Orleans and Baton Rouge, the Army Corps of Engineers opened the key Morganza Spillway, choosing to flood more rural areas with fewer homes.

Most residents in the path of the Morganza's floodwaters have heeded the call to leave their homes, with an estimated 4,800 people evacuated across the state.