15 miles of the flooded Mississippi River closed

Updated 6:49 a.m. ET

The Mississippi River flooding currently underway throughout the South and Midwest has already caused millions of dollars in damage, and now some of the region's businesses are facing their worst fear: the closure of the river to traffic.

The Coast Guard closed 15 miles of the river near Natchez, Miss., and gave no time frame for reopening the waterway, CBS News has confirmed. The closing is supposed to protect ships and flood structures from further damage, the Coast Guard said.

The river was projected to crest at 57.5 feet in Vicksburg, Miss., on Thursday and 63.5 feet in Natchez on Saturday.

Two vessels were waiting to move northbound on the river by the closure and one ship was waiting to move southbound, Reuters reports.

CBS News reported earlier that closing the river near New Orleans, a few hundred miles downstream from Natchez, could cost the U.S. upwards of $300 million per day.

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The river has yet to crest near New Orleans, which has one of the nation's busiest ports.

Reuters reports that the eight Louisiana refineries dependent on the river to bring crude oil from the Gulf of Mexico and to move refined products into the Gulf, are not affected by Monday's closure.

The Mississippi River is a vital lifeline of America's economy, but if it rises above 18.5 feet near New Orleans - two feet higher than it was on Saturday - the Coast Guard may have to shut down shipping traffic on the river. With 12,000 ships carrying 500-million tons of cargo annually and 700,000 cruise passengers, New Orleans is one of the busiest ports in the country.

"Every day that this river is closed, the detrimental economic consequence not to New Orleans but to the entire United States is $295 million a day and it grows exponentially after the fourth day," said Gary Lagrange, CEO of the Port of New Orleans.

The Mississippi was last closed in 2008 when a fuel barge and a tanker collided, spilling more than 400,000 gallons of fuel into the water. That cost the city of New Orleans an estimated $2 billion.