The Coast Guard closed the swollen Mississippi River north of New Orleans on Tuesday, halting cargo vessels on the nation's busiest waterway in the latest effort to reduce pressure from rising floodwaters.

The 15-mile closure at Natchez, Miss., blocked vessels heading toward the Gulf of Mexico and others trying to return north after dropping off their freight. If the channel remains closed, it could bring traffic to a standstill up and down the mighty river, which moves about 500 million tons of cargo each year.

And the interruption could cost the U.S. economy hundreds of millions of dollars for every day that it idles barges carrying coal, timber, iron, steel and more than half of America's grain exports.

Coast Guard officials said wakes generated by passing barge traffic could increase the strain on levees designed to hold back the river. Authorities were also concerned that barges could not operate safely in the flooded river, which has risen to the level of some docks and submerged others.

It was not clear how long the channel would be closed. Authorities suggested at least a few days.

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The river is expected to stay high in some places for weeks.

"Several days is not something anyone likes, but it will not be catastrophic," Port of New Orleans spokesman Chris Bonura said.

The Coast Guard did not have comprehensive figures on how many vessels were immediately affected, but the agency stopped at least 10 near Natchez.

In past closures, those numbers have grown quickly. In 2008, the agency halted 59 ships within a day of shutting down a stretch of the river near New Orleans because of a barge and tanker collision.

Shipping companies hoped for a swift reopening, but if the channel were to remain closed for a longer period, the economic pain was sure to intensify -- and spread well beyond the South.

On a typical day, 600 barges move up and down the river, according to Bob Anderson, spokesman for the Mississippi Valley Division of the Army Corps of Engineers. A single barge can carry as much cargo as 70 tractor-trailers or 17 rail cars.

"When it shuts, there's really no alternative," said Jim Reed, president of the Illinois Corn Growers Association.

Anderson said he expects barges to be asked to stay anchored in the middle of the river so they do not add to the flooding.

The Coast Guard's traffic-management division hoped to prevent barges from piling up on either side of the closed zone by requiring them to be at least 1,200 feet apart.

Also Tuesday, at least 10 freight terminals along the lower Mississippi between Baton Rouge and New Orleans suspended operations because of high water. Vessels scheduled to use the terminals will either have to wait out the high water or divert elsewhere. Delaying a vessel by even a single day often costs $20,000 to $40,000, port officials said.

A Port of South Louisiana spokesman expected many barge operators to stay in port after loading or unloading and then wait until the river reopens.

Throughout the spring, the Mississippi is a highway for barges laden with corn, soybeans and other crops headed from the Midwest to ports near New Orleans, where they get loaded onto massive grain carriers for export around the world.

The closure helped push corn, wheat and soybean prices higher Tuesday.

Traders were already nervous about tight supplies of all three crops. Farmers are behind on their planting because of cold, wet weather. At the same time, global supplies have been depleted by rising demand from ethanol makers and livestock producers.

The price of corn jumped 22.75 cents to $7.20 a bushel, while soybeans rose 14.5 cents, to $13.41 a bushel. Wheat rose 27.5 cents to settle at $7.64 a bushel.

While prices might spike in the short-term, flooding and port closures along the Mississippi River probably will not affect crop prices for long, said John Sanow, an analyst with DTN Telvent.

Traders are more worried that flooded acreage won't be replanted with corn, he said.

If the closure persists, the crop exports most likely to be affected are corn and soybeans, said Jason Ward, analyst with Northstar Commodity in Minneapolis.

The effect could be more acute for local buyers along the Mississippi, like poultry companies that buy bulk feed directly off the river, Ward said. They could wait weeks or months without being able to fill an order if barge traffic does not resume.

"It only affects that guy that's waiting for corn," Ward said. "The question is how long it's going to be down."

At the Port of South Louisiana, which stretches along both sides of the Mississippi for 54 miles, Operations Director Mitch Smith said the closure will have "a definite impact" on grain exports, though it was too early to say how much.

The port, the largest in terms of tonnage in the United States, handles 54 percent of the nation's annual grain exports.

A lot will depend upon how long the river is closed and how many barges are trapped upriver, Smith said.

The port handles about 60,000 barges a year, along with 4,500 to 5,000 deep-draft vessels that carry grain and other bulk cargo such as steel.

The Mississippi also conveys most of New England's home heating oil and gasoline, along with 20 percent of America's coal, according to the American Waterways Operators, the trade group of the barge operators.

American Commercial Lines, which operates more than 2,500 barges and 125 tow boats, feared the closure could last as long as a week in a worst-case scenario.

The closure was the third in a series of recent moves designed to protect homes and businesses behind levees and floodwalls along the river.

Over the weekend, the Army Corps opened the Morganza Spillway, choosing to flood rural areas with fewer homes to protect Baton Rouge and New Orleans. Another spillway near New Orleans was opened earlier, but it did not threaten homes.

The river is expected to crest Saturday in Natchez at 63 feet, down a half-foot than earlier predictions. But that level is still nearly five feet above a record set in 1937. It could take weeks for the water to recede.

Natchez Mayor Jake Middleton said if the city's levees were damaged, it could endanger hospitals, a convention center and historic buildings both in Natchez and across the river in Louisiana.

The floodwaters have displaced more than 4,800 people in Mississippi, including 80-year-old Leslie J. Sherwin, who was pushed out of his house in Tunica County three weeks ago and into a shelter.

"The road is cutoff. The house is flooded, and I can't even go home," Sherwin said. "But hey, nothing lasts forever. I've been knocked down many times. I'm just going to do what I've got to do, day by day."