JACKSON, Miss. - In the latest efforts to reduce pressure on levees from rising floodwaters, the Coast Guard first closed a 15-mile section of the Mississippi north of New Orleans to shipping for much of Tuesday, then allowed cargo vessels on the nation's busiest waterway to pass slowly and only one at a time.

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.

The closing of the stretch at Natchez, Miss., blocked vessels heading toward the Gulf of Mexico and others trying to return north after dropping off their freight.

Had the channel remained closed, it could have brought traffic to a standstill up and down the river, which moves 500 million tons of cargo each year.

That sort of interruption could have cost the U.S. economy hundreds of millions of dollars for every day of idled barges carrying coal, timber, iron, steel, and more than half of America's grain exports.

"We're closely monitoring traffic along the river, and all vessels must stay to the center of the river," Coast Guard Cmdr. Mark Moland said.

Moland said tests indicated that sandbagging and other measures to protect the area could withstand the wakes if the vessels moved through at the slowest possible speeds.

It was not clear how long barges would be allowed to move only one at a time through the section. The river is expected to stay high in some places for weeks.

The Coast Guard did not have comprehensive figures on how many vessels were immediately affected by the closing, but the agency stopped at least 19 near Natchez on Tuesday.

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

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

Natchez Mayor Jake Middleton said that 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.

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 have to either wait out the high water or divert elsewhere. Delaying a vessel by even a single day costs $20,000 to $40,000, port officials said.

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 grain carriers for export.

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