Authorities reopened the Mississippi River to some boat traffic Wednesday, hours after freeing a barge that ran aground in dangerously low waters caused by an extensive drought.

A trickle of barges had begun heading north as of late afternoon, but clearing all 33 northbound tows and 72 southbound barge strings was going to be a slow process, said Coast Guard Petty Officer Ryan Tippets.

Southbound barges were expected to begin moving overnight, said Lt. Cmdr. Rebecca Walthour. A safety zone remained in effect along the 11-mile stretch of river that first reopened late Tuesday after dredges cleared enough mud to make a passable channel. Thirty-three vessels had made it through the channel before the barge went aground near dawn Wednesday, Tippets said.

The river has closed several times this month, most recently on Monday, and low-water conditions are expected to affect the flow of cargo from the nation's heartland until October. In addition to the river closings, port operations along the river in Missouri, Tennessee, Arkansas and Mississippi have been closed while the Army Corps works to dredge channels.

Aboard the Dredge Jadwin near Greenville, assistant master Chuck Ashley and his crew were working to create a channel north of where the barge went aground. By Saturday, they hoped to move to the exact place where the vessel got stuck, which Coast Guard officials say has been a repeat trouble spot during low-water occurrences.

"We're working it just as fast as we can to get this thing opened up," Ashley said.

Capt. William Drelling didn't have a precise count of how many vessels have gone aground during the latest drought, but said it has been "far more than normal." Improvements in barge construction over the years — including double hulls — have helped to prevent cargo spillage when barges do go aground, he added.

Once channels are cleared in the Greenville area, the Coast Guard will stagger traffic, with southbound vessels moving during the daylight hours and northbound traffic — typically lighter this time of year — moving at night, Tippets said. It's a system the Coast Guard used in the Natchez, Miss., area in 2011 when the river's record flood spawned extreme currents that threatened to push towboats and barges out of the main channel and into bridges and levees.

A prolonged slowdown of traffic on the river could hamper operations of the busy grain port upriver from New Orleans. Much of the grain harvested in heartland states is moved down the Mississippi by barge to the Port of South Louisiana in LaPlace, where it is stored and later loaded aboard massive grain ships bound for foreign markets. More than 4,000 cargo ships and 55,000 barges use the port each year, carrying 48 million tons of cargo — much of it U.S. grain.

On Wednesday, heavy shipping was largely unaffected in the busy ports south of Baton Rouge, La., where the channel tends to be deeper.

South of New Orleans, however, a weakened river flow caused by the low water levels was allowing salt water to creep in from the Gulf of Mexico and raising concerns that drinking water could be affected. The Army Corps of Engineers was busy building an underwater dam to keep the salt water, which is heavier than fresh water, at the bottom of the river until the flow picks up enough to push it back to the Gulf.

Farther north in Memphis, officials have predicted that the river level could drop to within a foot of its record low if the drought persists. The river level was minus-9.1 feet Wednesday, according to the National Weather Service. The "minus" reading does not mean the river is dried up — it's just a measurement based on how the Memphis river gauge is designed. Essentially, the reading means the river level is far below normal.

The record for the lowest measured water level for the Mississippi River near Memphis is minus-10.7 feet, in 1988. A paper published in the Bulletin of the American Meteorological Society in September 1989 estimated that the drought caused the U.S. barge industry to suffer total losses of about $1 billion that year.


Associated Press writer Janet McConnaughey in New Orleans contributed to this report.