Experts say the stretch of Mississippi River where vessel traffic was halted after a barge hit a railroad bridge on Sunday is one of the most dangerous along the 2,500-mile-long river.
Late Monday, cleanup crews were skimming oily water near Vicksburg, a day after a barge struck a bridge, rupturing a compartment holding 80,000 gallons of oil.
Authorities said that the spill was light and that only a sheen had been spotted. Orange boom was stretched across part of the river downstream from the barge, and small boats patrolled the area as oil was pumped from the ruptured tank into another tank on the same barge. Officials hope to eventually transfer all the oil to another barge.
Tugs were holding the barge at the bank on the Louisiana side of the river, directly across from Vicksburg's Riverwalk and Lady Luck casinos.
Coast Guard spokesman Lt. Ryan Gomez said a tug was pushing two tank barges when the collision occurred about 1:30 a.m. Sunday. Both barges were damaged, but only one leaked. Authorities declared the bridge safe after an inspection.
Gomez said United States Environmental Services, an oil spill response company, was collecting oily water.
Officials did not yet have an estimate of how much oil had been pumped out, or how much spilled into the Mississippi.
Another Coast Guard spokesman, Petty Officer 3rd Class Jonathan Lally, said the oil was contained and skimmers would work through the night collecting it. He said a flyover by a Coast Guard helicopter from Vicksburg 50 miles to the south found no evidence of shoreline impact.
Authorities said a major environmental disaster was unlikely as the swift current dispersed the sheen. They were less certain when the river would reopen to vessels.
Drew Smith, a hydraulic engineer with the Army Corps of Engineers, wouldn't speculate on the specific cause of Sunday's crash, which is under investigation by the Coast Guard.
But he said the Mississippi at Vicksburg is challenging for southbound vessels, mostly barges carrying grain and other products from the nation's heartland.
Southbound tows must travel faster than the flow of the water for their rudders to steer effectively. At Vicksburg they must negotiate a 120-degree turn on the meandering Mississippi, then straighten up to pass under the railroad bridge and the Interstate 20 bridge.
The task is made more difficult by the Yazoo River, which empties into the Mississippi north of the bridges, increasing the speed of the current.
Herman Smith, superintendent of the Vicksburg Bridge Commission, said the railroad bridge is struck once or twice a year, usually during periods of high water. During the river's historic 2011 flood, the span took five hits over two weeks. The river isn't in flood stage now, he said.
"There's a curve to the north of us, about three-quarters to a mile away from us. But it's the current," Herman Smith said.
The river's other most dangerous stretch is at St. Louis. There, six bridges cross the river over a distance of four mils, Smith said.
On Monday, 24 tugboats, barges and other vessels were parked waiting for the river to reopen, said Army Corps spokesman Kavanaugh Breazeale. The river was closed to traffic for 16 miles — eight miles north and eight miles south of Vicksburg.
Ann McCullough, spokeswoman for the American Waterways Operators, a trade association for the U.S. tugboat, towboat and barge industry, said the shutdown is concerning. But she couldn't estimate the daily economic impact.
"It's a significant matter when the nation's waterborne superhighway is disrupted for any reason," she said.
During the 2011 flood, officials said delays in loading a ship — because barges can't move on the river — can cost shipping companies from $20,000 to $40,000 a day. But the river is busier at some times than others, so it's difficult to gauge the current impact.
The barges are owned by Corpus Christi, Texas-based Third Coast Towing LLC, Lt. Gomez said. A woman who answered the phone at the company Monday declined to comment.
Both vessels were being pushed by the tug Nature's Way Endeavor. The website for Nature's Way Marine LLC of Theodore, Ala., identifies the vessel as a 3,000-horsepower, 90-foot-long boat. It was built in 1974 and underwent a rebuild in 2011, according to the company.
A company manager referred calls to the Coast Guard in Vicksburg.
The last time an oil spill closed a portion of the lower Mississippi was in February 2012, when two barges collided, spilling less than 10,000 gallons. The river was closed for about a day. In 2008, a fuel barge collided with a tanker and broke in half, dumping 283,000 gallons of heavy crude and closing the river for six days.
On March 23, 2011, several barges broke loose, and some hit the U.S. 80 bridge and Interstate 20 bridge. One was hung up on the I-20 bridge for about three weeks before it was removed.
Sunday's spill was not expected to create environmental problems for the Gulf of Mexico, 340 miles to the south. The cargo of 80,000 gallons in the ruptured barge compartment doesn't compare with the more than 200 million gallons of oil that spewed from BP PLC's Macondo well after a blowout in 2010.
The BP well blew wild for months, while the Vicksburg spill has been contained and oil is being moved to safety.
Associated Press writer Janet McConnaughey in New Orleans contributed to this report.