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* River down 55 feet (16.8 metres) from 2011 flood level
* Commodities transport costs seen rising as river shrinks
By Emily Le Coz
TUPELO, Miss., July 26 (Reuters) - One year after its waters swelled to historic proportions, the drought-ravaged lower Mississippi River now sits so low that barge operators hauling some $180 billion in goods must lighten their loads for fear of getting stuck.
If water levels drop further on this main artery of the U.S. waterway system, prices could rise on the raw commodities commonly shipped by boat - coal, grain, petroleum and steel, to name a few.
The Mississippi River carries 566 million tons (513,467 tonnes) of freight per year, said Ann McCulloch, spokeswoman for American Waterways Operators, a national trade association representing tugboats, tow boats and barges.
"The main thing that they're doing now is voluntarily reducing the size of their tows ... so they're having to take more trips to carry their normal volume of commodities," McCulloch said. "This will drive up transportation costs if it continues over a long period of time."
Barges must unload 17 tons (15 tonnes) of cargo for every one-inch loss of water and 204 tons (185 tonnes) for every one-foot (30.5 centimetre) loss of draft, said Tom Allegretti, president of the trade association.
Draft is the vertical distance between the ship's waterline and the lowest point of its keel.
"When you consider that a typical tow on the upper Mississippi or Ohio Rivers has 15 barges, a one-foot loss of draft will decrease the capacity of that tow by 3,000 tons (2,720 tonnes)," Allegretti said in a statement.
"The tows on the lower Mississippi River are larger, consisting of 30-45 barges, resulting in decreased capacity of over 9,000 tons (8,165 tonnes)," he said.
He said it would take 130 semi trucks or 570 rail cars to haul the freight unloaded by one large barge under those conditions.
On Thursday, Kirby Corp, the largest U.S. inland tank barge operator, said it was adding capacity to its fleet that carries petrochemicals, gasoline and fertilizer, as it has been forced to reduce boat-loads because of low river levels.
The company said it has been incurring costs of about half a million dollars per month since mid-May because of reduced barge loads and delays.
During the 2011 floods, the Mississippi crested at nearly 48 feet (14.6 metres) above the baseline near Memphis, according to the National Weather Service.
Now with the U.S. Midwest locked in the most extensive drought in five decades, the river on Wednesdays had dropped to 7.1 feet (2.16 metres) below the baseline. That is 13 feet (4 metres) below normal for this time of year and 55 feet (16.76 metres) below last year's level.
It could drop another 2.5 feet (76.2 cm) by August, said Marlene Mickelson, meteorologist for the National Weather Service, calling that a "worst-case scenario."
The river typically is 6.1 feet (1.86 metres) above the baseline in July.
While the level has not yet hit the historic low of 10.7 feet (3.26 metres) below baseline, recorded in 1988, it is unusual to see it so dry this time of year.
"It normally happens in August and September that we're low," Mickelson said. "We're a month early."
To prevent closures along the Mississippi and keep commerce flowing, the Army Corps of Engineers has been dredging the waterway all summer, said its spokesman, Jim Pogue.
Dredging removes the silt that has fallen to the bottom of the river, which accumulates more when water levels drop.
"When the river is down," Pogue said, "it flows slower and so the sediment settles and ends up on the bottom."
Lessons from the 1988 drought, which forced week-long closures along the waterway, prompted changes in the way the Army Corps of Engineers maintains the river and, as a result, have lessened the impact of this year's drought, Pogue said.
The Corps has built dikes and other structures that direct the flow of water in key areas and help the river keep itself clean, he said.
"As a result we're in much better shape than in 1988," he said.