When it comes to maneuvering a powerful towboat in as little as four feet of muddy water, it helps to know the Mississippi River as intimately as Arthur Ward does.

"It's a slow process," the 72-year-old captain of the tricolor Ricky Robinson towboat explained as he delicately shunted utilitarian green and brown freight barges around the port of Memphis.

"You have to go about half-speed," he told AFP in his vessel's panoramic, air-conditioned wheelhouse, "and you have to stay in the center (of the channel) or else you'll knock off a rudder or pick up a rope."

Drought across much of the United States this summer has seen the mighty Mississippi -- a strategic waterway that runs 2,530 miles (4,070 kilometers) from Minnesota to the Gulf of Mexico -- drop to its lowest levels in years.

So drastic is the problem that even this past week's downpours from Hurricane Isaac -- downgraded to a tropical depression as it crept north from the Gulf of Mexico -- was unlikely to make much of an impact, officials said.

"It could provide a very brief respite... but we're not certain how big a difference it is going to make," said Donald Mayer, chief of the navigation section for the US Army Corps of Engineers in Memphis.

It's a 180-degree turn from last year, when too much rain in the American heartland swelled the Mississippi, provoking some of the worst floods in a century along the longest river in the United States.

"During the 2011 flood, the river was actually roughly three miles wide," said Mark Manning, a survey boat operator for the US Army Corps of Engineers, which dredges and maintains the Mississippi for shipping.

"Now... we are probably around 1,500 feet wide to 2,000 feet (450 to 600 meters) wide."

With the Mississippi at Memphis about 12 feet below its normal levels, towboat operators say they are feeling the pinch.

"It's hard on our equipment because the water is so shallow, we're damaging our equipment," said George Leavell, executive vice president of Wepfer Marine, which operates the Ricky Robinson and 21 other towboats at eight ports along the Mississippi and two of its tributaries.

"We're having to handle more barges because they can't load them so deep, and we've lost a significant amount of our fleeting area -- which is where we park the barges (along the sandy river bank) -- because the water's so low."

The last time the Mississippi dropped so low was in 1988, when barge industry losses were estimated at $1 billion. While this year has not yet seen such extremes, it is worryingly close.

Twice this month, barges have run around at Greenville, Mississippi, downriver from Memphis, forcing a halt to navigation and holding up hundreds of other barges making their way up or down the river.

Typically, a Mississippi river barge is 200 feet long by 35 feet wide. Fully loaded under normal conditions, it can hold as much product as 70 tractor-trailer highway trucks.

But lower water levels mean barges are going out only partially full, to avoid hitting the river bed.

Every one-inch (2.5-centimeter) drop in water level slashes the carrying capacity of a single barge by 17 tons of cargo, says the American Waterways Operators, an inland marine trade association.

"A lot of these are grain barges that will be used to haul corn, soybeans and rice," Leavell told AFP, pointing from the deck of the Ricky Robinson to the row of empty barges parked on the shore.

"These grain barges back here," he added, waving in another direction, "transport high fructose corn syrup. Then we have one right around the bend here that is a cement barge."

Chemicals, petroleum products and coal to fire power stations also travel by barge, so higher shipping costs brought on by the lower river levels can mean higher prices for the end consumer.

And this year, the increase comes on top of higher farm commodity prices resulting from the drought's impact on US grain production, much of which goes down the Mississippi to Asian and other global markets.

"Consumers have to be willing to pay the price, and the cost of getting to and from wherever (products) are processed is part of the price," said economics professor John Gnuschke of The University of Memphis.

Wepfer Marine's Leavell noted the problem was becoming "serious."

"We'll have to reduce drafts potentially more. The river channel may be able to handle it, but getting the barges in and out of these small ports will be the problem," he said.