PITTSBURGH — Pittsburgh's three rivers, the waterways largely responsible for the city's birth and prosperity, are expected to see increased traffic in the coming years, but some officials worry that the decaying system may not weather the increase.

Locks and dams built 100 years ago need as much as $2 billion in repairs, including $43 million in emergency work to an eroding base, said James McCarville, the Port of Pittsburgh's executive director.

Today, 43 million tons of cargo moves along the Allegheny, Monongahela and Ohio rivers. The cargo industry employs about 45,000 people, with another 150,000 employed in support industries, according to a 2006 study by the port.

"This whole system of locks and dams has so much to do with commerce in this region, with our quality of life," Sen. Bob Casey, D-Pa., said.

Port and shipping officials expect million of tons of additional traffic on the rivers in the coming years. Two planned ethanol plants will need tons of corn to turn into fuel, all of which will arrive by boat. New environmental regulations require coal-fired power plants to build scrubbers that need about 500,000 tons of limestone annually, also to be delivered by water.

"It's one of the foundations of the economy in the area," James Grech, senior vice president of Consol Energy, said of the rivers.

The federal government spent the late 1800s and early 1900s investing in the system needed to make Pittsburgh's three rivers an efficient water transport system. The locks and dams balance out river depths that would otherwise fluctuate between 20 feet and a shallow 18 inches, McCarville said.

Much of the century-old infrastructure was built to last about 50 years, he said.

"It's not new anymore. Now we've got to figure out how to sustain the investment," McCarville said.

In Louisville, Ky. when a lock was shut down for nearly two weeks it almost closed the airport, which had only two days of jet fuel left when the repair was completed.

The U.S. government spends about $400 million annually on inland waterway systems. The U.S. Senate has set aside $55 million in its draft of the budget for lock and dam repairs in Western Pennsylvania, but the money is not included in the U.S. House's draft.

Sixteen percent of the nation's freight is moved along inland waterways but is only 2 percent of the total costs, said John Moran, a lobbyist for the Arlington, Va.-based Waterways Council. Officials from other countries, most recently from China and South America, come to the United States to learn how to build such a system, he said.

"It was and still is a world-class transportation system," Moran said.