The barges that wind down the Mississippi River carry grain by the billions of bushels to be sold overseas, navigating an aging system of locks and dams. Delays can add days to the trip and thousands of dollars to the cost of shipping.

"Unless we take immediate steps," said corn farmer Greg Guenther of Belleville, Ill., "we will no longer be the lowest-cost provider to world markets. It's a very simple decision. We're either going to be in the world export market or we're not."

Guenther and other midwestern farmers are behind a $1.46 billion construction plan to replace seven upper Mississippi and Illinois River locks with longer, 1,200-foot gateways.

After more than 10 years of study, scandal, investigation and delays, the Army Corps of Engineers and its supporters in Congress are ready to make it happen. A Senate committee approved it last month. Similar legislation was recently introduced in the House.

Environmental organizations, taxpayer watchdog groups and other opponents are just as determined to keep blocking it, despite plans by the agency to spend an equal amount on ecosystem restoration.

Locks and dams were built decades ago to help barges smoothly travel a river that drops an estimated 350 feet along its upper reaches from Minneapolis to St. Louis.

Barges enter the lock chambers at more than two dozen locations, where water levels are lowered or raised depending on whether the boat is headed north or south.

Iowa corn grower Tim Burrack estimates that routine slowdowns on the river cost him about a nickel a bushel.

Then there are the delays. In the late 1990s, a barge just loaded with his grain bumped a lock gate and damaged it. The accident shut down traffic on that stretch of the river for days until the Corps of Engineers could replace the gate. By that afternoon prices had fallen 10 cents a bushel, he said.

The river's more modern locks are 1,200 feet long, about the length of today's barges. These locks can handle the process fairly smoothly in just minutes.

Older locks and dams are only half as long. At these points, two crews must separate the barges into pieces, send them through one at a time and then lash them back together on the other side.

"It's traditionally two hours, and that's just if you're the only one there," Burrack said. "You can be sitting there for hours, maybe even days, during the heavy harvest season if you get a bottleneck."

It is still cheaper to ship grain on the river, Guenther said. He bought his own truck because it costs 13 cents to 16 cents a bushel to take his grain to the river, which is just 25 miles from his Illinois farm. It costs roughly the same to ship the grain all the way down the river to New Orleans, he said.

About 85 million tons of grain from 19 midwestern states travel down the Mississippi to New Orleans every year. About half of all U.S. soybeans and an estimated 20 percent of all corn is sold overseas.

The potential to make river shipping cheaper -- and make U.S. grain more competitive -- is a rationale for the Corps and other supporters of the massive plan to build new locks.

Critics also use grain export figures to argue against the project.

"The future of U.S. grain exports has been grim for the past quarter of a century," said Philip Baumel, a recently retired agriculture economics professor from Iowa State University. "The odds are that exports will not increase. They certainly will not increase at the extent that the Corps' projections suggest. They may even decrease."

Baumel was on a National Academy of Sciences panel that in 2001 found flaws throughout the Corps' justification for the project. That review, and an Army internal investigation saying the Corps manipulated data to justify lock construction, followed whistle-blower allegations the corps fabricated its case for enlarging the locks.

The Corps went back to the drawing board. A subsequent academy review last year recommended the Corps make several changes to lock operations before it considers replacing the structures. Nonetheless, the Corps moved ahead in April, saying the system is so old it could suffer a catastrophic breakdown.

The main economic justifications for the new locks assume that barge traffic on the river will increase, although people on both sides of the issue say traffic is difficult to forecast.

The towboat Robin B. Ingram travels through a lock along the Mississippi River near LaCrosse, Wis.