John Schwartz / New York Times News Service

ABOARD THE DREDGE POTTER, on the Mississippi River — This ship is making sure that the Big River, shrinking under one of the worst droughts in modern history, stays deep enough.

The Potter is scooping this stretch of the Mississippi River’s navigation channel just south of St. Louis, the ship’s 32-foot-wide head sucking up about 60,000 cubic yards of sediment each day and depositing it via a long discharge pipe a thousand feet to the side in a violent, muddy plume that smells like muck and summer.

The Army Corps of Engineers has more than a dozen dredging vessels working the Mississippi this summer. Despite being fed by water flowing in from more than 40 percent of the United States, the river is feeling the ruinous drought affecting so much of the Midwest.

Some stretches are nearing the record low-water levels experienced in 1988, when river traffic was suspended in several spots.

That is unlikely this year, because of careful engineering work to keep the largest inland marine system in the world passable. But tow operators are dealing with the shallower channel by hauling fewer barges, loading them lighter and running them more slowly, raising their costs. Since May, about 60 vessels have run aground in the lower Mississippi.

The low water is not just affecting the 500 million tons of cargo like coal, grain and fertilizer that move up and down the river each year. The owners of the American Queen, a paddle-wheel steamboat that takes passengers on tours along the inland waterways, decided not to send the boat below Memphis on a trip to Vicksburg, Miss., this month. The water was deep enough, said Tim Rubacky, a company spokesman, but after conferring with the corps and the Coast Guard, the company decided that the likelihood of a barge accident and ensuing traffic closures would be too great.

“It’s kind of like a truckful of watermelons spilling over on the expressway,” Rubacky said. “Everything’s going to come to a halt.” The boat tied up at Memphis and sent the passengers on to Vicksburg by bus, he said.

The volume of water coming down the river is so much lower than normal this summer that a wedge of saltwater is creeping up the Mississippi toward New Orleans, imperiling local water supplies drawn from the river. The corps is building a sill — basically, a dam of sediment — in the river below New Orleans low enough to block the flow of saltwater while letting boats pass.

When the Mississippi is low, the flow slows and sediment settles, causing the river to silt up and obstructions to form, said James Pogue, a spokesman for the corps in Memphis. Since 1988, when record low water in the Mississippi caused navigation to shut down, the corps has engineered ways “to help the river keep itself open,” he said, building new features like dikes that stick out into the river and “sort of act like nozzles to speed up the flow of the river” to scour the bed.

Such river training structures help to reduce the amount of dredging necessary by making the river do much of the work. The result, he said, is that even if water reaches the levels that it did before, “we’ll still be in better shape than we were in ’88.”

The river’s problems are the main topic aboard the Motor Vessel Mississippi, a giant towboat fitted by the corps with meeting rooms and used during its annual low-water inspection trip, which included a public hearing in Alton, Ill., on Friday. Some of the speakers complained about the corps’ management of the river during last year’s floods, when water at Vicksburg was nearly 59 feet higher than it has been during this year’s drought. That is the nature of the river — an engineered system, managed but hardly controlled.

Col. Christopher Hall, the commander of the St. Louis district of the corps, in comments to the commission on Friday, said, “We feel we can provide safe and reliable navigation” under projections over the next 28 days.

While the corps is keeping the main navigation channel of the Mississippi open, the same cannot be said for the harbors along the river. Four of the 19 harbors that the corps is responsible for keeping open on the lower Mississippi have been closed, and the corps has estimated that eight more would probably be closed if the drought continues over the next month.

That is why the Potter is here, keeping the channel to the river’s authorized navigation depth of nine feet. Built in 1932, the Potter has been around so long that the corps upgraded it to diesel from steam power in 2001.