Low water levels in the Mississippi River have snarled barge traffic along one of the nation’s busiest waterways this week and have threatened the drinking water supply in Plaquemines Parish, La., just southeast of New Orleans.

The river is always relatively low in the early autumn, but this year the situation is especially pronounced after a very dry summer. Unless more rain falls soon in the Midwest, which is drained by the Mississippi, the southern reaches of the river could soon fall to some of their lowest levels in a decade.

The situation could persist for weeks. In a typical year, low water levels on the river begin to abate around early December, said Jeff Graschel, a National Weather Service hydrologist at the Lower Mississippi River Forecast Center in Slidell, La. “Basically, we’re not seeing any heavy rainfall over the next several weeks to indicate that we would get any relief from low water conditions for the lower Mississippi,” he said.

When the river’s flow dwindles, salty water from the Gulf of Mexico tends to creep upstream, underneath the fresh water. In response, the Army Corps of Engineers announced last week that it planned to build a sill — something like a speed bump made of sediment — on the riverbed near Myrtle Grove, La., to keep salt water from threatening drinking supplies that are drawn from the river farther north.

The sill will take about two weeks to complete, and the work may begin early next week, a corps spokesman, Ricky Boyett, said in an interview.

Plaquemines Parish officials also have plans to use reverse osmosis machines to desalinate drinking water there.

For the agriculture industry, “the timing of this couldn’t be more inopportune,” said Mike Steenhoek, the executive director of the Soy Transportation Coalition.

Lower water levels mean that fewer barges can travel the river at any one time, and those that do must carry lighter loads than they otherwise would. That has led to backups and bottlenecks in shipping soybeans and other commodities on the river this week, with long lines of trucks waiting at loading facilities. The delays hurt farmers and drive up retail food prices.

“It’s harvest time,” Mr. Steenhoek said. “We need to have that supply chain that can accommodate it, and clearly that’s not happening right now.”