The drought that's squeezing trade on the lower part of the Mississippi River is threatening to force barge lines to lighten cargos in Minnesota if it doesn't rain more soon.

A dangerously low river and narrow channels have already driven river shipping costs up by more than 30 percent on the lower Mississippi River. Barges ran aground earlier this summer near Vicksburg, Miss., parts of the river have closed temporarily and operators have been forced to reduce loads to get past shallow spots from St. Louis to New Orleans without running aground.

The shallow river causes delays, lowers profit margins for barge lines and grain companies and has forced the U.S. Army Corps of Engineers to continually look for and dredge low spots to keep channels open for massive shipments of grain, potash, scrap metal, cement and ethanol byproducts. The stakes are especially high for farmers and the companies that export their grain, since 60 percent of American grain exports are shipped out of the country by barge.

"The greatest effect that it's having is on the lower part of the Mississippi," said Ben Doane, barge freight coordinator for CHS Inc., which is based in Inver Grove Heights and operates 10 grain-loading locations in the Upper Midwest that feed into the Mississippi.

The system of locks and dams that governs the river between St. Paul and St. Louis has helped insulate this part of the trade route from the drought -- so far. To get through all the locks without running aground, the hull of a barge must be no more than 9 feet below the water's surface, and that hasn't changed with the severe dry spell.

But under normal conditions, a barge loaded in St. Louis can take on heavier loads and travel in water that's 12 feet deep with more powerful towboats pushing a wider raft of barges. That has changed.

Barge lines along the length of the rain-starved Mississippi are now pretty much operating on the 9-foot draft restriction, say officials with the Corps, and navigable channels are narrower, forcing transportation companies to push narrower rafts of barges with less cargo in them.

If the river lowers further it could mean tighter restrictions on barges leaving Savage, Minneapolis or St. Paul, and lower profit margins.

"If it starts running out down below, then we will be impacted," said Lee Nelson, president of Upper River Services Inc. in St. Paul, which empties, cleans, and loads barges for barge lines.

Draft restrictions matter because every inch a barge sinks below the water equals 17 tons of cargo, and empty space in a barge drives up the cost of shipping.

"The fact that it's partway empty, somebody's got to pay for that," Nelson said. "Whoever's commodity it is, they're the one who's going to bear the brunt of that."

Whether the impact extends northward to barges loaded in Minnesota will depend on whether the rest of the Midwest gets much-needed rain.

"Everyone is watching the weather," Nelson said, "and we all know how well we can affect that."

Adam Belz • 612-673-4405