ST. LOUIS (AP) — After months of drought, companies that ship grain and other goods down the Mississippi River are being haunted by a potential nightmare: If water levels fall too low, the nation’s main inland waterway could become impassable to barges just as the harvest heads to market.

Any closure of the river would upend the transport system that has carried American grain since before steamboats and Mark Twain. So shipping companies are scrambling to find alternative ways to move tons of corn, wheat and other crops to the Gulf Coast for shipment overseas.

“You can’t just wait until it shuts down and suddenly say, ‘There’s a problem,'” said Rick Calhoun, head of marine operations for Chicago-based Cargill Inc. “We’re always looking at Plan B.”

The mighty Mississippi is approaching the point where it may become too shallow for barges that carry food, fuel and other commodities. If the river is closed for a lengthy period, experts say, economic losses could climb into the billions of dollars.

It isn’t just the shipping and grain industries that will feel the pinch. Store prices and utility bills could rise. And deliveries of everything from road-clearing rock salt for winter and fertilizer for the spring planting season could be late and in short supply.

“The longer it lasts, the worse it gets,” said Don Sweeney, associate director of the Center for Transportation Studies at the University of Missouri-St. Louis. “It’s inevitable that it will mean higher prices down the road.”

The focus of greatest concern is a 180-mile stretch of the river between the confluences of the Missouri River near St. Louis and the Ohio River at Cairo, Ill.

The river depth is 15 to 20 feet less than normal, now about 13 feet deep in many places. If it dips to around 9 feet, rock pinnacles at two locations make it difficult, if not impossible, for barges to pass. Hydrologists for the National Weather Service predict the Mississippi will reach the 9-foot mark by Dec. 9.

The situation worsened last week when the Army Corps of Engineers began reducing the outflow from an upper Missouri River dam in South Dakota, where a group of experts said Thursday that the worst U.S. drought in decades had intensified sharply in the last week.

The flow is gradually being cut by more than two-thirds by Dec. 11 as part of an effort to ease the effects of the drought in the northern Missouri River basin.

Lawmakers from Mississippi River states are frustrated with the corps’ action and even requested a presidential emergency declaration to overturn it. So far, the White House has not responded.

Army Assistant Secretary Jo-Ellen Darcy told Sen. Dick Durbin of Illinois and some of his colleagues from Iowa and Minnesota Thursday that the corps would consider dialing back the amount of water being held back from the Mississippi.

Darcy also pledged to expedite removal of rock formations south of St. Louis, though that work would take at least two months after a contractor is hired.

Through the years, parts of the river have occasionally been closed because of low water, barge accidents, dredging, ice and flooding. But this shutdown, if it happens, would affect a pivotal stretch that is used for two-way traffic — shipments going south to the Gulf as well as transports from the Illinois and Ohio rivers headed north to Chicago and Minneapolis.

A two-month shutdown — the length of time that some observers fear given current conditions — would have an estimated impact of $7 billion, according to the river industry trade group American Waterways Operators.

It costs 30 to 35 cents more per bushel to send grain to the Gulf by rail instead of barge.

“When you think of all we buy at the grocery store that has grain and corn, consumers could really see it hit them in the pocketbooks,” said Ann McCulloch of American Waterways Operators.