The drought of 2012 has already caused restrictions on barge traffic up and down the Mississippi River. But things are about to get a lot worse.

As part of an annual process, the Army Corps of Engineers has begun reducing the amount of water flowing from the upper Missouri River into the Mississippi, all but ensuring that the economically vital river traffic will be squeezed even further. If water levels fall low enough, the transport of $7 billion in agricultural products, chemicals, coal and petroleum products in December and January alone could be stalled altogether.

“Without the river, we’re in a world of hurt,” said Kathy Mathers, a spokeswoman for the Fertilizer Institute. About half of the spring fertilizer that the industry sells to Midwestern farmers travels upriver, she said, and options to get the fertilizer to the fields by other means are few. “We know the rail cars aren’t there,” she said. The corps reduces water flow from the upper Missouri every year as part of its master plan for maintaining irrigation systems and meeting other water needs of the region, which stretches from Montana to St. Louis. This year the process began on Nov. 11, as the corps began reducing water flows from the Gavins Point Dam near Yankton, S.D. The flow has already been reduced from 37,500 cubic feet per second to 26,500, and will reach 12,000 by Dec. 11.

The plan, approved by Congress, has the power of law. “We do not have the legal authority to operate the Missouri River solely for the benefits of the Mississippi River,” said Monique Farmer, a spokeswoman for the corps.