DES MOINES, Iowa — Hot, dry weather has left the Mississippi River so low that barge companies are reducing their loads just as Midwest farmers are preparing to send corn and soybeans downriver.

The cost to transport America’s harvest from the Midwest to the rest of the world is soaring as shrinking water levels on the Mississippi River drive up barge freight rates.

A road partially funded by a $1 million U.S. government grant is enabling farmers to move freight from Interstate 29 directly to Iowa’s Port of Blencoe barge terminal on the Missouri River.

The Mississippi River isn’t out of the woods as falling water levels along its major tributaries threaten to deepen a crisis on the U.S.’s main artery for moving vital products. And now the Ohio River is seeing closures at multiple locations due to groundings and dredging work.

Prolonged drought is jeopardizing waterborne trade along the Mississippi River, a basin that produces 92% of the nation’s agricultural exports.

A logjam of more than 100 ships, tugboats and their convoys of barges in the shrinking Mississippi River is threatening to grind trade of grains, fertilizer, metals and petroleum to a halt.

The U.S. Coast Guard has reopened the Mississippi River to maritime traffic, ending a shutdown that stranded more than 1,000 barges on the key conduit for agriculture exports.

Tighter regulation of U.S. ships carrying record exports of diesel and gasoline is coming amid the worst year for oil spills from barges since 2008.