Barges were lined up along the Mississippi River earlier this week near Greenville, Miss. VICKSBURG, Miss.—Sections of the Mississippi River are approaching low water levels not seen in more than three decades, disrupting a vital supply lane for agriculture, oil and building materials and threatening businesses including barge and towboat operators, farmers and factories. The low water, caused by a lack of rain in the Ohio River Valley and the Upper Mississippi, has halted commercial traffic and river boat cruises at numerous spots below Illinois. Prices to ship goods have more than doubled in a matter of weeks. Barges are grounding on sandbars in unprecedented numbers and many ports and docks no longer have water deep enough for commercial boats to safely reach them.