The low levels have been months in the making, fueled by the nation's worst drought in decades, which has turned soils bone dry throughout much of the Midwest. Any precipitation is quickly soaked up in the ground and doesn't make its way into the Mississippi or other key rivers.
"Some places have gotten a lot of rain, but those weren't huge runoff events to jack up the river," said Bob Holmes, a Missouri-based hydrologist for the U.S. Department of the Interior's U.S. Geological Survey.
To compensate, many barge owners now are required to carry lighter loads, costing them more per ton to move cargo but also reducing chances of running aground as crews hustle to dredge silt and other sediments from the river to clear passages.
The waterways association said the reduction in releases on the Missouri would have particular impact on the Mississippi River from St. Louis south to Cairo, Ill., the area between where the Missouri and Ohio rivers merge into the Mississippi. Association officials cite concern about rock formations near Thebes, Ill., and Grand Tower, Ill., that would threaten to stop barge traffic by around Dec. 10 if water levels drop as much as anticipated.
"The Mississippi River is an economic superhighway that efficiently carries hundreds of millions of tons of essential goods for domestic use as well as national export. We need to address this situation swiftly, cut through bureaucratic red tape, and prevent the closure of the Mississippi," said Tom Allegretti, the American Waterway Operators' president and CEO.
Allegretti's trade group said barges on those corridors carry 20 percent of the country's coal and more than 60 percent of its grain exports. A mix of other cargo -- everything from petroleum products to lumber, sand, industrial chemicals and fertilizer -- also gets shipped by river.
Because of the drought, the Mississippi has received as much as 78 percent of its water from the Missouri this year, compared with 60 percent in a normal year, according to Nixon's office.
"I know that alternative measures are being considered to maintain the Mississippi channel, including dredging and clearing of rock pinnacles along the river," Nixon wrote. "I urge you to fast-track and fully implement those efforts."