Low Mississippi River levels are again putting the squeeze on shipping, with Arkansas’ 2012 harvest hobbling out on less-than-full barges.

The Mississippi, the nation’s largest river system, drains all or parts of 31 states and two Canadian provinces. Its watershed covers about 1.25 million square miles, or 41 percent of the United States’ landmass. For hundreds of years, the river has been a critical highway for agricultural and other products. According to the U.S. Army Corps of Engineers, the river and its tributaries moved 487 million tons of cargo in 2008.

However, widespread and prolonged drought this year meant there’s been little to drain into its 2,350-mile length. In August, levels were so low that shipping was halted briefly. (See “Grain piles up as drought-lowered Mississippi River slows barges,” http://bit.ly/11baqNY)

“All of our ports at Helena are open,” Robert Goodson, Phillips and Lee county extension agent for the University of Arkansas System Division of Agriculture, said Thursday. “Barges are leaving at about 70 percent full.”

When the navigation channel is shallow, shippers pack lighter loads allow the barges to sit higher in the water, known as reducing draft. Draft is the minimum depth at which a boat can navigate a waterway.