"U.S. Army Corps of Engineers Col. Chris Hall points to a vertical, rock-filled steel tube called a ""protector cell"" that serves as a buffer that Mississippi River barges rub against, aligning them before they actually enter the lock on Wednesday, Sept. 19, 2012, in Granite City, Ill. Emergency repairs are under way on that structure after workers discovered that an unarmored portion of it exposed by river levels dramatically lowered by the drought split open, spilling much of the rock into the channel and forcing the channel to be closed to shipping. (AP Photo/Jim Suhr)"
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