A backhoe removes chunks of limestone from the navigation channel on the Mississippi River near Thebes, Illinois in this handout photo

 
A backhoe removes chunks of limestone from the navigation channel on the Mississippi River near Thebes, Illinois in this handout photo
The United States Army Corps of Engineers' DREDGE POTTER, dredges a section of Upper Mississippi River, south of St. Louis Harbor in this December 14, 2012 handout photo obtained by Reuters December 18, 2012. REUTERS/U.S. Army Corps of Engineers/Handout