(AP) — Environmental groups sued the U.S. Army Corps of Engineers on Thursday over the agency's use of man-made structures meant to keep the Mississippi River navigable, claiming the techniques provoke flooding as seen during historic inundations four times in the last two decades. Combined with selective dredging, the structures help the Corps meet its congressional mandate to keep at least a 9-foot-deep channel for barge traffic on the river, officials with the Army Corps' St.