Every thousand years or so, the lower Mississippi changes course. It piles up enough silt at its delta that it ’spills over’ to a new shortest path to the ocean. At times, the outlet has been anywhere from Texas to the Florida Panhandle.
Since the early 20th century, the Mississippi has been trying to change course again—sending its main flow down the Atchafalaya river, which offers a much shorter, steeper path to the ocean. The Army Corps of Engineers was ordered by Congress to keep that from happening. The center of their effort is the Old River Control Structure, which limits the flow down the Atchafalaya to 30%.
The Morganza spillway has only been opened once (to take the stress off the failing ORCS in 1973), and then only partly. It’s fairly clear at this point that the Morganza spillway and the Bonnet Carré spillway will both be fully opened to route the flow away from New Orleans (which is expected to crest just a few feet below the tops of the levees there).In other words, the US government has tried to reroute the Mississippi River for decades now, and in an effort to control the flood at this point, they may end up letting nature take its natural course, and the geography of the America changed. Or as Ezzie would put it "The government tried to get involved, and failed"
I have no idea how likely the Old River Control and Morganza structures are to fail, or whether a rerouting of the Misssissippi through a new channel would be irreversible. You can read some speculation on this here.