The Mighty Mississippi is trying hard to change its course. Due to the buildup of silt in the lower Mississippi channel, a portion of the water in the lower Mississippi actually reaches the Gulf via the shorter, steeper route provided by the Atchafalaya, but a lot more wants to. This process is natural. Major river mouths, left to their own meanderings, frequently change their course.
The Old River Control Structure is an Army Corp of Engineers structure between Baton Rouge and Natchez that limits the amount of Mississippi water flowing into the Atchafalaya, but river silt is causing the river to flood more frequently and more severely. It is conceivable that a severe flood on the Mississippi could devastate the structure, causing a catastrophic flood on the Atchafalaya River while permanently changing the course of the Mississippi, stranding the ports in Baton Rouge and New Orleans on swampy, silt-ridden creeks too shallow for ocean liners.