John McPhee has written a fascinating history of the River Control Structure at Morganza. While it is not clear that the Mississippi still wants to go down the Atchafalaya, losing the River Control Structure in a flood would still be a catastrophe that could rival Katrina.
“Three hundred miles up the Mississippi River from its mouth—many parishes above New Orleans and well north of Baton Rouge—a navigation lock in the Mississippi’s right bank allows ships to drop out of the river. In evident defiance of nature, they descend as much as thirty-three feet, then go off to the west or south. This, to say the least, bespeaks a rare relationship between a river and adjacent terrain—any river, anywhere, let alone the third-ranking river on earth.”