It´s hard for people to usually understand how entire civilizations can collapse. How urban societies can swiftly be reduced to small groups of backward agriculturalists or wandering nomads? Look no farther than Louisiana and Mississippi. The thing that saves the people in there is that there isn´t a group of barbarian savages or a competing civilization next door ready to attack and finish them. In the modern world the federal government of US will eventually get things under control, how late it may be for many. But in the ancient world a civilization centered on New Orleans would have fallen. A total area of 234 000 square kilometers has been affected. The effects will have consequences that reach to a far larger area, in fact globally. Something like this - a natural disaster - could have destroyed the Mound builders along the northern Mississippi and the Anasazi(in their case, most likely severe and long drought) in the west.