There's always more to be said on the topic, but there have been some great posts about the Mississippian culture.
u/totallynotliamneeson in Almost all cradles of civilization happened to be around major rivers in large plains, except American ones, why did Pre-Columbian American civilizations develop on the Mexican and Andean mountains rather than, for example, the Mississippi basin?
u/Reedstilt in There was supposedly a reasonably complex Mississippi Valley civilization before Europeans arrived. What would the biggest settlement in the area, and what would it have looked like?
u/akyser and u/Reedstilt in Why did no civilization develop along the banks of the Mississippi?