It seems like a very large portion of native american tribes living in what is now the united states that we still hear about today, or that are well know, live/lived in 1) the dakotas 2) alabama and florida or 3) the four corners region.
Where these places actually populated more than other areas?
I can't speak for everything but in the region of what is now St. Louis to the Ohio river valley, there was a massive urban center that some experts place at a population of near 1,000,000. Might add to your conception of population locations