Do you find the “Mississippi culture” fort sites as suspect as I do?
They were big piles of Earth, built in flood plains, and I don't find that remarkable. I don't find piling up Earth to be some kind of unique cultural phenomenon.
I also don't find it surprising that large riverine settlements wound up parts of a far-flung trade network, since rivers were also great modes of transportation. Continent-wide trade dates as far back as 200 BC, and probably involved trade with coastal Mexico and Central America, which was already urbanized. Terraced ag ruins near the major rivers in eastern N America have been attributed to the Mayans or a Mayan influence by some.
The introduction of corn (maize) and beans from middle America (Mexico etc) came about by 800 AD, a mere seven hundred years before Columbus, and its arrival as a food source probably was the main impetus to that level of settlement, even though the mound building had been around for a while before that. Settled agriculture is always the necessary foundation for civilization, although a more nomadic culture persisted elsewhere -- mostly at the higher elevations and drier areas where large-scale ag would fail.
OTOH, Watson Brake in present-day Ouachita Parish, Louisiana is considered the oldest mound complex in North America, dating to circa 3500 BC, lon before the Mayans. As I said, piling up earth isn't a remarkable cultural development.