[Antoninus]

This was a major civilizational center for most of what is now the eastern US. The population of the US according to the 1860 census was in the neighborhood of 60 million.

Sure. And our agriculture and infrastructure was vastly more advanced then compared to what the Indians were doing. The term 'crop rotation' was unknown. They also tended to practice slash-and-burn agriculture and had no qualms about hunting game to local extinction. I highly doubt such a system could support more than a few hundred thousand at most.

At its height, the Iroquois Confederacy numbered about 30,000. If you lump in a few of the surrounding confederacies (the Hurons, Neutrals, Eries, Algonquins, Susquehannocks, Nanticokes, Delawares, and several smaller tribes) you find that no one of them numbered more than 40,000 and together they comprised perhaps 250,000 souls. And yet, they occupied an area roughtly that of modern New York, Quebec, Ontario, Pennsylvania, New Jersey, Maryland, and Delaware.

[Fraxinus]

You are comparing numbers for after the tribes you mention lost in excess of 50% of their population due to epidemic diseases. I also have severe doubts that the agriculture practices of the early American Republic were an order of magnitude more productive than that practiced by the Cahokians. There is also extensive, but often ancedotal data that there was significant reforestation in the perion of 1500 through 1600. This was formerly cultivated fields that were alowed to change to forest and were not cut down and returned to annual crops after a long (15+ years) fallow period. The long fallow period is often used in slash and burn agriculture.