There were two distinct major mound building cultures in the United States. The oldest was the Adena-Hopewell. The Adena were located along the middle Ohio. Their time in the sum was 1000 BC until 100 AD. To me, they are THE big mystery of precolumbian America. They seemed to come out of nowhere. They were round headed while most of their neighbors were long headed. There are reported stone tables with letters carved on them, 7’ tall skeletons with similar genetic defects. Very Art Bellish.
Their mounds were mostly conical and predominantly used for burial purposes. (They were also into making “sacred enclosure” circular mounds, long raised avenues and various other earthworks including effigy mounds like the above mentioned Serpent Mound.) Every Adena site that I know of (they were only religous sites not towns) is located next to a river. However, this is most likely a transportation requirement. The earthworks are usually on the second or third terrace above the river however. This is a nice tradeoff to avoid flooding but have access to a level area and lots of sand and clay.
The Hopewell people were probably the original inhabitants of the areas from western Penn. all the way to the Mississippi river. Those who were neighbors to the Adena in Ohio apparently adopted the Adena’s mound building religion and were dominant from 150 BC to 500 AD, centered in the Scioto Valley of south central Ohio. For some reason they never went south of the Ohio.
Excavations of both Adena and Hopewell mounds show a basal clay foundation and a fire area. This is usually explained as a preliminary preparation for the mound site but I always thought it more likely that this was the floor and firepit of the dead person’s “home”, the mound being built over the dwelling of the important deceased. The burials (about 20% were cremations) are accompanied with elaborate grave goods. Notable among these ARE copper ornaments. But the copper is from the upper peninsula of Michigan where it is found as native metal on the surface. There was mica from North Carolina, grizzly bear teeth from the plains, conch sheels from the Gulf, these guys got around. Anyway they disappeared but not before building walled enclosures in defensible locations which look for all the world like forts. My guess is that the proto-Iroquois/Cherokee marched eastward through this area and ended their culture.
What you are talking about at Etowah are the Mississippian mound builders. They started out about 700 AD, probably near St Louis where the largest mound, Monk’s Mound at Cahokia, is located. Mississippian mounds are sites of bona fide towns (some of the towns were actually palisaded.) These mounds are usually pyramidal and flat topped. While there are some burials in them, Mississippian mounds are predominantly platforms for the top guy’s house and maybe a temple or two. At its height, the Mississippian culture had spread up to southern Wisconsin, up the Ohio to southwest Indiana, down the Mississippi to Lousiana, up the Arkansas River to Oklahoma, across the south into Georgia amd down to the Florida Gulf Coast. It is very unlikely that these were all the same people but they did have very similar cultural patterns. Trade was a major factor for the location of these mound site towns. Again, they were always on rivers but since the people actually lived at these sites, it was unlikely that they flooded. They were agriculturalists (the Adena-Hopewell were not) and farmed the bottomlands probably intensively (there was probably over 10,000 people living at Cahokia). Again, the sites contain copper from the upper peninsula of Michigan, shells from the Gulf Coast, flint hoes from Illinois, these guys got around too.
I usually avoid cultural diffusionist theories but I think it likely that the similar shape of the Mississippian mounds to the Mexican pyramids shows some kind of interaction. However, if so, it was very early, possibly Teotihuacan traders. It is a big jump from Mexico City to St Louis, but Teotihuacan did fall just about when the Mississipian culture started and in the later Totltec and Aztec times there was an established tradition of long distance trading.
By 1250, Cahokia was no longer the dominant focus. There are indications that a number of smaller mound towns were founded south along the Mississippi River about this time probably by Cahokia immigrants.
De Soto marched through their territory and skirmished with Indians still living in palisaded mound towns in 1541-1542. The last surviving mound people were the Natchez at Natchez, Mississippi. The French occupied their territory and from their records of the Natchez we have a small idea of what Mound culture might have been like. The Natchez rose up against the French in 1729 but within two years were defeated and most sold as slaves in Santo Domingo. A few fled to the Chikasaws, Creeks, and Cherokees (themselves probably the descendants of the Georgia and Alabama mound builders).
The ghosts lie thick on any mound site if you want to get a sense for these people. But, if you are not familiar with this facet of Indian culture and civilization, go to Cahokia in East St Louis and you will have your socks knocked off.