I have been reading a book by Timothy R. Pauketat called "Cahokia," about a Native American city called Cahokia on the Mississippi River.
My question is, why was Cahokia abandoned and depopulated? From what I've read, Cahokia was a powerful and populous city, with extensive agriculture, huge buildings, production of a lot of tools and pottery, and many other features. It lasted for over two hundred years, and was one of the biggest cities north of Mesoamerica. Yet Cahokia declined during the thirteenth century, to the point that eventually very few Native Americans lived in or around its ruins.
Why would such a large city be so completely abandoned? Was it war, disease, internal strife, ecological devastation, or some other cause? (Or a combination of these factors?)
The short answer is that no one knows for sure. Cahokia was abandoned by about the 13th century, long before any whites would have been able to catalog for posterity the reasons.
Contemporary scholars have proposed a variety of possible reasons. Over-hunting, over-fishing, and over-logging have been posited -- the site was indeed extensive, and would have required a huge amount of resources, by stone-age standards, to maintain. It's possible that either the Indians either used up all the resources, there was an environmental or climate event (or a mass animal disease) that reduced the number to untenable proportions, or some combination of those factors.
The huge number of fires that would have been built throughout the settlement could, theoretically, have led to deforestation. Deforestation, in turn, could also have affected the Indians' ability to cultivate corn, as it might have led to a sloughing off of topsoil or a proliferation of predatory animals or insects, etc.
Disease is another possibility. Cahokia would have represented a relatively rare area of North America that was densely populated enough for diseases to run rampant. As I said above, as well, animal or corn disease or a plague of insects could also have had the secondary effect of killing the humans who depended upon those resources.
Outside invasion by other Indian peoples has also been posited by many scholars. There's no real reason to believe this happened, as there's no evidence of massive warfare (arrowheads, defensive structures, knives, skulls, etc.), but there's no reason to believe it didn't, either. Stone-age battles may not necessarily have left large amounts of evidence, especially if the attackers or later scavengers took a lot of the stuff with them.
Of course, internal strife is another possibility. We know that Cahokia had a variety of different Indian cultures living together, to say nothing of wandering traders, etc., that might have interacted with it. It's possible that these different cultures eventually had a grievance that couldn't be settled and that led to a violent struggle.
And, as you suggest, it could have been a combination of any of these factors -- and it also may have occurred very gradually, over time. In 400 years, people may wonder what became of Detroit, once a grand city on Lake Michigan. That city did not go from being one of the biggest in North America to its (hypothetical for the purposes of this analogy) ultimate abandonment overnight.
Source: Thomas Emerson and Barry Lewis, Cahokia and the Hinterlands, 1999 Link