500 years takes the drought to 1850. In reading up on great plains\midwest droughts, the whole 19th century was considered dry with periods of drought.
The prior century as a whole was considered wet.
Which makes me wonder how Cahokia was in a 500 year drought ?
Here’s an older article with some different spin.
1200 to 1700. Cahokia was long gone before getting through the 500 years.
Not that “500 year drought” means there was a uniform drought affecting all localities for precisely 500 years; it means a widespread drought. Here and there, there could have been even short heavy rains, but insufficient to make much of a difference, or ill-timed. Some areas would fare better than others... in that region there’s a notable difference in temp depending on which side of the Mississippi you live on, or whether you live in the bottoms or on the highlands beyond the bluffs.
Once dried out that bottomland clay would be rock hard and if they did get a cloudburst now and then, it’d drain away before soaking in. It would take prolonged rain to soften it up. The clay there is called “gumbo” and it is rich and black when wet but pancakes with deep fissures when dry, very hard to work it dry. The sand ridges would be easier to work but even drier and not very fertile. Upland loes would blow away as it blew in. In fact, that tan-brown upland loess along the bluffs and interior of Illinois blew in from the far west in the first place, millions of years ago.
this looks better researched than the drought theory.