Also, interesting that you mention those rivers. While the US is huge on a map, remember that those early peoples were completely tied to those water sources (unlike later whites who irrigated massive sections of the country).
If you get away from the rivers, the Great Plains was a nearly impossible place to live year round. The cold fronts coming down from Canada with 50 to 60 mile hour winds, which are still quite common, would have been awful out on the open prairie. Along the rivers there was at least some firewood and timber for building decent shelters. Those people must have been extremely tough to have survived with only stone age technology.