As a geologist who studied in the midwest and also spent time as a student archaeologist in that region, I would say that I think you'd be surprised about all the cliffs around the Great Plains. I just spent a few minutes looking around the USGS National Topographic Map and if you look at basically any river valley you'll see a sharp drop (maybe you'd call it a bluff, a wall, something other than a cliff, but those terms are very squishy) that could conceivably kill a bison. Also think of the added effect of a stampede of animals going over the bluff and how many might be crushed.