I thought I had the answer at hand but (for once) decided to check my facts BEFORE I wrote. Wikipedia has a really interesting article about this, well worth a (very short) read:
https://en.wikipedia.org/wiki/Bison_hunting#Native_American_...
https://en.wikipedia.org/wiki/Bison_hunting#Native_American_...
Supposedly there were many more people on the plains but the human population crashed and the bison took over. This would have coincided with “the Little Ice Age” for further speculation fodder
>The buffalo jump was used for 5,500 years by the indigenous peoples of the plains to kill buffalo by driving them off the 11 metre (36 foot) high cliff.
Further...
>The cliff itself is about 300 metres (1000 feet) long, and at its highest point drops 10 metres (33 ft) into the valley below. The site was in use at least 6,000 years ago, and the bone deposits are 12 metres (39 feet) deep.
Big damn herds, for a damn long time.