We arrived at the entrance of the park before sunrise and waited in our car until the gates opened at about 6:15 am. There was a long line of cars both directions and there seemed to be much competition to get the best spot to viewing and photography. There were two viewing areas and we were at the North viewing area on the side of a hill facing the valley. We had a long wait, but a great view as the herd of Bison came thundering over the hill across the valley and were eventually herded in to the pastures adjacent to the corrals. It was a cloudy, drizzly, dreary day and had rained the night before. I had imagined the Bison stirring up clouds of dust in the morning sun as they stampeded across the prairie, but it didn't quite happen that way. The photos were taken with telephoto lenses, because they never did get really close, but I still got some good shots. You can do a bit of searching online and learn much more about the event.
See more photos of the "Buffalo Roundup" and Custer State Park.