Thundering herd of buffalo or American Bison
A herd of bison moves quickly along the Firehole River in Yellowstone National Park (near Midway Geyser Basin). They were not spooked or stampeding, but they were in a hurry to get somewhere early that morning. I cautiously move out of their way as they passed by me and through this narrow stretch of grassland that laid between the river and the lodge pole forest to my left. I noticed later that their hooves had beaten a path through the area, so this might be a periodic migration pattern.