There’s a sound like distant thunder and the ground begins to shake. In a scene reminiscent of a classic western movie, dust and hooves fly as Custer State Park’s buffalo population — about 1,300 strong — is herded into corrals.

The 71,000-acre park is home to one of the world’s largest publicly-owned bison herds, and the annual event held on the last Friday of September draws a huge crowd of spectators.

The annual roundup is not just for show. While it’s a spectacular sight to see, there’s an important purpose: it’s a critical management tool in maintaining a strong and healthy herd. The park can only sustain a certain number of bison based on grassland conditions and the amount of food available for the animals.

The herd roams freely throughout the park for most of the year and can often be seen along the 18-mile Wildlife Loop Road in the southern part of the park. And while these animals appear to be docile, don’t be fooled: they weigh as much as 2000 pounds, can run very fast and can turn on a dime, making them a force of nature to be reckoned with.¬†

Once the buffalo have been moved into the corrals, park staff sort out approximately 200 animals to be sold at auction in November. They vaccinate new members of the herd, brand the new calves, and check the females for pregnancy. Working the entire herd takes at least four days.

The horseback riders taking part in the event are a mixed group. Some are Custer State Park staff, others are longtime riders, and up to 20 volunteer cowboys and cowgirls are selected each year to participate through an application process. The park staff sets up two safe viewing areas for the public and provides a shuttle service available after the roundup for those who want to go to the corral area for a closer look at these majestic animals.