Have you ever seen someone ride on a surfboard in the ocean? Well, riverboarding is kind of like that, except instead of waves, they ride on rapids in a river.
Instead of standing up on the board like on a surfboard, the person lays down on their stomach while holding onto handles on the board. This helps them stay steady as they go down the rapids.
This sport can be dangerous, so people wear special gear like helmets, wetsuits, and life jackets to protect themselves. It's important to be very careful and know what you're doing before trying it out!