The water in the river carries small particles of debris which become trapped in the natural fractures in the rock. The force of the running water causes these small particles to move around in the fracture, grinding away part of the rocks like sandpaper. As the hole increases in size, larger debris becomes trapped and these also begin grinding. The result of this grinding away at the rock forms a pothole.
At times, potholes in the most popular areas of the park can be a notorious place to find litter, especially broken glass. Please use caution and help keep the river clean. Also, it is best to leave potholes filled with trapped, stagnant water alone, unless you are conducting a science project.