A pothole is a deeply-scoured hole in the rock of a river bed. It is produced by pebbles getting caught in eddies in the water and being swirled around. AS they swirl around, so they wear away the bed as well as getting worn away themselves. New stones get trapped in the hole and are whirled around, scouring the bed deeper. You can see pebbles in a pothole in this picture. The pebbles are not moving because the water level is low. When the water level is high you could not see the pebbles, but that is when they would start to move because the water will be swirling around the pothole.

Potholes can only form in rivers with rocky beds, so you only find them in mountain regions.