Potholes are round to oval shaped holes in the bedrock of a river bed. They are created where sediment accumulates within naturally occurring small depressions on the rock surface on the river bed. Turbulent flow swirls the stones (called grinders) around in the depressions, widening and deepening them through the prolonged process of abrasion. As the holes gets bigger even bigger debris can become trapped in the pothole, and this material is again used as an abrasive tool.