Potholes form when sediment carried by a river scour the bed. Where depressions exist in the channel floor turbulent flow can cause pebbles to spin around and erode hollows through abrasion. As the holes gets bigger even bigger debris can become trapped in the pothole and this material further supports erosion.
Potholes in Maine, are often associated with glacial melt water. These photos were taken just below the Brunswick Fishway and Hydroelectric Station.