Potholes are round / oval shaped holes in the bedrock of a river bed.
They are formed when sediment and other material carried by a river scours the bed. Where depressions exist in the channel floor turbulent flow can cause pebbles to spin around and erode hollows through abrasion/corraision. As the holes gets bigger even bigger debris can become trapped in the pothole and this material further supports erosion.
The video above shows the formation of a pothole
The video above shows potholes in the bed of the River Ure close to Aysgarth Falls.