Potholes
Potholes = cylindrical holes drilled into the rocky bed of a river - turbulent high-velocity water loaded with pebbles.
The pebbles = trapped in slight hollows + vertical eddies in the water= the sediment grinds a hole into the rock by abrasion (corrasion).
Attrition = rounds + smooths - pebbles caught in the hole = reduces the size of the bedload.
Potholes = vary in width from a few centimetres to several metres.
Found in the upper or early-middle course of a river= valley lies well above base level= more potential for downcutting + river bed = likely to be rocky.
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