Pot holes - cylindrical holes drilled into the rocky bed of a river by turbulent high velocity water loaded with pebbles.

what happens?

A pebble gets trapped in slight hollows (these are known as vertical eddies)in the river bed.

 A very strong flow of water then has a powerful force on the pebble in the bed an causes cylindrical movements of the pebble and this causes a drill effect.

This process is dynamic and goes deeper and deeper.

This is due to the erosional processes as a result of the force of the eddie, (hydraulic action), as the pebble being forced strongly against the river bed causes abrasion, (rubbing of rocks that causes a sand paper effect). Also, as it doesn't just have to be one pebble but could be several small rocks in a whole being rotated, this causes attrition as they are colliding together at the same time.

Pot holes also vary in width, (for mm to metres).

where?

They're usually found in the upper/middle course of the river.

THIS IS BECAUSE they're more likely to have a rocky river bed for the load to be trapped in the first place. Also, these points in the river are well above base level and has more potential for down cutting.

I'm a bit unsure of how to explain the last few sentances simpler and about base level so edit? :)