Waterfalls form where there are alternating layers of hard rock and soft rock. The soft is eroded faster than soft rock resulting on a step eg (knick point) The water flowing over the step speeds up due to lack of friction. Speed increases means more erosive power causing further erosion resulting in the undercutting on hard rock.
Under cut hard rock collapsed a deep plunge pool in carved from the rock due to abrasion occurring as the collapsed rock is swirled around by turbulence. over time this process reoccurs and leads to the waterfall retreating leaving behind a steep sided gorge.
Potholes- small hollows in river bed due to heavy material that has been deposited the bounder is too heavy to be transported by traction so eddie currents cause it become abrasive and erode a pothole in the river bed.
Rapids- Steep sections due to alternating resistant and less resistant rock
in a river that cause turbulent flow.
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