We saw last time that as the river moves the pebbles around, they crash into each other and bits break off. The pebbles become smaller, smoother and more rounded by this process, which is called attrition.
In some places the river drags its pebbles across solid rock on the river bed. This rubs both the pebbles and the river bed, gradually smoothing the surfaces. The process is called abrasion.
If the pebbles get trapped into a swirl by the current, then they whirl in a circle and the abrasion makes a depression in the river bed.
The circular depression is called a pothole.
Once the pothole shape has started to form, pebbles will tend to collect in it. They will sit there in position, all ready to be whirled round when the river flow is strong enough.
So, the pothole will quickly get deeper, which means it will collect even more pebbles etc.
Abrasion on the river bed will make the river channel deeper and will cut a deeper valley through the landscape.