This video was taken in the Lake District on a stream flowing down to Ullswater. There is no voice-over, you simply see and hear what was there at the time. It is up to you to work out what is going on – just as it would be if you were there.

However, here are some hints: it is taken in fine weather when the river level is low, so the stones in the potholes would not move without my helping hand. I move some pebbles to the lip of the pothole to show how the pebbles can be carried by fast-flowing water along smooth rocky beds.

The largest pebbles stay in the pothole because they are too heavy to be moved when river levels are low, but they are rounded, so they must move when the water is faster – after wet weather.

Compare what you see with the pothole diagram.