The stones carried by a river are dragged along on the river bed causing abrasion. The river bed is smoothed and worn away producing potholes.
This means that the channel gets deeper.
So why doesn’t every river have deep vertical sides like this?
The steeper the slope the more likely it is that the soil and rocks will fall.
They will land in the river and the river will break up the big pieces and gradually wash everything away.
But slopes don’t have to be vertical for soil to find its way into the river.
Here you can see how the layer of soil and plants is gradually sliding down the hill, leaving bare areas where the ground has slipped away.
The result is that the valley usually takes a V shape. The river is deepening in the middle and the side slope slide determines the angle of the V.
It could be quite narrow like this…
…or wider like this.
Next week we will look at a situation where you will find some vertical slopes.