V-shaped valleys and waterfalls are the main features of the upper course of a river.
This is a feature of erosion.
It occurs when the river erodes downwards into the land by abrasion and hydraulic action. This is called vertical erosion.
The valley sides are then shaped by the weather, plants and animals (weathering).
This is a feature of erosion. It occurs when a river flowing over hard rock meets a band of softer, less resistant rock.
Hydraulic action and abrasion erode the softer rock forming a 'step' in the river bed.
The softer rock is undercut and the hard rock is left as an overhang. A plunge pool is formed at the base of the waterfall; this plunge pool is deepened by hydraulic action, corrosion and abrasion as the pebbles erode its base.
The overhang eventually collapses and in this way the waterfall retreats towards the source of the river.
As the erosion continues, a gorge is formed which is a steep-sided valley.