How do waterfalls help to form gorges?

In a waterfall the river falls over a cliff edge into a deep plunge pool at the bottom, where there are weak levels of rock which have been excavated more quickly than that overlying rock. The force of the swirling water around the rocks causes hydraulic action and abrasion which deepens the plunge pool. This leads to the undercutting of overlying rocks, this rock eventually collapses and the waterfall goes upstream leaving a gorge ahead of it.