How do waterfalls form?

Waterfalls form when a river flows over an area of softer rock after flowing over an area of harder rock. Due to the softer rock being more easily eroded than the harder rock, the soft rock undercuts the harder rock. The hard rock is left overhanging and due to continual erosion and the force of gravity, eventually collapses. This produces a steep drop called a waterfall. The collapsed rock falls to the base of the waterfall and is swirled around with other rocks, causing one of the processes of erosion called abrasion to take place. This creates a deep plunge pool. Overtime, more undercutting causes more hard rock to collapse so the waterfall retreats upstream and creates a narrow and steep-sided gorge.