According to the GCSE Geography curriculum, a waterfall is a prominent feature of river erosion typically found in the "upper course" of a river. The key characteristics and formation steps include: 1. Differential Erosion: A waterfall forms where a river flows over an area of hard, resistant rock (the cap rock) that sits on top of a layer of softer, less resistant rock. 2. Undercutting: The softer rock is eroded more quickly by processes like abrasion and hydraulic action, creating an overhang of hard rock. 3. Plunge Pool: As the water falls over the edge, the force of the falling water and the boulders it carries create a deep depression at the base called a plunge pool. 4. Collapse and Retreat: Eventually, the overhanging hard rock becomes unsupported and collapses into the plunge pool. 5. Gorge Formation: As this cycle of undercutting and collapse repeats over thousands of years, the waterfall "retreats" upstream, leaving behind a steep-sided valley known as a gorge. These features are classic indicators of a "high-energy" river environment where vertical erosion is more dominant than lateral erosion.