A waterfall is an upper course river landform in which a river cascades across an abrupt change in bed elevation.
A change in bedrock geology is the 'textbook' cause of waterfall formation. (They are also found for other reasons, e.g. hanging valleys caused by glacial erosion). The upper section of the waterfall is composed of a resistant caprock, while the lower section is formed from a softer, more erodible rock type.
Over time, the waterfall is undercut by abrasion and hydraulic action from the highly turbulent flow in the plungepool at its base. Erosion is most rapid during powerful flood events. Over many centuries, the waterfall may retreat to form a gorge of recession.
High Force on the River Tees is one of England's largest waterfalls (21m high). It has formed where the Tees flows from the resistant caprock above (volcanic whinstone), onto softer limestones beneath. It has gradually retreated to form a gorge 700m long. The Tees is a powerful upland river which rises rapidly after rain (a flashy river hydrograph) to flows of over 20 cumecs.