A waterfall occurs where a stream’s course is interrupted by a sudden, near-vertical drop. These striking geological features are typically found where a river flows over rock that is significantly more resistant to wear than the material beneath it. The tremendous energy of the water, which has been building up as it moves across the landscape, is suddenly released downward as it descends. This powerful, concentrated flow immediately begins to reshape the landscape directly beneath the fall, carving out a characteristic depression that marks the termination of the drop.
The Specific Term: Plunge Pool
The deep basin or depression carved out directly at the base of a waterfall is known by the specific hydrological and geological term, the plunge pool. This feature is a type of stream pool created by the relentless erosional forces generated by the cascading water impacting the riverbed below. The term may refer to the water-filled pool itself, or the excavation in the rock that contains the water.
Plunge pools are a common and defining characteristic of many waterfalls. Their depth is often directly related to the height of the fall and the volume of water flowing over the edge. The existence of a plunge pool is concrete evidence of the immense power the water possesses as it strikes the base rock, demonstrating that the river’s work of erosion continues.
Geological Processes That Form Plunge Pools
The formation of a plunge pool is an intricate process driven by three primary mechanisms of water-based erosion: hydraulic action, abrasion, and cavitation.
Hydraulic Action
Hydraulic action is the sheer force and pressure of the water impacting the bedrock at the base of the fall. The immense weight and velocity of the descending water compress the air and water within cracks and fissures in the rock. This can cause the rock to weaken and break apart as the pressure is cyclically released.
Abrasion
This erosion is significantly amplified by the process of abrasion, where sediment, pebbles, and even large boulders carried by the turbulent water act as grinding tools. As the water swirls violently in the developing pool, these rock fragments are forcefully scraped against the bed and sides of the depression, effectively scouring the rock into a deeper, wider basin. The material broken off by hydraulic action is immediately incorporated into this abrasive load, further deepening the pool.
Cavitation
The third mechanism, cavitation, occurs as the water’s extreme turbulence creates and collapses tiny air bubbles, which releases shockwaves that exert intense localized pressure on the rock surface. The combination of these forces is most effective where the river flows over a resistant rock layer—known as caprock—that overlies a softer, less resistant layer beneath. The softer rock is eroded more quickly, creating the initial depression and allowing the erosion processes to continue deepening the plunge pool.
Related Landscape Features at the Waterfall Base
The continuous erosion at the base of the waterfall leads to the formation of other distinct landscape features closely associated with the plunge pool.
Undercutting and Collapse
As the softer rock beneath the hard caprock is eroded by the combined action of the pool’s turbulence and splashback, a hollow or shallow cave known as a rock shelter is formed underneath the hard layer. This process, called undercutting, leaves the overlying, more resistant rock unsupported. Eventually, the weight of the unsupported caprock becomes too great, and the rock layer collapses into the plunge pool below. The fallen rock fragments are then broken down and incorporated into the pool’s abrasive load, further deepening the basin.
Headward Erosion and Gorge Formation
This cycle of undercutting and collapse causes the waterfall to retreat upstream over a long period, a process known as headward erosion. The steep-sided, narrow valley left behind as the waterfall moves back is the resulting landform called a gorge or canyon.