A waterfall is a geological feature where a river or stream’s water flows over a steep, near-vertical drop or a sequence of sharp descents. The presence of a waterfall indicates an imbalance between the river’s erosive power and the resistance of the underlying rock layers.
The Mechanics of Formation
Waterfalls typically form through differential erosion, which occurs when a river flows over rock layers of varying hardness. A layer of hard, resistant rock, often called a caprock, rests on top of softer, less resistant rock like shale or sandstone. The river’s flow erodes the softer rock underneath much more quickly than the hard caprock above it.
The river utilizes several erosional processes, including hydraulic action (the force of water pressing into cracks) and abrasion (the grinding action of sediment). This rapid erosion under the softer layer eventually undercuts the hard rock, creating an overhang. A deep basin, known as a plunge pool, forms at the base due to the turbulent action of the falling water.
As undercutting continues, the unsupported caprock overhang eventually collapses into the plunge pool. This debris is broken down and carried away, and the process repeats. This cycle causes the waterfall to retreat upstream, a phenomenon known as headward erosion. The location where the river’s gradient abruptly changes is called a nickpoint, and its migration leaves behind a steep-sided gorge.
Categorizing Waterfall Types
Waterfalls are classified based on the appearance of the descending water and its interaction with the rock face. One recognized form is the Plunge waterfall, where the water completely loses contact with the bedrock surface as it drops vertically. This type often creates a cavern behind the curtain of water.
A Cascade waterfall features water descending over a series of small, irregular steps or sloping surfaces, maintaining constant contact with the rock face. When a wide river flows over a broad ledge, it forms a Block waterfall, which is typically wider than it is tall. Niagara Falls is a famous example of this wide, sheet-like descent.
A Segmented waterfall is distinguished by the stream splitting into two or more distinct channels or flows as it descends. These separate flows are usually divided by rock outcrops before they rejoin in a pool below.
Measuring and Understanding Water Flow
Two primary metrics are used to quantify a waterfall: height and flow rate. Height refers to the total vertical distance the water drops from the lip to the plunge pool or receiving water body.
The flow rate is the volume of water passing over the lip in a given time, commonly expressed in cubic feet per second (cfs) or cubic meters per second (cumecs). Scientists typically measure this volume upstream of the actual fall, where the river channel is more stable, by multiplying the water’s velocity by the cross-sectional area of the river.
Waterfalls also vary based on their consistency of flow, categorized as either perennial or seasonal. Perennial waterfalls flow continuously throughout the year, supported by a consistent water source. Seasonal, or ephemeral, falls only flow during specific times, usually following heavy precipitation or significant snowmelt.