At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens slowly, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall, it plucks material from the riverbed. Whirlpools created in the turbulence as well as sand and stones carried by the watercourse increase the erosion capacity. This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as one and half meters per year.
Often, the rock stratum just below the more resistant shelf will be of a softer type, meaning that undercutting due to splashback will occur here to form a shallow cave-like formation known as a rock shelter under and behind the waterfall. Eventually, the outcropping, more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool or gorge.
A river sometimes flows over a large step in the rocks that may have been formed by a fault line. Waterfalls can occur along the edge of a glacial trough, where a stream or river flowing into a glacier continues to flow into a valley after the glacier has receded or melted. The large waterfalls in Yosemite Valley are examples of this phenomenon, which is referred to as a hanging valley. Another reason hanging valleys may form is where two rivers join and one is flowing faster than the other. Waterfalls can be grouped into ten broad classes based on the average volume of water present on the fall (which depends on both the waterfall's average flow and its height) using a logarithmic scale. Class 10 waterfalls include Niagara Falls, Paulo Afonso Falls and Khone Falls.
Classes of other well-known waterfalls include Victoria Falls and Kaieteur Falls (Class 9); Rhine Falls and Gullfoss (Class 8); Angel Falls and Dettifoss (Class 7); Yosemite Falls, Lower Yellowstone Falls and Umphang Thee Lor Sue Waterfall (Class 6); Sutherland Falls (Class 5).
Types
Ledge Waterfall: Water descends vertically over a vertical cliff, maintaining partial contact with the bedrock.
Block/Sheet: Water descends from a relatively wide stream or river.
Classical: Ledge waterfalls where fall height is nearly equal to stream width, forming a vertical square shape.
Curtain: Ledge waterfalls which descend over a height larger than the width of falling water stream.
Plunge: Fast moving water descends vertically, losing complete contact with the bedrock surface. The contact is typically lost due to horizontal thrust of the water before it falls. It always starts from a narrow stream.
Punchbowl: Water descends in a constricted form and then spreads out in a wider pool.
Horsetail: Descending water maintains good contact with bedrock most of the time.
Slide: Water glides down maintaining continuous contact.
Ribbon: Water descends over a long narrow strip.
Chute: A large quantity of water forced through a narrow, vertical passage.
Fan: Water spreads horizontally as it descends while remaining in contact with bedrock.
Cascade: Water descends a series of rock steps.
Tiered/Multi-step/Staircase: A series of waterfalls one after another of roughly the same size each with its own sunken plunge pool.
Cataract: A large, powerful waterfall.
Segmented: Distinctly separate flows of water form as it descends.
Catadupa: A cataract or waterfall, originally those of the Nile. The term catadupae refers to people inhabiting near such cataracts; there are suppositions that these people are deaf due to the constant din.
Tide Fall: A waterfall that directly empties into the sea or ocean.
Frozen: Any waterfall which has some element of ice.
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