A waterfall is an area where water flows over a vertical drop or a series of steep drops in the course of a stream or river. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls are commonly formed in the upper course of a river in steep mountains. Because of their landscape position, many waterfalls occur over bedrock fed by little contributing area, so may be ephemeral and flow only during rainstorms or significant snowmelt. The further downstream, the more perennial a waterfall can be. Waterfalls can have a wide range of widths and depths.
When the river courses over resistant bedrock, erosion happens slowly and is dominated by impacts of water-borne sediment on the rock, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall, it may pluck material from the riverbed, if the bed is fractured or otherwise more erodible. Hydraulic jets and hydraulic jumps at the toe of a falls can generate large forces to erode the bed, especially when forces are amplified by water-borne sediment. Horseshoe-shaped falls focus the erosion to a central point, also enhancing riverbed change below a waterfalls. A process known as "potholing" involves local erosion of a potentially deep hole in bedrock due to turbulent whirlpools spinning stones around on the bed, drilling it out. Sand and stones carried by the watercourse therefore increase 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-a-half metres 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 in the gorge downstream.
Streams can become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep area just below the waterfall because of the kinetic energy of the water hitting the bottom. However, a study of waterfalls systematics reported that waterfalls can be wider or narrower above or below a falls, so almost anything is possible given the right geological and hydrological setting. Waterfalls normally form in a rocky area due to erosion. After a long period of being fully formed, the water falling off the ledge will retreat, causing a horizontal pit parallel to the waterfall wall. Eventually, as the pit grows deeper, the waterfall collapses to be replaced by a steeply sloping stretch of river bed. In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can cause a differential in land heights which interfere with the natural course of a water flow, and result in waterfalls.
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.
An example of an ephemeral waterfall. This one, when flowing, feeds into the Chagrin River (Ohio).
Ledge waterfall: Water descends vertically over a vertical cliff, maintaining partial contact with the bedrock. (e.g. Niagara Falls)
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 velocity of the water before it falls. It always starts from a narrow stream. (e.g. Angel Falls)
Punchbowl: Water descends in a constricted form and then spreads out in a wider pool. (e.g. Punch Bowl Falls)
Horsetail: Descending water maintains contact with bedrock most of the time. (e.g. Jog Falls)
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.
Dettifoss in northeast Iceland is the second largest waterfall in Europe in terms of volume discharge, having an average water flow of 193 cubic metres per second (6,800 cu ft/s). The falls are 100 metres (330 ft) wide and have a drop of 44 metres (144 ft) down to the Jökulsárgljúfur canyon.
Rhine Falls near Schaffhausen, Switzerland, is the most powerful, with the highest flow rate (average winter rate: 250 cubic metres per second (8,800 cu ft/s), average summer rate: 600 cubic metres per second (21,000 cu ft/s), max rate(1965): 1,250 cubic metres per second (44,000 cu ft/s)), at a height of 23 metres (75 ft) and with 150 metres (490 ft) wide among the widest in Europe.
Ribbon Fall, a seasonal waterfall in Yosemite National Park, is the highest single-drop fall in North America, at 1,612 feet (491 m).
Venta Rapid in Latvia, said to be Europe's widest waterfall, is more than 800 feet (240 m) wide but only about 6 feet (1.8 m) high.
Victoria Falls, on the Zambezi river along the border between Zimbabwe and Zambia, is among the largest waterfalls in the world. During periods of high flow, it creates an unbroken sheet of water more than a mile wide.