A waterfall is a place where water flows over a vertical drop in the course of a stream or river. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Formation of a waterfall Waterfalls are commonly formed when a river is young. 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.
Baatara gorge waterfall near Tannourin, Lebanon Streams 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. 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. 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).
Dark Hollow Falls, near Skyline Drive, Virginia, is an example of a cascade waterfall Block: Water descends from a relatively wide stream or river. Cascade: Water descends a series of rock steps. Cataract: A large, powerful waterfall. 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. Frozen: Any waterfall which has some element of ice. Horsetail: Descending water maintains some contact with bedrock. Plunge: Water descends vertically, losing contact with the bedrock surface. Punchbowl: Water descends in a constricted form and then spreads out in a wider pool. Segmented: Distinctly separate flows of water form as it descends. Tiered: Water drops in a series of distinct steps or falls. Multi-step: A series of waterfalls one after another of roughly the same size each with its own sunken plunge pool. 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.
List of waterfalls by height and List of waterfalls by flow rate
A tributary of the Ribeira de Cheleiros that near the mouth takes the name of Rio Lizandro the Ribeira de Cabrela born in the northern municipality of Sintra at an altitude of 280m, under the name of Ribeira Blacksmith. Next to the air base Nº 1, the river takes the name of Ribeira da Granja to reach the EN9. After this crossing the national highway the river takes the name of Ribeira Fervença, and about 3 km further called Ribeira de Cabrela. In this zone the Ribeira has a high conservation status with a well developed riparian gallery. The Lizandro River basin covers an area of 175 km2, covering a large area of the municipality of Mafra. In Sintra basin occupies 80km2. This area of the basin is occupied by farmland, where some farming. The high growth of the mining and processing of marble and granite has negatively influenced the quality of water resources. Abandoned quarries with water levels, is a common factor in the landscape of this region. The exception to the rule, belongs to the Ribeira Cabrela, which presents some sections naturalized and very well preserved. Fonte : Wikipédia
In order to log your find, your task is answered the following questions:
1- Measure and/or calculate the dimension of the waterfall.
2- Tell us the colour of hard limestone on top of the fall.
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Português
Para poder fazer o log desta cache, deverá responder ás seguintes perguntas:
1- Meça e/ou calcule as dimensões da queda de água.
2- Diga-nos qual a cor dos calcários mais duros no topo da queda de água.
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Por favor coloquem no log a referência/nome da cache nas Vossas respostas.
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