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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 [[outcrop]]ping, 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 (weathering)|attrition]] as they collide with each other, and they also erode the base of the waterfall by [[Abrasion (geology)|abrasion]], creating a deep plunge pool or gorge.
 
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 [[outcrop]]ping, 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 (weathering)|attrition]] as they collide with each other, and they also erode the base of the waterfall by [[Abrasion (geology)|abrasion]], creating a deep plunge pool or gorge.
 
[[Image:Baatara gorge.JPG|thumb|right|[[Baatara gorge waterfall]] near [[Tannurin]], [[Lebanon]]]]
 
[[Image:Baatara gorge.JPG|thumb|right|[[Baatara gorge waterfall]] near [[Tannurin]], [[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.<ref name="fenh"/> In addition to gradual processes such as erosion, earth movement caused by [[earthquakes]] or [[mudslides]] or [[volcano]]es can cause a differential in land heights which interfere with the natural course of a water flow, and result in waterfalls.
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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.<ref name="fenh"/> In addition to gradual processes such as erosion, earth movement caused by [[earthquakes]] or [[mudslides]] or [[volcano]]es can cause a differential in land heights which interfere with the natural course of a water flow, and result in waterfalls. AMAR BAFOR GOO KHA
   
 
A river sometimes flows over a large step in the rocks that may have been formed by a [[Fault (geology)|fault line]]. Waterfalls can occur along the edge of a [[glacial trough]], whereby 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.<ref name="fenh"/>
 
A river sometimes flows over a large step in the rocks that may have been formed by a [[Fault (geology)|fault line]]. Waterfalls can occur along the edge of a [[glacial trough]], whereby 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.<ref name="fenh"/>

Revision as of 15:47, 6 May 2011

Angel Falls in Venezuela is the world's tallest waterfall at 979 m (3,212 ft).
Frozen waterfall in southeast New York

A waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff.

Formation

Formation of a waterfall

Waterfalls are most commonly formed when a river is young.[1] 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.[1][2] 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.[1] 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.[3] The rate of retreat for a waterfall can be as high as one and half meters per year.[1]

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.

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.[1] In addition to gradual processes such as erosion, earth movement caused by earthquakes or mudslides or volcanoes can cause a differential in land heights which interfere with the natural course of a water flow, and result in waterfalls. AMAR BAFOR GOO KHA

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, whereby 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.[1] 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).[4]

Types

Dark Hollow Falls, near Skyline Drive, Virginia, is an example of a cascade waterfall
  • Block: Water descends from a relatively wide stream or river.[2][5]
  • Cascade: Water descends a series of rock steps.[2][5]
  • Cataract: A large, powerful waterfall.[5]
  • Chute: A large quantity of water forced through a narrow, vertical passage.[5]
  • Fan: Water spreads horizontally as it descends while remaining in contact with bedrock.[5]
  • Frozen: Any waterfall which has some element of ice.[5]
  • Horsetail: Descending water maintains some contact with bedrock.[5]
  • Plunge: Water descends vertically, losing contact with the bedrock surface.[5]
  • Punchbowl: Water descends in a constricted form and then spreads out in a wider pool.[5]
  • Segmented: Distinctly separate flows of water form as it descends.[5]
  • Tiered: Water drops in a series of distinct steps or falls.[5]
  • Multi-step: A series of waterfalls one after another of roughly the same size each with its own sunken plunge pool.[5]

Examples of large waterfalls

Havasu Falls, near Supai, Arizona, is an example of a plunge waterfall
Powerscourt Waterfall, near Enniskerry, Wicklow County, Ireland, is an example of a horsetail waterfall
Aerial view of Niagara Falls in the state of New York, US, and province of Ontario, Canada
Duruitoarea waterfall in Ceahlău, Romania
Plitvice lakes, Croatia

Significant waterfalls,[6] listed alphabetically:

Some waterfalls are constructed by artificial means indoors. The largest of these is in the lobby of the International Center, in Detroit.[7]

See also

References

  1. ^ a b c d e f Carreck, Rosalind, ed. (1982). The Family Encyclopedia of Natural History. The Hamlyn Publishing Group. pp. 246–248. ISBN 011202257 Check |isbn= value: length (help). 
  2. ^ a b c http://geography.howstuffworks.com/terms-and-associations/waterfall.htm/printable
  3. ^ http://www.classzone.com/books/earth_science/terc/content/visualizations/es1305/es1305page01.cfm?chapter_no=visualization
  4. ^ Richard H. Beisel Jr., International Waterfall Classification System, Outskirts Press, 2006 ISBN 1-59800-340-2
  5. ^ a b c d e f g h i j k l http://worldwaterfalls.com/waterfall_types.php
  6. ^ World Waterfall Database
  7. ^ Scholastic Book of World Records 2009. Scholastic. 2008. p. 267. ISBN 978-0-545-08211-2.