Tuesday, December 15, 2009

Waterfall

Waterfall

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

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

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[edit]Formation

Formation of a waterfall

Typically, a river flows over a large step in the rocks that may have been formed by a fault line. As it increases its velocity at the edge of the waterfall, it plucks material from the riverbed. 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.

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 or plunge pool 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 byattrition 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 Tannurin, Lebanon.

Streams become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep pool 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.

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. The rivers are flowing from hanging valleys.

[edit]Classifying Waterfalls

Waterfalls are grouped into ten broad classes based on the average volume of water present on the fall 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, Gullfoss and Sutherland Falls (Class 8); Angel Falls and Dettifoss (Class 7); Yosemite Falls, Lower Yellowstone Falls and Umphang Thee Lor Sue Waterfall (Class 6).[1]

[edit]Types of waterfalls

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.
  • Fan: Water spreads horizontally as it descends while remaining in contact with bedrock.
  • Horsetail: Descending water maintains some contact with bedrock.
  • Plunge: Water descends vertically, losing contact with the bedrock surface.
Frozen waterfall in southeast New York
  • 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.

[edit]Examples of large waterfalls[2]

Havasu Falls, near Supai,Arizona, is an example of a plunge waterfall
Powerscourt Waterfall, nearEnniskerry, Wicklow County, Ireland, is an example of a horsetail waterfall
Aerial view of Niagara Falls in the state of New York, USA, and province of Ontario, Canada

Significant waterfalls are listed alphabetically:

[edit]See also

Plitvice lakes, Croatia

[edit]External links

[edit]References

  1. ^ Richard H. Beisel Jr., International Waterfall Classification System, Outskirts Press, 2006 ISBN 1598003402
  2. ^ http://www.world-waterfalls.com/ World Waterfall Database