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Victoria Falls - Реферат

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Victoria Falls
Victoria Falls with relatively low water in the dry season
Africa's Victoria Falls or Mosi-oa-Tunya are, by some measures, the largest waterfall on the planet, as well as being among the most unusual in form, and having arguably the most diverse and easily-seen wildlife of any major waterfall site.
Although being neither the highest nor widest waterfalls in the world, the claim for being largest is based on a width of 1.7 km (1 mile) and a height of 108 m (360 ft), forming what may be the largest sheet of falling water in the world. No waterfalls are both wider and higher. Their maximum flow rate compares well with other major waterfalls (see table below).[1]
The unusual form of Victoria Falls enables virtually the whole width of the falls to be viewed face-on, at the same level as the top, from a distance as close as 60 m (200 ft), because the whole Zambezi River drops into a deep, narrow slot-like chasm, connected to a long series of gorges. Few other waterfalls allow such a close approach on foot to the heart of their power.[2]
Many of Africa's animals and birds can be seen in the immediate vicinity of Victoria Falls, and the continent's range of river fish are well represented in the Zambezi, enabling wildlife viewing and sports fishing to be combined with sightseeing.[3]
Victoria Falls are one of Africa's major tourist attractions, and a UNESCO World Heritage Site (see box below). They are shared between Zambia and Zimbabwe, and each country has a national park to protect them and a town serving as a tourism centre: Mosi-oa-Tunya National Park and Livingstone in Zambia, and Victoria Falls National Park and the town of Victoria Falls in Zimbabwe.[4]
Physical features
For a considerable distance above the falls, the Zambezi flows over a level sheet of basalt, in a shallow valley bounded by low and distant sandstone hills. The river's course is dotted with numerous tree-clad islands, which increase in number as the river approaches the falls. There are no mountains, escarpments, or deep valleys which might create a waterfall, only flat plateau extending hundreds of kilometres in all directions.[2][3]
The falls are formed as the full width of the river plummets in a single vertical drop into a chasm 60-120 m (200-400 ft) wide, carved by its waters along a fracture zone in the basalt plateau. The depth of the chasm, called the First Gorge, varies from 80 m (262 ft) at its western end to 108 m (360 ft) in the centre. The only outlet to the First Gorge is a 110 m (360 ft) wide gap about two-thirds of the way across the width of the falls from the western end, through which the whole volume of the river pours into the Victoria Falls gorges.[2][3]
There are two islands on the crest of the falls large enough to divide the curtain of water even at full flood: Boaruka Island (or Cataract Island) near the western bank, and Livingstone Island, near the middle. At less than full flood, islets divide the curtain of water into separate parallel streams. The main ones are named, going from Zimbabwe (west) to Zambia (east): Leaping Water (called Devil's Cataract by some), Main Falls, Rainbow Falls (the highest) and the Eastern Cataract.[3]
Flood and dry season flow rates
The Zambezi basin above the falls experiences a rainy season from late November to early April, and a dry season the rest of the year. The river's annual flood season is February to May with a peak in April,[5] and this is when the falls is the most impressive in terms of power and its thunderous rumbling sound. The spray from the falls rises typically to a height of over 400 metres (1300 ft), and sometimes even twice as high, and is visible from up to 50 km (30 miles) away. At full moon, a "moonbow" can be seen in the spray instead of the usual daylight rainbow. During the flood season however it is impossible to see the foot of the falls and most of its face, and the walks along the cliff opposite it are in a constant shower and shrouded in mist. Close to the edge of the cliff, spray shoots upwards like reversed rain, especially at Zambia's Knife-Edge Bridge.[3]
As the dry season takes effect, the islets on the crest become wider and more numerous and in September to January up to half of the rocky face of the falls may become dry, and the bottom of the First Gorge can be seen along most of its length. At this time it becomes possible, though not necessarily safe, to walk across some stretches of the river at the crest. It is also possible to walk to the bottom of the First Gorge at the Zimbabwean side. The minimum flow which occurs in November is around a tenth of the April figure, a larger variation than for other major falls, which brings Victoria Falls' annual average flow rate well down.[3]
The best time to see Victoria Falls depends on what you want to see. May to August offers the best compromise between a flow rate which impresses with its power, and the falls not being obscured by spray.
The Victoria Falls are roughly twice the height of North America's Niagara Falls and well over twice the width of its horseshoe falls. In height and width Victoria Falls is only rivalled by South America's Iguazu Falls which is divided into over 270 smaller falls and cataracts. See table for comparisons.
The Victoria Falls Gorges
Satellite image showing the broad Zambezi falling into the narrow cleft and subsequent series of zigzagging gorges (top of picture is north)
The whole volume of the river pours through the First Gorge's 110 m (360 ft) wide exit for a distance of about 150 m (500 ft), then enters a zigzagging series of gorges named in order from the first. Water entering the Second Gorge makes a sharp right turn and has carved out a deep pool called