Victoria Falls with relatively low water in the dry season

's Victoria Falls or Mosi-oa-Tunya are, by some measures, the largest on the planet, as well as being among the most unusual in form, and having arguably the most diverse and easily-seen of any major waterfall site.
Introduction
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).
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 drops into a deep, narrow slot-like , connected to a long series of . Few other waterfalls allow such a close approach on foot to the heart of their power.
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.
Victoria Falls are one of 's major tourist attractions, and a (see box below). They are shared between and , and each country has a national park to protect them and a town serving as a tourism centre: and in Zambia, and and the town of in Zimbabwe.
Physical features
For a considerable distance above the falls, the Zambezi flows over a level sheet of , in a shallow bounded by low and distant hills. The river's course is dotted with numerous tree-clad , which increase in number as the river approaches the falls. There are no mountains, , or deep valleys which might create a waterfall, only flat extending hundreds of kilometres in all directions.
The falls are formed as the full width of the river plummets in a single vertical drop into a chasm 60–120(200–400wide, carved by its waters along a fracture zone in the basalt plateau. The depth of the chasm, called the First Gorge, varies from 80(262 ft) at its western end to 108(360 ft) in the centre. The only outlet to the First Gorge is a 110 m (360wide 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 .
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.
Flood and dry season flow rates
The Zambezi basin above the falls experiences a from late November to early April, and a the rest of the year. The river's annual season is February to May with a peak in April, 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 (30away. 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.
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.
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 and well over twice the width of its horseshoe falls. In height and width Victoria Falls is only rivalled by South America's 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 the Boiling Pot. Reached via a steep footpath from the Zambian side, it is about 150(500 ft) across, its surface is smooth at low water, but at high water is marked by slow, enormous swirls and heavy boiling turbulence. Objects which are swept over the falls are frequently found swirling about here or washed up at the north-east end of the Second Gorge, such as the occasional dead hippo, or more tragically, people. This is where the bodies of Mrs Moss and Mr Orchard, mutilated by crocodiles, were found in 1910 after two canoes were capsized by a hippo at Long Island above the falls.
The principal gorges are (see reference for note about these measurements):
First Gorge: the one the river falls into now
Second Gorge: (spanned by the Victoria Falls Bridge), 250 m south of falls, 2.15 km long (270 yd south, 2350 yd long)
Third Gorge: 600 m south, 1.95 km long (650 yd south, 2100 yd long)
Fourth Gorge: 1.15 km south, 2.25 km long (1256 yd south, 2460 yd long)
Fifth Gorge: 2.55 km south, 3.2 km long (1.5 mi south, 2 mi long)
Songwe Gorge: 5.3 km south, 3.3 km long, (3.3 mi south, 2 mi long) named after the small Songwe River coming from the north-east, and the deepest at 140 m (460 ft), at the end of the dry season.
: below the Songwe, the gorge is called the Batoka Gorge (which is also used as an umbrella name for all the gorges). It is about 120 km (75 mi) long (the straight line distance to its end is about 80 km (50 mi) east of the falls) and takes the river through the to the valley in which now lies.
The walls of the gorges are close to vertical and generally about 120(400 ft) high, but the level of the river in them varies by up to 20 metres (65 ft) between wet and dry seasons.
How the Victoria Falls formed
"Leaping Waters", the westernmost cataract of Victoria Falls and the start of a line of weakness where the next falls will form.
The recent geological history of Victoria Falls can be seen in the form of the gorges below the falls. The basalt plateau over which the Upper Zambezi flows has many large cracks filled with weaker sandstone. In the area of the current falls the largest cracks run roughly east to west (some run nearly north-east to south-west), with smaller north-south cracks connecting them.
Over at least 100,000 years, the falls have been receding upstream through the Batoka Gorges, eroding the sandstone-filled cracks to form the gorges. The river's course in the current vicinity of the falls is north to south, so it opens up the large east-west cracks across its full width, then it cuts back through a short north-south crack to the next east-west one. The river has fallen in different eras into different chasms which now form a series of sharply zig-zagging gorges downstream from the falls.
Ignoring some dry sections, the Second to Fifth and the Songwe Gorges each represents a past site of the falls at a time when they fell into one long straight chasm as they do now. Their sizes indicate that we are not living in the age of the widest ever Mosi-oa-Tunya.
The falls has already started cutting back the next major gorge, at the dip in one side of the 'Leaping Waters' section of the falls. This is not actually a north-south crack, but a large east-north-east line of weakness across the river, and that is where the next full width falls will eventually form.
Further geological history of the course of the is in the article of that name.
Precolonial History
Archaeological sites around the falls have yielded Homo habilis stone artefacts from 3 million years ago, 50,000-year-old Middle Stone Age tools and Late Stone Age (10,000 and 2,000 years ago) weapons, adornments and digging tools. Iron-using (bushmen) displaced these Stone Age people and in turn were displaced by tribes such as the southern Tonga people known as the , who called the falls Shungu na mutitima. The , later arrivals, named them aManza Thunqayo, and the (whose language is used by the ) called them Mosi-oa-Tunya. All these names mean essentially the same thing, 'the smoke that thunders'.
The first European to see the falls was on , during his 1852-1856 journey from the upper Zambezi to the mouth of the river. The falls were well known to the , and , but Europeans were sceptical of their reports, perhaps thinking that the lack of mountains and valleys on the plateau made a falls unlikely. The Arabs may have known of them under a name equivalent to 'the end of the world'.
Livingstone had been told about the falls before he reached them from upriver and was paddled across to a small island that now bears the name Livingstone Island. Livingstone had previously been impressed by the further upstream, but found the new falls much more impressive, and named them after . He wrote of the falls "No one can imagine the beauty of the view from anything witnessed in England. It had never been seen before by European eyes; but scenes so lovely must have been gazed upon by angels in their flight".
In 1860, Livingstone returned to the area and made a detailed study of the falls with . Other early European visitors included explorer , explorer , who made the first detailed plan of the falls and its surroundings in 1875 (published in 1880), and British artist , who executed some of the earliest paintings of the falls. Until the area was opened up by the building of the railway in 1905, though, the falls were seldom visited by other Europeans.

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