The Okavango Delta (or Okavango Grassland) (formerly spelled Okovango or Okovanggo) in Botswana is a very large, swampy inland
delta formed where
the Okavango River reaches a tectonic trough in the central part of
the endorheic basin of the Kalahari. All the water reaching the Delta is
ultimately evaporated and transpired, and
does not flow into any sea or ocean. Each year approximately 11 cubic
kilometres of water spreads over the 6,000-15,000 km2 area. Some flood-waters drain
into Lake Ngami.The Moremi Game Reserve, a National Park, is on the eastern side of the Delta. The scale and
magnificence of the Okavango Delta helped it secure a position as one of
the Seven Natural Wonders of Africa, which were officially declared on
February 11, 2013 in Arusha, Tanzania. On 22 June 2014, the Okavango Delta became the 1000th
site to be officially inscribed on the UNESCO World Heritage List.
The area was once part of Lake Makgadikgadi,
an ancient lake that mostly dried up by the early Holocene. Although the Okavango Delta is widely
believed to be the world's largest inland delta, it is not. In Africa alone
there are two larger similar geological features: the Sudd on the Nile in South
Sudan, and the Inner Niger Delta in Mali.
Floods
The Okavango is produced by seasonal
flooding. The Okavango River drains the summer (January–February) rainfall from
the Angola highlands and the surge flows 1,200 kilometres in
approximately one month. The waters then spread over the 250 km by
150 km area of the delta over the next four months (March–June). The high
temperature of the delta causes rapid transpiration and evaporation, resulting
in a cycle of rising and falling water level that was not fully understood
until the early 20th century. The flood peaks between June and August,
during Botswana’s dry winter months, when the delta swells to three times
its permanent size, attracting animals from kilometres around and creating one
of Africa’s greatest concentrations of wildlife.
The delta is very flat, with less than 2 metres
variation in height across its 15,000 km2.
Water flow
Every year, approximately 11 cubic
kilometres (11,000,000,000,000 litres) of water flow into the delta.
Approximately 60% is consumed through transpiration by
plants, 36% by evaporation, 2% percolates into the aquifer system; and 2% flows into Lake
Ngami. This turgid
outflow means that the delta is unable to flush out the minerals carried by the
river and is liable to become increasingly salty and uninhabitable. Water salinity is reduced by salt collecting
around plant roots as most of the incoming water is transpired by plants. Peat
firesmight contribute
to deposit salt into layers below the surface. The low salinity of the water also means that the
floods do not greatly enrich the floodplain with nutrients.
Salt islands
The agglomeration of salt around plant
roots leads to barren white patches in the centre of many of the thousands of
islands, which have become too salty to support plants, aside from the odd salt
resistant palm tree. Trees and grasses grow in the sand around the edges of the
islands that have not become too salty yet.
Approximately 70% of the islands began
as termite mounds (often Macrotermes spp.), where a tree then takes root on the mound of earth.
Chief’s Island
Chief’s Island, the largest island in
the delta, was formed by a fault
line which
uplifted an area over 70 km long and 15 km wide. Historically it was
reserved as an exclusive hunting area for the chief. It now provides the core
area for much of the resident wildlife when the waters rise.
Climate
The Delta's profuse greenery is not the
result of a wet climate; rather, it is an oasis in an arid country. The average
annual rainfall is 450mm (approximately one third that of its Angolan catchment
area) and most of it falls between December and March in the form of heavy
afternoon thunderstorms.
December to February are hot wet months with daytime
temperatures as high as 40 °C, warm nights, and humidity levels
fluctuating between 50% and 80%. From March to May, the temperature becomes far
more comfortable with a maximum of 30 °C during the day and mild to cool
nights. The rains quickly dry up leading into the dry, cold winter months of
June to August. Daytime temperatures at this time of year are mild to warm, but
the temperature begins to fall after sunset. Nights can be surprisingly cold in
the Delta, with temperatures barely above freezing.
September to November sees the heat and atmospheric
pressure build up once more as the dry season slides into the rainy season.
October is the most challenging month for visitors - daytime temperatures often
push past 40 °C and the dryness is only occasionally broken by a sudden
cloudburst.
Wildlife
The Okavango delta is both a permanent
and seasonal home to a wide variety of wildlife which is now a popular tourist
attraction.
Species include African bush elephant, African buffalo, hippopotamus, lechwe, tsessebe, sitatunga, blue wildebeest, South African giraffe, Nile crocodile, Southwest African lion, South African cheetah, African leopard, brown
hyena, spotted hyena, springbok, greater
kudu, sable antelope, impala, south-central black rhinoceros, southern white rhinoceros, Burchell's zebra, common warthog, chacma baboon and vervet monkey.
Notably the endangered Cape wild dog survives
within the Okavango Delta, exhibiting one of the richest pack densities in
Africa. The delta also includes over 400 species of birds, including African fish eagle, Pel's fishing owl, crested crane, lilac-breasted roller, hammerkop, South African ostrich, and sacred
ibis.
The majority of the estimated 200,000 large mammals in
and around the delta are not year-round residents. They leave with the summer
rains to find renewed fields of grass to graze on and trees to browse, then
make their way back as winter approaches. Large herds of buffalo and elephant
total about 30,000 beasts.
Fish
The Okavango Delta is home to 71 fish
species including tigerfish, tilapia and various species of catfish. Fish sizes range from 1.4 m African sharptooth catfish to 3.2 cm sickle
barb. The same species
are to be found in the Zambezi River,
indicating a historic link between the two river systems.
Lechwe
The most populous large mammal is
the lechwe antelope, with more than 60,000. It is a little larger
than an impala with elongated hooves and a water repellent substance
on their legs that enables rapid movement through knee deep water. They graze
on aquatic plants and, like the waterbuck, take to water when threatened by
predators. Only the males have horns.
Plants
Papyrus and
reed rafts make up a large part of the Okavango's vegetation. During the flood
season they float well above the sandy river bed with roots dangling free in
the water. This gap between bed and roots is utilised as shelter by crocodiles.
The plants of the Delta play an important role in providing cohesion for the
sand. The banks or levees of a river normally have a high mud content and this combines with the
sand in the river’s load to continuously build up the river banks. In the
Delta, because of the clean waters of the Okavango, there
is almost no mud and the river’s load consists almost entirely of sand. The
plants capture the sand, acting as the glue and making up for the lack of mud
and in the process creating further islands on which more plants can take root.
This process is not important in the
formation of linear islands. They are long and thin and often curved like a
gently meandering river. The reason for that is that they are actually the
natural banks of old river channels which over time have become blocked up by
plant growth and sand deposition, resulting in the river changing course and
the old river levees becoming islands. Due to the flatness of the Delta, and
the large tonnage of sand flowing into it from the Okavango River,
the floor of the delta is slowly but constantly rising. Where channels are
today, islands will be tomorrow and then new channels may wash away these
existing islands.
Game lodges
The Botswana Okavango Game Lodges (2011) cater to
small numbers of guests, each one operating in its own Okavango Delta private
concession area. There are many lodges with low-environmental-effect policies.
People
The Okavango Delta peoples consist of
five ethnic groups, each with its own ethnic identity and language. They are
Hambukushu (also known as Mbukushu, Bukushu, Bukusu, Mabukuschu, Ghuva,
Haghuva), Dceriku (Dxeriku, Diriku, Gciriku, Gceriku, Giriku, Niriku), Wayeyi (Bayei, Bayeyi, Yei), Bugakhwe (Kxoe, Khwe,
Kwengo, Barakwena, G/anda) and ||anikhwe (Gxanekwe, //tanekwe, River Bushmen, Swamp Bushmen, G//ani, //ani, Xanekwe). The Hambukushu,
Dceriku, and Wayeyi have traditionally engaged in mixed economies of
millet/sorghum agriculture; fishing, hunting, and the collection of wild plant
foods; and pastoralism.
The Bugakhwe and ||anikwhe are Bushmen who have traditionally practised
fishing, hunting, and the collection of wild plant foods; Bugakhwe utilized
both forest and riverine resources while the ||anikhwe mostly focused on
riverine resources. The Hambukushu,
Dceriku, and Bugakhwe are present along the Okavango River in Angola and in
the Caprivi Strip of Namibia, and there are small numbers of Hambukushu
and Bugakhwe in Zambia as well. Within the Okavango Delta, over the past 150
years or so Hambukushu, Dceriku, and Bugakhwe have inhabited the Panhandle and
the Magwegqana in the northeastern Delta. ||anikhwe have inhabited the
Panhandle and the area along the Boro River through the Delta, as well as the
area along the Boteti River.
The Wayeyi have inhabited the area
around Seronga as well as the southern Delta around Maun, and a few
Wayeyi live in their putative ancestral home in the Caprivi Strip. Within the
past 20 years many people from all over the Okavango have migrated to Maun, the
late 1960s and early 1970s over 4,000 Hambukushu refugees from Angola were
settled in the area around Etsha in the western Panhandle.
The Okavango Delta has been under the
political control of the Batawana (a Tswana nation)
since the late 18th century.[13] Led by the house of Mathiba I, the
leader of a Bangwato offshoot, the Batawana established complete control
over the Delta in the 1850s as the regional ivory trade exploded.[14] Most Batawana, however, have
traditionally lived on the edges of the Delta, due to the threat that tsetse
fly poses to their cattle. During a hiatus of some forty years, the tsetse fly
retreated and most Batawana lived in the swamps from 1896 through the late
1930s. Since then the edge of the Delta has become increasingly crowded with
its growing human and livestock populations.
Possible threats
The Namibian government has presented
plans to build a hydropower station in the Zambezi Region,
which would regulate the Okavango's flow to some extent. While proponents argue
that the effect would be minimal, environmentalists argue that this project
could destroy most of the rich animal and plant life in the Delta. Other
threats include local human encroachment and regional extraction of water in
both Angola and Namibia.
The award-winning South African
filmmaker and conservationist Rick
Lomba warned in
the 1980s of the threat of cattle invasion to the area. His documentary The End of Edenvividly
portrays this and his lobbying on behalf of the Delta helped to preserve its
integrity.