541
wildlife. The delta is very fl at, with <2 m variation
in height across its 15,000 km
2 .
Where the Water Goes
Every year, c 11 km
3 (11,000,000,000,000 l) of
water fl ows into the delta. Approx. 60 % is transpired by plants and 36 % by evaporation, 2 %
percolates into the aquifer system and 2 % fl ows
into Lake Ngami. This turgid outfl ow means that
the delta is unable to fl ush out the minerals carried by the river and is liable to become increasingly salty and uninhabitable. But this effect is
reduced by the low salt content which collects
around the roots of the plants. The low salinity of
the water also means that the fl oods do not greatly
enrich the fl oodplain with nutrients.
24.4.1.2 Salt Islands
The agglomeration of salt around plant roots
means that many of the thousands of islands have
barren white patches in their centre, which have
become too salty to support plants, aside from the
odd salt resistant palm tree. Trees and grasses
grow in sand near the edges of the islands that has
yet to become too salty. c 70 % of the islands,
perhaps, began as termite mounds, where a tree
then takes root on the mound of earth.
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 fl atness of
the delta, and the large tonnage of sand fl owing
into it from the Okavango River, the fl oor 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.
24.4.2 Climate
The delta’s profuse greenery is not the result of a
tropical climate. Rather, it is an oasis in an arid
country. The average annual rainfall is 450 mm.
December to February are hot wet months with
day temperatures as high as 40 °C, warm nights
and humidity levels fl uctuating 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. September
to November sees the heat and atmospheric pressure build up once more as the dry season slides
into the rainy season.
24.4.3 Wildlife
The Okavango Delta is both a permanent and
seasonal home to a wide variety of wildlife and is
now a popular tourist attraction. Species include
African bush elephant, African buffalo, hippopotamus, lechwe, tsessebe, sitatunga, blue wildebeest, giraffe, Nile crocodile, lion, cheetah,
leopard, brown hyena, spotted hyena, greater
kudu, sable antelope, black rhinoceros, white rhinoceros, plains zebra, warthog and chacma
baboon. Notably the endangered African wild
dog still survives within the Okavango Delta,
exhibiting one of the richest pack densities in
Africa. The delta also includes >400 species of
birds, including African Fish Eagle, Pel’s Fishing
Owl, Crested Crane, Lilac-breasted Roller,
Hammerkop, Ostrich and Sacred Ibis. Further,
most of the c 200,000 large mammals in and
around the delta are not year-round residents.
They leave with the summer rains to fi nd renewed
fi elds of grass to graze on and trees to browse and
then make their way back as winter approaches.
There are also large herds of buffaloes and
elephants.
24.4.4 Lechwe
The most populous large mammal is the lechwe
antelope, which possibly number > 60,000.
They have large legs which enable rapid movement through knee-deep water. They graze on
aquatic plants and, like waterbuck, take to water
when threatened by predators. Only the males
have horns.
24.4 Wetlands of Botswana
wildlife. The delta is very fl at, with <2 m variation
in height across its 15,000 km
2 .
Where the Water Goes
Every year, c 11 km
3 (11,000,000,000,000 l) of
water fl ows into the delta. Approx. 60 % is transpired by plants and 36 % by evaporation, 2 %
percolates into the aquifer system and 2 % fl ows
into Lake Ngami. This turgid outfl ow means that
the delta is unable to fl ush out the minerals carried by the river and is liable to become increasingly salty and uninhabitable. But this effect is
reduced by the low salt content which collects
around the roots of the plants. The low salinity of
the water also means that the fl oods do not greatly
enrich the fl oodplain with nutrients.
24.4.1.2 Salt Islands
The agglomeration of salt around plant roots
means that many of the thousands of islands have
barren white patches in their centre, which have
become too salty to support plants, aside from the
odd salt resistant palm tree. Trees and grasses
grow in sand near the edges of the islands that has
yet to become too salty. c 70 % of the islands,
perhaps, began as termite mounds, where a tree
then takes root on the mound of earth.
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 fl atness of
the delta, and the large tonnage of sand fl owing
into it from the Okavango River, the fl oor 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.
24.4.2 Climate
The delta’s profuse greenery is not the result of a
tropical climate. Rather, it is an oasis in an arid
country. The average annual rainfall is 450 mm.
December to February are hot wet months with
day temperatures as high as 40 °C, warm nights
and humidity levels fl uctuating 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. September
to November sees the heat and atmospheric pressure build up once more as the dry season slides
into the rainy season.
24.4.3 Wildlife
The Okavango Delta is both a permanent and
seasonal home to a wide variety of wildlife and is
now a popular tourist attraction. Species include
African bush elephant, African buffalo, hippopotamus, lechwe, tsessebe, sitatunga, blue wildebeest, giraffe, Nile crocodile, lion, cheetah,
leopard, brown hyena, spotted hyena, greater
kudu, sable antelope, black rhinoceros, white rhinoceros, plains zebra, warthog and chacma
baboon. Notably the endangered African wild
dog still survives within the Okavango Delta,
exhibiting one of the richest pack densities in
Africa. The delta also includes >400 species of
birds, including African Fish Eagle, Pel’s Fishing
Owl, Crested Crane, Lilac-breasted Roller,
Hammerkop, Ostrich and Sacred Ibis. Further,
most of the c 200,000 large mammals in and
around the delta are not year-round residents.
They leave with the summer rains to fi nd renewed
fi elds of grass to graze on and trees to browse and
then make their way back as winter approaches.
There are also large herds of buffaloes and
elephants.
24.4.4 Lechwe
The most populous large mammal is the lechwe
antelope, which possibly number > 60,000.
They have large legs which enable rapid movement through knee-deep water. They graze on
aquatic plants and, like waterbuck, take to water
when threatened by predators. Only the males
have horns.
24.4 Wetlands of Botswana