92
inundated area is called the fl oodplain . Within the
fl oodplain area, the river may change its course
over the centuries and leave oxbow lakes to mark
its previous line of fl ow.
Concomitant to above, the upper portion of a
dam will, perhaps, remain inundated forever. But
the lower portion of the dam may have fl uctuating water levels. One of the main aims for controlling rivers is the expectation that it would
prevent fl ooding of agricultural lands and human
settlements. For this purpose, generally, the
banks are raised and channels are deepened along
the rivers, such as the Mississippi, the Danube
and many great lowland rivers all over the developed countries. It may be noted here that control
of the Hwang Ho, as it crosses the Great Plain of
central China, was the basis of state organisation
in ancient China. For centuries, defences against
the power of the river have involved many thousands of people building dykes. Care of these
dykes refl ected the strengths and weaknesses of
the ruling dynasty. A weak regime and neglect of
the dykes led to fearful disasters and enormous
loss of lives.
6.7.3 Lake Nasser/Nubia
Lake Nasser (24° N, 32° E; altitude 183 m; lake
area 6,216 km
2 ; lake volume 156.9 km
3 , catchment area 2.88 million km
2 ; maximum depth
130 m; mean depth 25.2 m; principal infl ow:
River Nile) is a reservoir situated in Africa around
Egypt and Sudan. The Lake Nasser/Nubia, on the
River Nile at the border between Egypt and
Sudan, took about 10 years to fi ll up to its fi nal
maximum depth of 130 m. This lake is in an
almost unvegetated region of the Sahara Desert.
The lake, which was formed behind the high dam
at Aswan, is called Lake Nasser in Egypt and
Lake Nubia in Sudan.
The high dam at Aswan on the River Nile is
well below the confl uence of the White Nile and
the Blue Nile at Khartoum. The White Nile drains
the high land of East Africa. The most southerly
source is in Burundi and fl ows via the Kagera
River into Lake Victoria, out via the Victoria Nile
into Lake Mobutu Sese Seko (formerly Lake
Albert), then northwards through the Sudd
swamps in Southern Sudan and so to Egypt.
Another source is the water fl owing from the
permanent snowfi elds of the Ruwenzori Mountains
into Lake George, in Western Uganda, and then
to Lake Edward whose outfl ow, the Semliki, also
fl ows into Lake Mobutu. Despite this enormous
catchment, the White Nile supplies only 16 % of
the water entering Egypt.
The water is very turbid despite the two dams.
Its highly seasonal fl ow generates the Nile fl ood
during July to September. This enormous
amount of water used to fl ow directly into the
Mediterranean. The silt was deposited seasonally
along the banks of the lower Nile and on the
delta, where the land increased in height each
year. This deposition has more or less ceased
since the closure of the high dam. The delta
suffers from increased erosion on the seaward
side. It may be noted here that much of the sediment brought down by the Blue Nile now generally accumulates on the bottom of Lake Nasser/
Nubia and has gradually been fi lling it up.
Vegetation is almost totally absent in Lake
Nasser/Nubia. So the problem of deoxygenation
was minimal after the closure of the dam.
However, Lake Nasser/Nubia is deep enough to
have annual cycle of thermal stratifi cation and
mixing. Stratifi cation of both temperature and
oxygen is gradually broken down as the fl ood
arrives in July.
A plankton community is able to develop
which is, however, quite different in its dominant
components. Both the headwater lakes and the
upstream reservoirs provide species which
develop their populations within the reservoir.
The Nile water has a higher conductivity (>200
μS cm
−1 ) than many African reservoirs. Hence, it
is hardly surprising that the productivity of Lake
Nasser/Nubia is also higher. Consequently, this
had led to gradual increase in fi sh catch in the
lake. This could be due to lack of drowned vegetation and the long fi lling time of the reservoir
(>10 years). Fishes, such as Distichodus and
Bagrus , which had dominated the catches during
the early years, are now much less abundant. On
the other hand, ichthyospecies, such as Lates
niloticus and Oreochromis niloticus , are now
6 Lakes of the World
inundated area is called the fl oodplain . Within the
fl oodplain area, the river may change its course
over the centuries and leave oxbow lakes to mark
its previous line of fl ow.
Concomitant to above, the upper portion of a
dam will, perhaps, remain inundated forever. But
the lower portion of the dam may have fl uctuating water levels. One of the main aims for controlling rivers is the expectation that it would
prevent fl ooding of agricultural lands and human
settlements. For this purpose, generally, the
banks are raised and channels are deepened along
the rivers, such as the Mississippi, the Danube
and many great lowland rivers all over the developed countries. It may be noted here that control
of the Hwang Ho, as it crosses the Great Plain of
central China, was the basis of state organisation
in ancient China. For centuries, defences against
the power of the river have involved many thousands of people building dykes. Care of these
dykes refl ected the strengths and weaknesses of
the ruling dynasty. A weak regime and neglect of
the dykes led to fearful disasters and enormous
loss of lives.
6.7.3 Lake Nasser/Nubia
Lake Nasser (24° N, 32° E; altitude 183 m; lake
area 6,216 km
2 ; lake volume 156.9 km
3 , catchment area 2.88 million km
2 ; maximum depth
130 m; mean depth 25.2 m; principal infl ow:
River Nile) is a reservoir situated in Africa around
Egypt and Sudan. The Lake Nasser/Nubia, on the
River Nile at the border between Egypt and
Sudan, took about 10 years to fi ll up to its fi nal
maximum depth of 130 m. This lake is in an
almost unvegetated region of the Sahara Desert.
The lake, which was formed behind the high dam
at Aswan, is called Lake Nasser in Egypt and
Lake Nubia in Sudan.
The high dam at Aswan on the River Nile is
well below the confl uence of the White Nile and
the Blue Nile at Khartoum. The White Nile drains
the high land of East Africa. The most southerly
source is in Burundi and fl ows via the Kagera
River into Lake Victoria, out via the Victoria Nile
into Lake Mobutu Sese Seko (formerly Lake
Albert), then northwards through the Sudd
swamps in Southern Sudan and so to Egypt.
Another source is the water fl owing from the
permanent snowfi elds of the Ruwenzori Mountains
into Lake George, in Western Uganda, and then
to Lake Edward whose outfl ow, the Semliki, also
fl ows into Lake Mobutu. Despite this enormous
catchment, the White Nile supplies only 16 % of
the water entering Egypt.
The water is very turbid despite the two dams.
Its highly seasonal fl ow generates the Nile fl ood
during July to September. This enormous
amount of water used to fl ow directly into the
Mediterranean. The silt was deposited seasonally
along the banks of the lower Nile and on the
delta, where the land increased in height each
year. This deposition has more or less ceased
since the closure of the high dam. The delta
suffers from increased erosion on the seaward
side. It may be noted here that much of the sediment brought down by the Blue Nile now generally accumulates on the bottom of Lake Nasser/
Nubia and has gradually been fi lling it up.
Vegetation is almost totally absent in Lake
Nasser/Nubia. So the problem of deoxygenation
was minimal after the closure of the dam.
However, Lake Nasser/Nubia is deep enough to
have annual cycle of thermal stratifi cation and
mixing. Stratifi cation of both temperature and
oxygen is gradually broken down as the fl ood
arrives in July.
A plankton community is able to develop
which is, however, quite different in its dominant
components. Both the headwater lakes and the
upstream reservoirs provide species which
develop their populations within the reservoir.
The Nile water has a higher conductivity (>200
μS cm
−1 ) than many African reservoirs. Hence, it
is hardly surprising that the productivity of Lake
Nasser/Nubia is also higher. Consequently, this
had led to gradual increase in fi sh catch in the
lake. This could be due to lack of drowned vegetation and the long fi lling time of the reservoir
(>10 years). Fishes, such as Distichodus and
Bagrus , which had dominated the catches during
the early years, are now much less abundant. On
the other hand, ichthyospecies, such as Lates
niloticus and Oreochromis niloticus , are now
6 Lakes of the World