86
Evaporation exceeds 2 m per year. Ninety- two
percent of the water entering the lake on an average year is lost through evaporation and the rest
through seepage. Gains and losses are thus almost
equal. Chad is a shallow lake with no outlet and a
single inlet called Chari River, which provides c
87 % of the incoming water, while the rest come
from rain. The volume of the lake and the area it
covers are closely related to the volume of the river
infl ow. This, in turn, refl ects changes in rainfall in
the catchment. The Lake Chad is said to have
almost dried up three times since the beginning of
this century. However, in between, there had been
periods of extensive fl ooding also. In the early
1980s, Lake Chad suffered a period of drought.
There seems to be fl uctuations in the water
level (WL) of Lake Chad. Each year, the River
Chari and its principal tributary, the Logone, have
a period of reduced fl ow from March to June, and
they fl ood during the period from September to
December. Then, they spill over to inundate vast
areas of fl oodplain in Cameroon and Nigeria.
Thus, the lake itself increases in depth and area.
At the turn of the century, the lake appeared to
be shrinking as compared to levels observed by
earlier explorers. It may be noted here that by
1908, it had fallen to half of its 1903 level and
many feared that it could dry up altogether. This
had paved the way to a detailed study of the fl uctuations in its WL. Meanwhile, the lake had
expanded between 1912 and 1919. The fact that
the lake had fi lled again put an end to the worst
fears of its drying up, at least for the time being.
It, further, revealed that the lake has three phases,
namely, (a) Greater Chad, when it had an area of
c 25,000 km
2 during the nineteenth century; (b)
Normal Chad, with an area of c 21,000 km
2 which
could draw down to c 15,000 km
2
; and (c) Lesser
Chad, when the lake shrinks below this. It is
important to note here that when a team of French
hydrobiologists started to study the lake intensively in 1964, it was in the ‘Normal Chad’ phase.
However, by 1972, a ‘Lesser Chad’ phase had
clearly started. And by 1974, the north basin had
dried up although it had greater depth. This phase
was still evident in 1981 and almost certainly continued through the drought suffered by the whole
of the Sahel region in 1984. Thus, over a period of
40 years, it could be visualised that the lake had
been expanding and contracting annually. It was
also gradually getting larger from year to year
then progressively shrinking again.
Lake Chad has been further dealt with in
Chap. 20 .
6.5.4 Lake Naivasha
It is situated in the Eastern Rift Valley of Kenya in
Africa. It is also an unusual lake in that it is an
apparently endorheic basin which contains freshwater. It was once a part of a much larger basin
which included the present lakes Nakuru and
Elementaita. It had an outlet through the Njorowa
Gorge. The level of this outlet is now high above
the present lakes. Both Nakuru and Elementaita
contain high saline water now. However, Lake
Naivasha has remained fresh. This is, probably,
because it loses water and salts by seepage through
the highly porous volcanic rocks which underlie.
There have been many dry phases alternating
with wet periods in this part of the Rift Valley.
The basin, now occupied by Lake Naivasha, has
certainly been completely dry about 300 years ago.
Perhaps, for this reason, the lake has very impoverished fauna at the beginning of the twentieth century. Its present fauna is almost completely
composed of introductions. This includes the
North-American black bass ( Micropterus salmoides ) and several species of tilapia. The introduction
of fi shes has invited several species of piscivorous
birds to the lake. This has added to the biodiversity
of the lake. The lake also has fringing vegetation.
Further, large varieties of bare mud are also
exposed on the lake ward side. These are rapidly
colonised by some plants and also by a number of
ephemeral semi-terrestrial species. These newly
germinated plants provide food for terrestrial animals, both wild and domestic.
6.5.5 Loch Leven
This is said to be one of the shallow lakes as
much of its area is very shallow. This lake has
been dealt with in detail under Sect. 6.2.5.3 .
6 Lakes of the World
Evaporation exceeds 2 m per year. Ninety- two
percent of the water entering the lake on an average year is lost through evaporation and the rest
through seepage. Gains and losses are thus almost
equal. Chad is a shallow lake with no outlet and a
single inlet called Chari River, which provides c
87 % of the incoming water, while the rest come
from rain. The volume of the lake and the area it
covers are closely related to the volume of the river
infl ow. This, in turn, refl ects changes in rainfall in
the catchment. The Lake Chad is said to have
almost dried up three times since the beginning of
this century. However, in between, there had been
periods of extensive fl ooding also. In the early
1980s, Lake Chad suffered a period of drought.
There seems to be fl uctuations in the water
level (WL) of Lake Chad. Each year, the River
Chari and its principal tributary, the Logone, have
a period of reduced fl ow from March to June, and
they fl ood during the period from September to
December. Then, they spill over to inundate vast
areas of fl oodplain in Cameroon and Nigeria.
Thus, the lake itself increases in depth and area.
At the turn of the century, the lake appeared to
be shrinking as compared to levels observed by
earlier explorers. It may be noted here that by
1908, it had fallen to half of its 1903 level and
many feared that it could dry up altogether. This
had paved the way to a detailed study of the fl uctuations in its WL. Meanwhile, the lake had
expanded between 1912 and 1919. The fact that
the lake had fi lled again put an end to the worst
fears of its drying up, at least for the time being.
It, further, revealed that the lake has three phases,
namely, (a) Greater Chad, when it had an area of
c 25,000 km
2 during the nineteenth century; (b)
Normal Chad, with an area of c 21,000 km
2 which
could draw down to c 15,000 km
2
; and (c) Lesser
Chad, when the lake shrinks below this. It is
important to note here that when a team of French
hydrobiologists started to study the lake intensively in 1964, it was in the ‘Normal Chad’ phase.
However, by 1972, a ‘Lesser Chad’ phase had
clearly started. And by 1974, the north basin had
dried up although it had greater depth. This phase
was still evident in 1981 and almost certainly continued through the drought suffered by the whole
of the Sahel region in 1984. Thus, over a period of
40 years, it could be visualised that the lake had
been expanding and contracting annually. It was
also gradually getting larger from year to year
then progressively shrinking again.
Lake Chad has been further dealt with in
Chap. 20 .
6.5.4 Lake Naivasha
It is situated in the Eastern Rift Valley of Kenya in
Africa. It is also an unusual lake in that it is an
apparently endorheic basin which contains freshwater. It was once a part of a much larger basin
which included the present lakes Nakuru and
Elementaita. It had an outlet through the Njorowa
Gorge. The level of this outlet is now high above
the present lakes. Both Nakuru and Elementaita
contain high saline water now. However, Lake
Naivasha has remained fresh. This is, probably,
because it loses water and salts by seepage through
the highly porous volcanic rocks which underlie.
There have been many dry phases alternating
with wet periods in this part of the Rift Valley.
The basin, now occupied by Lake Naivasha, has
certainly been completely dry about 300 years ago.
Perhaps, for this reason, the lake has very impoverished fauna at the beginning of the twentieth century. Its present fauna is almost completely
composed of introductions. This includes the
North-American black bass ( Micropterus salmoides ) and several species of tilapia. The introduction
of fi shes has invited several species of piscivorous
birds to the lake. This has added to the biodiversity
of the lake. The lake also has fringing vegetation.
Further, large varieties of bare mud are also
exposed on the lake ward side. These are rapidly
colonised by some plants and also by a number of
ephemeral semi-terrestrial species. These newly
germinated plants provide food for terrestrial animals, both wild and domestic.
6.5.5 Loch Leven
This is said to be one of the shallow lakes as
much of its area is very shallow. This lake has
been dealt with in detail under Sect. 6.2.5.3 .
6 Lakes of the World