91
devastating fl oods in many places, which could
be unprecedented in the history of mankind.
Notwithstanding the above, the reservoirs
held back by dams in many countries serve the
secondary use of recreation, whereas in less
developed and developing countries, the lentic
systems serve as sites of aquaculture as a means
of nutrition and earning livelihood.
6.7.1 Dammed Valley Reservoirs
There are signifi cant differences between a reservoir and a natural lake. A reservoir fi lls a previous river valley. It usually has an unusually long
shoreline as compared to its area. The reservoirs
are often fed primarily by one large infl ow river.
The ratio of the drainage area to the surface
area of the lake is usually larger for a reservoir
than for a natural lake. Then, the events in the
catchment area have a great infl uence on the
reservoir, which responds very quickly to changes.
Further, the retention time of reservoirs is generally much shorter than that of natural lakes. There
are also other differences. Maximum and mean
depths of reservoirs are generally lesser than
those of natural lakes. In most reservoirs, the
deepest part is close to the dam wall. The height
of the dam sets an upper limit to the depth of the
lake. However, as with natural lakes, the shallower the water and the more exposed the surface
area to wind, the more the lake water will be
mixed. This would make it less likely to be stratifi ed for any duration of time.
It may be noted here that some new reservoirs
fi ll within months, others take years. For example, Lake Kainji in Nigeria in Africa fi lled only
in 3 months. It is because the basin is small in
relation to the enormous infl ow of the River
Niger. Its retention time is about 3 months. Most
of the big dams built in Africa have fl ooded large
areas of savanna woodland, as at Kariba on the
Zambezi, or tropical forest, as in the Volta Lake
in Ghana. The drowned vegetation starts to
decompose almost immediately, and the activity
of the bacteria not only releases nutrients into
the water but also uses up oxygen. It may be noted
here that Lake Kariba has a distinctly seasonal
climate which leads to thermal stratifi cation of
the water column. During the fi rst few years, the
hypolimnion was completely deoxygenated for
several months of the year after the dam was
closed. After the experience of Kariba, a similar
effect was anticipated when the dam was closed
on the Volta River and the lake (reservoir) began
to form.
Notwithstanding the above, most of the reservoirs in Great Britain have been built in the
uplands from which the tree cover has since
long been cleared. Also, here, the cool environment does not encourage rapid decomposition. Hence, deoxygenation had not been a
problem. On the other hand, the problem of
deoxygenation was aggravated in Lake Kariba by
the prolifi c growth of exotic fl oating fern,
Salvinia molesta , which covered large areas of
the lake within a month of closing of the dam.
The whole depth of water column under this
fl oating mat of vegetation was without oxygen
because the Salvinia blocked the light and prevented algae from performing photosynthesis.
Notwithstanding the above, another prolifi c
coloniser in the newly created lakes in the tropics
is the pestilential weed, water hyacinth
( Eichhornia crassipes ). The plant originated in
slow-fl owing and stagnant rivers in Brazil. Its
international spread began when some specimens
were sent to a horticultural exhibition in New
Orleans in 1884. People admirably took some
plants for their own ornamental ponds. The surplus plants were soon dumped in the warm waters
of the southern USA. It had spread from Florida
to Texas in just 6 years. It was taken to Australia
in 1895. It arrived in India by 1902.
6.7.2 The Downstream Effects
of Damming a River
The rate of fl ow of a river is drastically reduced
when the gradient of a river suddenly fl attens.
The river consequently becomes wider and shallower. At times of high water, as the snow melts,
or during the rainy season, the river overfl ows the
banks of its main channel and fl oods out across
the fl at country on one or both the sides. The
6.7 Man-Made Lakes
devastating fl oods in many places, which could
be unprecedented in the history of mankind.
Notwithstanding the above, the reservoirs
held back by dams in many countries serve the
secondary use of recreation, whereas in less
developed and developing countries, the lentic
systems serve as sites of aquaculture as a means
of nutrition and earning livelihood.
6.7.1 Dammed Valley Reservoirs
There are signifi cant differences between a reservoir and a natural lake. A reservoir fi lls a previous river valley. It usually has an unusually long
shoreline as compared to its area. The reservoirs
are often fed primarily by one large infl ow river.
The ratio of the drainage area to the surface
area of the lake is usually larger for a reservoir
than for a natural lake. Then, the events in the
catchment area have a great infl uence on the
reservoir, which responds very quickly to changes.
Further, the retention time of reservoirs is generally much shorter than that of natural lakes. There
are also other differences. Maximum and mean
depths of reservoirs are generally lesser than
those of natural lakes. In most reservoirs, the
deepest part is close to the dam wall. The height
of the dam sets an upper limit to the depth of the
lake. However, as with natural lakes, the shallower the water and the more exposed the surface
area to wind, the more the lake water will be
mixed. This would make it less likely to be stratifi ed for any duration of time.
It may be noted here that some new reservoirs
fi ll within months, others take years. For example, Lake Kainji in Nigeria in Africa fi lled only
in 3 months. It is because the basin is small in
relation to the enormous infl ow of the River
Niger. Its retention time is about 3 months. Most
of the big dams built in Africa have fl ooded large
areas of savanna woodland, as at Kariba on the
Zambezi, or tropical forest, as in the Volta Lake
in Ghana. The drowned vegetation starts to
decompose almost immediately, and the activity
of the bacteria not only releases nutrients into
the water but also uses up oxygen. It may be noted
here that Lake Kariba has a distinctly seasonal
climate which leads to thermal stratifi cation of
the water column. During the fi rst few years, the
hypolimnion was completely deoxygenated for
several months of the year after the dam was
closed. After the experience of Kariba, a similar
effect was anticipated when the dam was closed
on the Volta River and the lake (reservoir) began
to form.
Notwithstanding the above, most of the reservoirs in Great Britain have been built in the
uplands from which the tree cover has since
long been cleared. Also, here, the cool environment does not encourage rapid decomposition. Hence, deoxygenation had not been a
problem. On the other hand, the problem of
deoxygenation was aggravated in Lake Kariba by
the prolifi c growth of exotic fl oating fern,
Salvinia molesta , which covered large areas of
the lake within a month of closing of the dam.
The whole depth of water column under this
fl oating mat of vegetation was without oxygen
because the Salvinia blocked the light and prevented algae from performing photosynthesis.
Notwithstanding the above, another prolifi c
coloniser in the newly created lakes in the tropics
is the pestilential weed, water hyacinth
( Eichhornia crassipes ). The plant originated in
slow-fl owing and stagnant rivers in Brazil. Its
international spread began when some specimens
were sent to a horticultural exhibition in New
Orleans in 1884. People admirably took some
plants for their own ornamental ponds. The surplus plants were soon dumped in the warm waters
of the southern USA. It had spread from Florida
to Texas in just 6 years. It was taken to Australia
in 1895. It arrived in India by 1902.
6.7.2 The Downstream Effects
of Damming a River
The rate of fl ow of a river is drastically reduced
when the gradient of a river suddenly fl attens.
The river consequently becomes wider and shallower. At times of high water, as the snow melts,
or during the rainy season, the river overfl ows the
banks of its main channel and fl oods out across
the fl at country on one or both the sides. The
6.7 Man-Made Lakes