The basic idea of a dam is simple: block the passage of a river down
a valley, and its waters will rise to form a man-made lake. Then the
water can be piped or channelled to dry areas where it is needed for
irrigation , wjuch is supplying crops with water. Besides this, flooding
can be controlled and the water-power can be used to turn turbines
which generate electricity - the cheapest way of producing it.
Many great dams have been built throughout the world. When they
were built, especially in developing countries, their planners predicted a
tremendous improvement in the living conditions of the people who
would receive the water and electric power. Many of these hopes have
been realized and there has been a great improvement in the living
conditions, but experience shows that dams can create new problems
while solving others.
A survey was recently carried out into the effects of dams that have
been built in the past 25 years. It was found that in some cases the dams
provided less irrigation and less hydro-electric power than the planners
had anticipated. And in certain instances, the presence of the dams was
actually causing damage to the environment.
Damming a river does a great deal more than create a huge reservoir
of water. Because the water trapped in the lake is no longer flowing
rapidly, much of the silt - sand, soil or mud carried along by a river - is
deposited on the bottom of the lake. When this happens, the land below
the dam may actually lose its fertility because it is silt that makes the
land fertile , or productive. An outstanding example of the unforeseen
ill-effects can be observed in the Nile Valley. The lands north of the
Aswan High Dam are no longer getting the vast quantities of silt which
the annual flood used to deposit on them. Instead, the large quantities
of silt are accumulating on the bottom of the man-made Lake Nasser
behind the dam.
On the other hand, while losing rich silt and minerals, the dam water
may pick up some chemicals which may have extremely harmful
effects. For example, the waters of the Volta Dam in Ghana have
^' ?/ caused the destruction of some of the country's richest cocoa
. .cations.
