The damming of rivers to form reservoirs was common practice in Roman times,
and probably much earlier. Over the intervening centuries the practice has
evolved for three main purposes; human consumption, irrigation, and industrial
use. Since rainfall is never sufficiently uniform to provide a reliable water
source from normal river flow, the reservoir has always been a great boon,
spanning as it does lengthy periods of drought. There are, of course, other
reasons for damming a river. Following heavy storms in mountainous areas more
water than a river can carry will flow down to the plain, inundating large areas
often destroying crops, homes and animal and human life. Mountain passes, if
dammed, give the opportunity not only to control the flow of flood water at its
peak but also to retain large quantities for controlled use in dry weather. This
control of water works from sea to river as well as from to sea. An example is
the recently constructed Thames barrier below Woolwich on the River Thames in
England. This consists of solid pillars across the river with massive steel
plates between each pillar which can be raised or lowered mechanically. The
object is to protect the low-lying parts of London when a certain combination of
wind and tide raises the water level by as much as thirty feet.
A further use of the dam is to produce hydroelectric power, the power station,
of course, being built below the level of the dam. Water is channeled through
conduits and used as mechanical power for the turbines. It is then returned to
the river. Reservoir water is also used today in vast quantities in nuclear
power stations for cooling and other purposes and is similarly returned to the
river or to the sea.
Some rivers in their natural state are not navigable by large vessels, or run
dry in seasons of drought. Where such rivers can be dammed they can be given a
constant depth for purposes of navigation, and this benefits a country's
More often than not a dam may be central in a multipurpose scheme including most
of the functions outlined above. This is particularly true in countries lying in
temperate zones, where water shortage can be a nuisance but is rarely a
Matters are different in underdeveloped countries which are hot, dry and barren.
In these areas populations are often nomadic, driving flocks and herds from
waterhole to waterhole, or scratching a mere subsistence from the soil. Drought
means disease, hunger, dehydration and death to animals and humans alike. The
damming of major rivers such as the Nile is crucial to life itself, and the
control of their floodwater for irrigation purposes essential, but even this is
not preventing large tracts of North East Africa from reverting to desert.
Perhaps of least importance is the social value of a reservoir. Some stocked
with trout, provide good fishing, and most are used by sailing clubs, and for
other aquatic sports. A reservoir is also a great attraction to water-loving
birds and often becomes an interesting staging-post for migrations.
Damming a river can also cause problems. A great area of land, with its trees,
flora and fauna, farms and houses, has to be inundated. Neither the resident,
who have to resettle elsewhere, nor the environmentalists, appreciate this. The
problem is more acute when the countryside about to be spoilt is environmentally
unique, as in a recent case in Australia, where attempts were made to involve
Prince Charles and Princess Diana in the dispute. Creating an artificial lake
inevitably changes the character of a large area. Farmland is destroyed, natural
scenery, in the opinion of some, despoiled. People have to move, inconveniently.
A second drawback may be political. Many rivers flow through more than one
country, that lying higher up the river having the whip-hand. A dam built for
one country's benefit may be much to the detriment of another. The USSR has been
in dispute with Turkey over this matter as has Egypt with the Sudan.
Dams are vulnerable to air attack in wartime. The destruction of the Mohne See
dam in the Second World War brought the industry of the Ruhr to a virtual
standstill, as well as causing loss of life. There have also been cases of the
collapse of dams due to inadequate strength and imperfect design.