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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 river 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 arc not navigable by large vessels, or run dry in sea-
sons 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 communication system.
More often than not a dam may be central in a multi-purpose scheme including most of
the functions outlined above. This is particularly true in countries lying in temporate zones,
where water shortage can be a nuisance but is rarely a disaster.
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 residents, 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.
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