Geoscience Reference
In-Depth Information
One of the most profound ways in which people alter rivers is by damming them. Ob-
structing a river and controlling its flow in this way brings about a raft of changes. A dam
traps sediments and nutrients, alters the river's temperature and chemistry, and affects the
processes of erosion and deposition by which the river sculpts the landscape. Dams cre-
ate more uniform flow in rivers, usually by reducing peak flows and increasing minimum
flows. Since the natural variability in flow is important for river ecosystems and their biod-
iversity, when dams even out flows the result is commonly fewer fish of fewer species.
Although dams have been built on rivers for thousands of years, the past 50 years or so has
seen a marked escalation in the rate and scale of construction of dams all over the world,
thanks to advances in earth-moving and concrete technology. At the beginning of the 21st
century, there were about 800,000 dams worldwide, some towering more than 200 metres
in height. Certain rivers have been intensively manipulated in this way. North America's
River Columbia, for example, has, since the mid-19th century, become the site for no fewer
than 80 dams. In some large river systems, the capacity of dams is sufficient to hold more
than the entire annual average discharge of the river. The reservoirs behind dams on the
Volta River in West Africa can store more than four times the river's annual average flow.
Globally, the world's major reservoirs are thought to control about 15% of all runoff from
the land. The volume of water trapped worldwide in reservoirs of all sizes is no less than
five times the total global annual river flow, and this huge redistribution of water is thought
to be responsible for a very small but measurable change in the orbital characteristics of the
Earth.
The very first dams were constructed to control floods and to supply water for crop irriga-
tion and domestic use. Modern dams still provide these services, plus a number of others,
including hydroelectricity generation and industrial water supply. There is no doubt that
many dam schemes have been very successful in achieving their objectives, and in many
respects have made substantial contributions to the sustainable use of river resources. In
Egypt, the Aswan High Dam has been perceived as a great symbol of economic advance-
ment and national prestige since its completion in 1970. It generates about 20% of the
country's electricity, and water held in its reservoir, Lake Nasser, has enabled irrigated ag-
riculture to expand on to 5,000 square kilometres of new land. This is particularly import-
ant for a desert country with only a very small area suitable for cultivation. The creation of
Lake Nasser has also given rise to a new fishing industry. The dam allows management of
the highly seasonal variations in discharge, evening out the Nile's flow to protect against
both floods and droughts. The stability of water levels in the river's course has also brought
benefits for navigation and tourism.
