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Newsletter and Technical Publications
<Lakes and Reservoirs - Similarities, Differences and Importance>
How are Lakes and Reservoirs Formed and
Located?
Lakes and reservoirs are dramatic, often visually pleasing, features of the
landscape that comprises river drainage basins (also called watersheds or
catchments). They range from pond-sized water-bodies to those containing vast
quantities of fresh or saline water (Photos 1 and 2) and stretching for hundreds
of kilometres. For the purposes of this booklet, lakes are water-bodies formed
by nature whereas reservoirs are artificial water bodies constructed by humans,
either by damming a flowing river or by diverting water from a river to an
artificial basin (impoundment). Many characteristics of lakes and reservoirs are
a function of the way in which they were formed and the use to which humans put
their waters. However, lakes and reservoirs also share a number of common
features, involving some that fundamentally control both the quantity and the
quality of the water contained in them.
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Phote 1: Lake Victoria, in Tanzania,
Uganda and Kenya
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Phote 2: Lake Eyre, a saline lake in Australia
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Natural Lake
Simply stated, lakes are naturally formed, usually “bowl-shaped” depressions
in the land surface that became filled with water over time. These depressions
(also called basins) were typically produced as a result of the catastrophic
events of glaciers, volcanic activity, or tectonic movements. The age of most
permanent lakes usually is of a geological time frame, but with most not much
older than 10,000 years. A few are much older, and some ancient lakes may be
millions of years old.
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Photo 3: Glacier Perito Moreno, descending from
the Andes
mountains and melting in the Lago Argentino, Argentina |
The most significant past mechanism for the formation of lakes in the
temperate areas was the natural process of “glacial scour”, in which the slow
movement of massive volumes of glacial ice during and after the Ice Age produced
depressions in the land surface that subsequently filled with water. The North
American Great Lakes (Superior, Michigan, Huron, Erie, Ontario), lakes in the
Lake District in the United Kingdom, and the numerous lakes in Scandinavia and
Argentina (Photo 3) are prominent examples of this type of lake formation.
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Phote 4: Lake Baikal, the deepest
lake in the world, Russia
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Photo 5: Lake Nakuru, a rift valley lake rich in colonies of
flamingos and white pelicans, Kenya
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 Another major lake
formation process was “tectonic movement”, in which slow movements of the
Earth’s crust produced depressions over time, which subsequently filled with
water. Lake basins also formed as a result of volcanic activity, which also
produced depressions in the land surface. Most of the Earth’s very deep lakes
resulted from either volcanic or tectonic activity. Lake Baikal in Russia (Photo
4), the world’s deepest lake and one which contains approximately 20 percent of
the world’s liquid fresh water, and the African Rift Valley lakes (Photos 1 and
5) are prominent examples of this type of lake formation.
Other natural processes that produced lake basins include (i) seepage of
water down through layers of soluble rock, (ii) erosion of the land surface by
wind action, and (iii) plant growth or animal activity (e.g., beavers, Photo 6)
that resulted in damming of the outlet channels from shallow depressions in the
land surface. There are literally millions of small lakes around the world,
concentrated largely in the temperate and sub-arctic regions. These regions are
also characterized by a relative abundance of fresh water. Many more millions of
temporary lakes occur in semi-arid and arid regions.
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