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<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.

Phote 1: Lake Victoria, in Tanzania,
Uganda and Kenya

Phote 2: Lake Eyre, a saline lake in Australia

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.

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.

Phote 4: Lake Baikal, the deepest 
lake in the world, Russia

Photo 5: Lake Nakuru, a rift valley lake rich in colonies of flamingos and white pelicans, Kenya

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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