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Newsletter and Technical Publications

Lakes and Reservoirs vol. 2

The Watershed: Water from the Mountains into the Sea

Lakes And Reservoirs: Water Pools on the Land Surface

Lakes are some of the most picturesque features of the natural landscape (Photo 29). They range from pond-sized water-bodies to those stretching for hundreds of kilometres and containing vast quantities of fresh water. Lakes are water-bodies formed when natural depressions or basins in the land surface become filled with water over time. In contrast to flowing streams and rivers, lakes provide a means for pooling or storing water for varying periods of time.

Lakes are of both natural and human origin. Many natural lake basins were formed by massive glaciers creating depressions in the land surface (“glacial scour ”) as they slowly moved over it during the past glacial periods. Other formative processes for lake basins include volcanic activity and the slow movement of different portions of the land surface (“tectonic movements ”). Whatever their origin, the basins subsequently became filled with runoff over time, producing the familiar water-bodies known as lakes.

Photo 29: Lake Poso, one of the most beautiful and deepest lakes in Indonesia.

There are millions of small natural lakes around the world. Most of the world’s great lakes are also natural. Prominent examples include Lake Baikal in Russia, containing about 23,000 cubic kilometres of water (equivalent to about 20% of the world’s liquid fresh water), and the Laurentian Great Lakes of North America (Superior, Huron, Michigan, Erie and Ontario), comprising the world’s largest contiguous freshwater volume (containing 24,600 cubic kilometres of water, also equivalent to about 20% of the world’s freshwater), Lakes Victoria and Tanganyika in Africa, and Lake Titicaca in Latin America. Interestingly, most natural lakes are relatively shallow geologic features and even some of the largest lakes have maximum depths of less than 30m. However, a few natural lakes have maximum depths of up to 1.5 km at their deepest parts (e.g., Lakes Baikal and Tanganyika). A relative comparison of the surface area of many of the world’s larger lakes is provided in Fig. 5.

Fig. 5: Comparative size of some lakes in the world.

Arid and semi-arid regions are characterized by having low annual rainfall, high evaporation and sparse vegetation. Nevertheless, these regions also can have large waterbodies seasonally or unpredictably. They can range in size from shallow lakes which contain water only intermittently, to large, permanent and deep lakes. Most are relatively shallow. As discussed in a following section, many arid and semi-arid watersheds may contain “terminal lakes ”, which have no natural drainage outlets to the oceans. They are found on all continents and often have more saline (salty) water than lakes in temperate regions. The total volume of freshwater lakes comprises about 0.009% of the world’s liquid freshwater. The equivalent value for saline lakes is about 0.008%. Interestingly, some of the world’s largest natural lakes exist in dry areas. For example, the Caspian Sea alone contains about 70% of the world’s inland saline water. Other large saline lakes include the Aral Sea, Great Salt Lake, and Lake Chinghai. Saline lakes also exist at the highest and lowest altitudes (e.g., Tibetan Plateau lakes and the Dead Sea; Photo 30, respectively).

Photo 30: View of the Dead Sea, Jordan side.

Lakes also can be artificially con- structed. Human-made lakes, also known as reservoirs or impoundments, typically are constructed by erecting a dam structure across a flowing river, thereby trapping or pooling the previously flowing water (Photo 6). They are normally constructed to address situations of water scarcity or water excess. In regard to water scarcity, reservoirs store precipitation received during one time of the year for subsequent human use during other times of the year when it is more needed. For areas subject to excessive precipitation and runoff, reservoirs temporarily store the excess water, thereby alleviating downstream flooding problems. As might be expected, reservoirs are more prominent in areas with relatively few natural lakes.

Because they are of human construction, reservoirs are not of a geologic age. The first small reservoirs were constructed about 4,000 years ago in China, Egypt and Mesopotamia, primarily for drinking water supply and irrigation purposes. The total volume of impounded water increased about 12-fold around the world after World War II, including a 40-fold increase in Latin America and a 100- fold increase in Africa and Asia. On a global scale, the building of reservoirs peaked in the late 1960s, and since then construction of new reservoirs has essentially ceased in North America and Europe. More than one-half of the world ’s reservoirs (including most of the largest ones) are located in the United States, Canada, Mexico, Brazil, China and India. Nearly all the new reservoirs scheduled to come into operation in the 21st century are located in Asia, Africa and Latin America.

Fig. 6: Characteristic shape of reservoirs and natural lakes.

A few comments on the similarities and differences of lakes and reservoirs are relevant. Natural lakes are generally more or less circular, bowl-shaped, natural depressions in the land surface. The center of the lake water basin is usually the deepest part, and the lake usually has one inflowing and outflowing river channel. In contrast, reservoir basins usually have multiple tributary inputs (“arms ”) along their lengths, and are more irregular and dendritic in shape (Figure 6). The depth increases from the upstream end to the dam end of the water basin. Because the dams are the physical structures that facilitate the pooling of the inflowing water, they also offer increased management options. Many dams contain water-discharge outlets at multiple depths, for example, allowing for the selective discharge of water from specific layers in a reservoir (Fig. 7).

Fig. 7: Cross-sectional dam wall showing multiple water withdrawal possibilities. The thermocline shows the drop of temperature from the surface down to deeper layers. The anoxic layer is a volume of water without oxygen.

Volume I of this UNEP/ILEC Short Series discusses the similarities and differences of natural lakes and reservoirs, as well as their management implications, and the reader may refer to it for additional information.

Photo 31: Characteristic algal blooms during summer time in a polluted reservoir. The fact that the velocity of inflowing river water decreases as the water enters lake and reservoir basins can have both positive and negative impacts on water quality. The water quality will typically increase, for example, when the inflowing tributary water enters the deeper, pool-like environment of a lake or reservoir, mainly because the decreased water velocity will allow the sediments and sediment- associated materials (e.g. nutrients) in the water column to settle to the bottom of the lake/ reservoir basin. This will also typically result in increased water clarity. At the same time, however, decreased water velocity enhances natural processes that flourish in non-flowing or pooled water systems. An example is the growth of microscopic, free-floating algae (phytoplankton). Lakes and reservoirs enhance the possibility for phytoplankton to accumulate to nuisance levels that interfere with beneficial human water uses (i.e., algal blooms Photo 31). The phytoplankton can also grow in flowing waters, but are subsequently swept downstream before they can accumulate to visible nuisance levels. Thus, the cumulative positive and negative effects of pooled water environments must be balanced in evaluating the water quality of lakes and reservoirs. The increased potential for controlling water quality in reservoirs, via selective water withdrawal, is another factor to be considered in developing water-quality management programs for these water-bodies. Because sediments and other materials carried in inflowing waters will typically settle to the bottom as they enter the more pool-like environment of natural lakes and reservoirs into which they drain, lakes and reservoirs are essentially transitory features of the Earth ’s surface. They will slowly cease to exist over time as they continue to fill with sediment and other materials.

Table of Contents

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• International Year of Forests
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