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<Lakes and Reservoirs - Similarities, Differences and Importance>

Man-made Lakes (Reservoirs)

In contrast to natural processes of lake formation, reservoirs are man-made water-bodies, usually formed by constructing a dam across a flowing river. Upon completion of the dam, the river pools behind the dam and fills the artificially created basin. A dam also may sometimes be constructed on the outlet channel of a natural lake as a means of providing better control of the lake’s water-level (examples being Lake Victoria, Africa, and Lake Tahoe, USA, Photo 7). However, these latter water-bodies typically retain their natural lake characteristics.

Reservoirs are found primarily in areas with relatively few natural lakes, or where the lakes are not suitable for human water needs. They are much younger than lakes, with life spans expressed in terms of historical rather than geological time. Although lakes are used for many of the same purposes as reservoirs, a distinct feature of reservoirs is that they are usually built by humans to address one or more specific water needs. These needs include municipal and drinking water supplies, agricultural irrigation, industrial and cooling water supplies, power generation, flood control, sports or commercial fisheries, recreation, aesthetics and/or navigation.

The reasons for constructing reservoirs are ancient in origin, and initially focused on the need of humans to protect themselves during periods of drought or floods. Accordingly, reservoirs are usually found in areas of water scarcity, or where a controlled water facility was necessary. Small reservoirs were first constructed some 4,000 years ago in China, Egypt and Mesopotamia, primarily to supply drinking water and for irrigation purposes. Simple small dams were constructed by blocking a stream with soil and brush, in much the same manner as beavers dam a stream. Larger reservoirs were constructed by damming a natural depression, or by forming a depression along the river and digging a channel to divert water to it from the river. Early irrigation practices were linked largely to land adjacent to streams. It required the construction of larger dams, which allowed humans to impound larger volumes of water, before irrigation of agricultural land located distant from the water source could occur. Later reservoirs also were used as a source of power, first for moving waterwheels, and subsequently to produce hydroelectric power.

Like lakes, reservoirs range in size from pond-like to very large water-bodies. The variations in type and shape, however, are much greater than for lakes. The term ‘reservoir’ includes several types of constructed water-bodies and/or water storage facilities; namely, (1) valley reservoirs – created by constructing a barrier (dam) perpendicular to a flowing river (Photo 8), and (2) off-river storage reservoirs – created by constructing an enclosure parallel to a river, and subsequently supplying it with water either by gravity or by pumping from the river. The latter reservoirs are sometimes also called embankment or bounded reservoirs, and have controlled inflows and outflows to and from one or more rivers. In addition to single reservoirs, reservoir systems also exist, and include (i) cascade reservoirs - consisting of a series of reservoirs constructed along a single river, and (ii) interbasin transfer schemes – designed to move water through a series of reservoirs, tunnels and/or canals from one drainage basin to another (Fig. 2)

Fig 2: Dirrerent types of reservior systems

Because they have both river-like and lake-like characteristics, reservoirs constitute an intermediate type of water-body between rivers and natural lakes (Fig. 3). Their flushing rate and the degree of river influence ultimately determines the specific characteristics and potential uses of reservoirs.

Several types of dams are constructed to make reservoirs, including earth-fill, gravity, arch and buttress. Very small dams (dam height of three – six metres above the natural river bed) could be used to strore river water for drinking purposes (Photo 9) or to divert the water flow of smaller rivers for various purposes, including the operation of mill water wheels. The most common type of dam is earth-filled, and approximately 85 percent of the dams of heights between 15 – 60 meters are of this type. Arched dams are usually constructed where very high dam walls are required, and account for 40 - 50 percent of the very large dams (dam height of 150 meters or more) around the world.

An estimated 800,000 dams were in operation worldwide in 1997. By one count, about 45,000 of these were large dams (i.e., dam height of 15 meters or more above the natural river bed and containing any water volume, or a minimum dam height of 10 meters and a volume of at least one million cubic metres). Approximately 1,700 more large dams are currently under construction, particularly in developing countries. Most reservoirs are concentrated in the temperate and sub-tropical zones of the northern and southern hemisphere. Nearly all major river systems in the world have reservoirs in their drainage basins, and a number of river systems (e.g., Columbia, Dnieper, Volga, Angara, Parana, Missouri) also have cascades of reservoirs within their basins. It has been estimated that approximately 25 percent of all water previously flowing to the oceans is now impounded in reservoirs. In fact, reservoirs exist on all continents and in all countries (except Antarctica), although their distribution within specific countries and regions is irregular. Construction of new reservoirs has essentially ceased in North America and Europe. In contrast, reservoir construction is continuing in developing countries, with nearly all new reservoirs scheduled to enter operation in the 21st Century located in Asia, Africa and Latin America.

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