Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 395

New Hampshire and Vermont combined (Devine, 1995) . The 0.3% of the global land surface which has
been submerged represents a much greater loss than the raw statistic implies—the floodplain soils which
reservoirs inundate provide the world's most fertile farmlands; their marshes and forests the most diverse
wildlife habitats.
As listed in Table 7.4 the dams on the rivers provide a huge amount of power to the industries of the
countries and serve as the engine of the economy. The dams also provide benefits of flood control, water
supply and navigation facilities.

T able 7.4 Dams with largest capacity hydroplants (after International Water Power, 1995)
Rank Dam Country Completed Installed capacity (MW)

1 Itaipú Brazil/Paraguay 1983 12,600

2 Guri (Raul Leoni) Venezuela 1986 10,300

3 Sayano-Shushensk Russia 1989 6,400

4 Grand Coulee U.S.A. 1942 6,180

5 Krasnoyarsk Russia 1968 6,000

6 Churchill Falls Canada 1971 5,428

7 La Grande 2 Canada 1979 5,328

8 Bratsk Russia 1961 4,500

9 Ust-Ilim Russia 1977 4,320

10 Tucurut Brazil 1984 3,960

11 Ilha Solteira Brazil 1973 3,200

12 Tarbela Pakistan 1977 3,046

13 Gezhouba China 1981 2,715

14 Nurek Tadjikistan 1976 2,700

15 Mica Canada 1976 2,660

16 La Grande 4 Canada 1984 2,650

17 Volgograd Russia 1958 2,563

18 Paulo Afonso IV Brazil 1979 2,460

19 Cabora Bassa Mozambique 1975 2,425

20 W.A.C. Bennet Canada 1968 2,416

Note: The data are updated to 1995

7.1.2 Water Quality in Reservoirs

7.1.2.1 Thermal Stratification
The relatively small volume of water in any river section, together with turbulent mixing and the large
surface-area in contact with the atmosphere, allows a rapid response of stream-water temperature to the
prevailing meteorological conditions. In reservoirs, however, the increased mass of relatively still water
allows heat storage to take place and this produces density differences. From spring through summer, a
well-defined temperature gradient develops and highly stable summer stratification may become
established in reservoirs (Fig. 7.2).
In the zone near the surface the temperature is roughly uniform because surface waves keep the water
in the layer moving, this is the epilimnion . Whereas in the zone near the bottom, there is a layer also
with a uniform temperature distribution, but much lower, which is called hypolimnion . An intermediate
layer of maximum temperature-gradient may be established between the epilimnion and the cooler,
denser, water of the hypolimnion . The rapid decrease of temperature with depth through the thermocline
is associated with a rapid increase in density of the water. This density-discontinuity is poorly developed
in spring, but becomes well defined in summer. At its full development, the thermocline commonly