The construction of two or more large dams within a river basin has marked effects on the aquatic environments. Multiple impounding can take several forms (Fig. 40):
In this simple situation the two dams may operate cooperatively. For example, on the Great Ruaha River in Tanzania, Kidatu dam has good hydraulic head for electricity generation but insufficient storage capacity for development of the available potential. Thus Mtera dam was constructed upstream to act as a storage head tank for Kidatu (Bernacsek, 1980a). Kafue Gorge/Flats and Itezhitezhi dams in Zambia interact in a similar manner, although in this case the upstream Itezhitezhi reservoir is required because evaporation loss on the Kafue Flats is too high to use it for storage purposes. Another common situation results after a major hydroelectric dam is built on a river. A second smaller dam with low-head turbines is constructed downstream to take advantage of the strongly regulated flow being discharged from the large dam upstream (i.e. ‘run-of-river’ mode of operation). Examples of this situation are Volta (Akosombo)/Kpong on the Volta River in Ghana and Kossou/Tabor on the Blanc River in Ivory Coast.
Cascade and parallel impounding
This situation is more often than not a transient situation soon replaced by the following.
Full hydrological regulation of a river basin usually requires numerous dams. Examples from Africa are shown in Fig. 41.
The overall effect of multiple impounding is to reduce floodpeaks and generally smooth out river flow (Fig. 42). There is also some temporal retardation of peaks in the flow pattern (if not eliminated) which increases progressively downstream. Sediment trapping is magnified if several of the dams have a storage function. Conductivity increases progressively downstream if several of the reservoirs have a long retention time and are situated in areas with high solar evaporation rates. The latter will also result in a net loss of water from the basin if the combined reservoir surface area is large. Control of flooding on floodplains is increased with the result that more intermediate stretches of river are converted to the less productive ‘reservoir’ type. Reoxygenation of turbinated discharge water (which will become the dominant species of discharge water if the basin water management policy is centred on hydroelectricity generation - and it usually is) becomes more critical because the mean distance between reservoirs generally decreases as the number of dams in a basin or sub-basin increases. Waterfalls and cataracts if present may become permanently inundated by reservoirs, and there may be insufficient time for deoxygenated discharge water (in some cases containing toxic H2S) to reoxygenate naturally via atmospheric and photosynthetic diffusion before entering the next reservoir downstream, where biological productivity can be expected to be adversely affected by the poor affluent water quality.
The overall effect of multiple impounding on the fish yield of a river basin will depend mainly on the extent to which reservoir yields can compensate or exceed the flood-plain yields which are lost. Maximization of total reservoir surface area is critical, as well as the creation of as many major and large reservoirs as possible (since above a critical reservoir surface area and mean depth combination, the additional benefit of a pelagic fishery resource can be realized).
Fig. 39. Egyptian marine fish landings from 1962 to 1979 to illustrate the effect of Aswan High Dam. Data from George (1972) and Latif (in press).
Fig. 40. Types of multiple impounding of a river system
Fig. 41. Schematic diagram of multiple impoundments on the Oum er Rbia River (Morocco), Nile River (Egypt/Sudan) and the Zambeze River (Mozambique, Zambia and Zimbabwe)
Fig. 42. Effect of multiple impounding on discharge and water level in the Nile system. Modified from Lewis (1956).