3. REGISTER OF AFRICAN DAMS AND RESERVOIRS

In order to obtain a comprehensive appreciation of the importance of large dams to African hydrological basins a register of existing dams was assembled from diverse sources (reproduced in the Appendix). As far as the author is aware only large dams ² are included in the register and with reservoirs greater than 1.0 ha in surface area at USL ³.

Number of reservoirs per country and total known surface area impounded are given in Table 2. Three hundred and twenty dams/reservoirs were identified and a total maximum storage surface area of 40 809 km² was determined for 231 of these dams. Most of the reservoirs for which surface area data were not obtained are probably small or minor reservoirs. Unfortunately it was not possible with the time and resources available to the author to assemble a complete list of large dams for each country. The actual number of existing large dams is known to be significantly greater than 320. For example, Blin (1979) notes that there are 80 permanent reservoirs in Upper Volta being exploited by fishermen with a combined surface area of 130 km². The register however lists only 4 reservoirs by name with a known total surface area of 22.00 km² for only 3. Similarly, 120 large dams exist in Zimbabwe but only 37 were identified by name in the register (Van der Lingen, 1981). Van der Leeden (1975) totalled 76 reservoirs in the Canary Islands but the register lists none by name. In contrast almost complete coverage is thought to have been achieved for some countries (i.e. Tunisia, Algeria, Morocco, Tanzania, South Africa). The actual number of large dams/reservoirs in Africa is likely to be between 600 and 800.

² As defined by the International Commission on Large Dams a large dam is one in which the height from the lowest portion of the general foundation to the crest is greater than 15 m. Some dams between 10 and 15 m high which meet special conditions are also considered large dams (Thomas, 1976).

³ USL = upper storage level in m a.s.l.

Table 2. Numbers and combined surface areas of reservoirs in the African region (data from Appendix).

¹ Nasser/Nubia reservoir was counted only once in subregion total.
² Kariba reservoir was counted only once in subregion total.

Reservoirs are ranked according to surface area in Table 3. The following 5 category groupings were adopted:

The geographical positions of the 8 major and 24 large reservoirs of Africa are indicated in Fig. 1.

The emphasis in this paper is on capture fisheries. In the African context small to major reservoirs can usually only be managed as capture fisheries. Minor reservoirs need to be carefully assessed as to the best method of fishery management, either laisser-faire capture or some form(s) of extensive aquaculture. Reservoirs of less than 1.0 ha are to be classified as ‘ponds’ which ideally should be managed on intensive aquaculture principles.

In order to estimate the total impounded surface area for Africa as a whole, cumulative surface area was plotted in Fig. 2. The curve tends towards an asymptote of approximately 41 000 km². The 32 major and large reservoirs together account for 94.7% of the asymptotic estimate and it is unlikely that any reservoirs have been overlooked from these two categories. It also seems unlikely that the large number of medium, small and minor reservoirs missing from the register and Table 3 would have an important effect on the asymptote if they had been included in the plot.

Table 3. African reservoirs ranked by surface area

¹ Itezhitezhi and Kafue Gorge dams (situated at the upstream and downstream ends of the Kafue Flats flood plain, respectively) together have altered and now strongly regulate flooding patterns on the Kafue Flats. The area flooded during the rainy season is now smaller (4 340 km²) and more regular. Flood duration is longer and slightly delayed. During the dry season a larger area remains permanently flooded (about 1 000 km²). The amplitude of water level fluctuation has been reduced from a mean of 5.1 m to a new mean of 3.3 m (Dudley, 1979; Werger and Ellenbroek, 1980; Chipungu, 1981). For these reasons the Kafue Flats is here accepted for hydrological classification as an ‘artificial’ reservoir, albeit a very atypical one.

Fig. 1. Geographical location of 32 major and large African reservoirs

Fig. 2. Cumulative surface area of African reservoirs

The asymptotic estimate is based on USL surface areas. Mean annual surface area is however rather less due to drawdown. The combined mean annual surface area of the 8 major reservoirs is estimated at 27 408 km² or 85.6% of the combined USL surface area. This suggests that the mean annual surface area asymptote for Africa is at about 35 000 km². Even this estimate is probably rather high since 2 major reservoirs have not yet filled completely (Nasser/Nubia and Kossou) and others are currently operating at well below design water levels (Volta, Kariba and Cahora Bassa).

Welcomme (1979) also used the asymptote method to estimate total river drainage basin area for Africa. It should be noted that unlike river basins, construction of dams in the future will necessarily raise the asymptote and that the latter will be most sensitive to the formation of new major and large reservoirs.

Although the total surface area of impounded waters may seem small in comparison to Africa's natural lakes (Lakes Victoria, Tanganyika and Niassa/Malawi alone have a combined surface area of 132 500 km²), the total impounded area can be expected to increase substantially in the future as many member countries have dam projects in the design or construction stages. Of greater significance than surface area is the volume of water stored, since from a utilitarian viewpoint it is volumes (= masses) of water which have a use potential (rather than surface area which is a predetermined resultant of impounding a specific volume of water over a particular basin surface). The ratio of volume impounded-to-mean total annual river runoff is already fairly high for some countries: 25% for Zimbabwe, 40% for South Africa, 49% for Morocco, 58% for Tunisia and in the special case of Egypt exceeds 100%. In effect, the goal of river basin development in most African countries is to increase the national storage volume-to-runoff ratio towards the maximum potential.

The historical trend of dam construction in Africa is presented in Table 4 and Fig. 3. The first major dam/reservoir was Gebel Aulia, closed in 1937. Since the Second World War there has been a steady increase in the number of dams closed per decade. Total surface area impounded peaked sharply in the 1960's (due to the closure of several major dams) and has dropped significantly since then. The trend would appear to be towards construction of numerous smaller dams/reservoirs. This suggests that fishery biologists might be called upon more frequently than in the past to provide advice on dam projects both at the planning and the implementation/operation stages.

Table. 4. Historical trend of large dam construction in Africa

Fig. 3. Number of dams closed and surface area impounded in Africa over time