River and Stream Ecosystem of Northwestern Africa
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Chapter 17 RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA CHRISTIAN LBVEQUE INTRODUCTION African rivers is still partly focused on taxonomic inventories, many species being unknown or still to West Africa lies between the Sahara and the Gulf be described, despite a real improvement in knowl- of Guinea, and includes the Chad basin on its edge (Durand and Lévêque, 1980-1981; Teugels eastern border. The southern limit is the Cross et al., 1988; Lévêque et al., l989,1991a,b). For econ- river, on the Nigeria-Cameroun border. As de- omic reasons, most of the data collected deal with fined above, tropical West Africa covers a vast area hydrology or fisheries for use in management pro- (3 million km') with a distinct climatic gradient jects. This explains why the state of the knowledge of from north to south. river system is very poor compared with North The available information on West African river European or American countries. There is still a ecology is rather poor, often qualitative or descrip- great need for field data as well as for experimental tive, with the few quantitative data generally scat- manipulation (Lowe-McConnell, 1988). tered in research papers, obscure reports or unpub- lished theses. In fact, the hydrobiological investiga- tions were for a long time, as elsewhere in the world, GEOMORPHIC SETTING focused on lakes. Lake Chad, an endorheic lake which could partly be considered as an extension of Geomorphology and catchment characteristics the riverine environment, as well as the lower courses of the Chari and Logone rivers and their Most of the area consists of flat sedimentary associated flood plains, were well investigated dur- basins and upland plains ranging from 150 to 600 m ing the International Biological Programme (IBP) a.s.l., with patches of highlands above 1OOOm and and later on (Carmouze et al., 1983). The large distinct from one another. Among the most impor- interior Niger delta also was investigated by many tant, the Fouta Djalon and the Guinea range along scientists because of its economic importance and its the western coast separate those rivers flowing di- peculiar situation at the limit of the desert (Gallais, rectly into the Atlantic Ocean from those flowing 1967; Grove, 1985). Short-term pre-impoundment northwards, which are the sources of the Niger, surveys often were conducted before the damming Sénégal and Gambia rivers. The rivers on the which created major man-made lakes such as the Atlantic side (in Guinea, Sierra Leone and Liberia) Akosombo on the Volta, and the Kainji on the are relatively short, and partly torrential in their Niger. Since 1974, information also has been col- upper courses, which often are punctuated by rapids lected on West African rivers to evaluate the impact and falls and flow through narrow valleys. The main of chemicals used in the Onchocerciasis Control headwaters of the Chari, Logone and Benue rivers Programme in West Africa (Lévêque, 1989; are on the Adamawa plateau in the east (Fig. 17.1). Yameogo et al., 1989). Although the northern fringe of the area is dry, Diversity of the fauna and flora is generally con- there is a dense river drainage. The Niger is the sidered to be greater in tropical than in temperate longest West African river. It rises in the Fouta river systems, and the research effort on West Djalon mountains in Guinea, and flows in a loop .i CHRISTIAN Li\ ÊQVE passing northeactiurds through the estensive in- I+ith :ì catchment Lirea greater than 10000km' ;ire terior central delta (20OOOkm'). then southwards given in Table 17. I. through the coastal delta (36000 km2)before enter- ing the Atlantic Ocem lts major tributrìry is the Benue river ivhich rises in the Adamawa m Cameroun. Roth rivers have fringing flood plains which cover more than 10000km2 at peak floods (\Velcomme. 1979. 1989). The Kainji dam. on the loiver Niger. forms one of the major impoundments in Africa. The Chari river Lind its major tributar>-. the Logone. drain large ureas of Central Africa sav;inn;i into the endorheic Lake Chad. lt includes an czten- si\e Rood-plain system of about 90 OOCI km'. includ- ing the 7000 hi2 Yaere in northern Cameroun. which is cnnnected to the Chari. Logone and Lake Chad. The Sinigal river originates at the confluence of the Bafing hvhich rises in the Foutri Djrilonl and the Bakoye rikerc. The middle coiirse of the SCnegal is tiow under control of two dams: hlanantali up- stream. and Diama in the estuary. The Voltiì is within the great meander of the Niger river. The river sy<tem coniprihes four major headwaters: the Black \.olt;i. the \+'hire Voltrl. the Red Volta and the Oti which contribute.; -31 to 3(1",, of the mnual flcm of the Volta. The closing of the Akosombo dnni in 1964 strongly modified the hy- drology of the lower course of the Volta. The main characteristic5 nf \Vest .African river5 . L 1O0 1 1O0 50" O0 50" 4 1O0 80 80 80 40" 80 60 60 60 30' 60 40 40 40 20" 40 20 20 1 O' 20 20 O O O .. n JFMAMJJASOND JFMAMJJASOND JFMAMJJASOND RAINFALL (mm) > 3000 ..... ..... 1500-3000 .. ..1000-1 500 500-1000 0 ......o- 250-500 II. 0 0 0 0. o o .-.....- YI .-. 0.0 R ("1 AD'OPODOUME 700 T 'C 1O0 50" 1O0 80 40" 80 60 30" 60 40 200 200 -p 40 20 I OD E 20 O Io 'FMAMJJASOND J C M A M J J A S O N D Fig. 17.2. Distribution of rainfall over West Africa (distribution of isohyets from Cazenave and Valentin, 1990; climatic diagrams according to Walter and Lieth, 1960-1967). 527 CHRISTIAN L F V 6 Q Li t Climate the northern regions. near thc lQ(l-nim isohyet. thc rain) season is short (a few weeks in July ,August 1 The regular seasonal pattern of climate chnnge is and there i.; a very long dry season. Furthtrr south. a consequence of the seasonal migration of the near the 750-mm ihohyet, the rainy season extends intertropical front or intertropical convergence from June to September. follnwd by iì long dry se;ì- zone (ITCZI. This front is :i Lone of climatic instahil- son. Near the coastiil regions. there is a distinct four- ity sepnriiting two air masw: the soiitherlq mass of se:ison climatic regime. Ivith ;i ni;i.ior dry season moist niaritinle air. and thc northerly mass of hot (November to hlarch~.a mri-jor net seiison (April ta and dry continental air. From December through July). ;i minor dr>-se;ison (August- September). and hlarch. the ITCZ i5 located south. reaching 5 N: the ;I minor- wet seiison (October-Noveniberi. Iivrlherlj air mas< doniinntes. and thi\ is conse- 1 he yarly amount of rainfull nnd seasonal dis- quently the period of the major dry sason in the trihution pattern shw local variations. But the coastal forebt zmc. .A drj- 15 ind. the H;irmattan. .;triking feature is the large pxr-Ic,-ye;ir variation in blons northeast across the reginn. o totnl rainfall. and the existence of a drought period reaching the coast. and bringing ;i fine duzt from the through the 1970.; Lind 19SOs. relative to earlier Sahnru. Then the ITCZ migrates clo\vlj northu.ards records. :tnd rexhes 170 N b>- .Id)- through August. In September. the TTCZ once agnin begin.. its south- Vegetation \vard migration. Locally. the seasonal pattern and ;ìmount nfrain- The decreae in tot:ì1 r;iinf:ill :ind the length of'the fall (Fig. 17.3depend on the latitude and the position rainy season from the coii.;t to the north. result in ;i of ITCZ. The length of the rain) seiiwn and the more or less regular arrangement of climatic belts amnunt of rain increase from north to south. In and vegetation mnes i Fig. 17.31. In the south. some a Semi-desert steppe Semi-desert grassland and shrubland a Drier savanna types rbloister savanna types a Lowland rain forest -A RIVER AND STREAM ECOSYSTEMS OF NORTHWESTERN AFRICA 523 native rain-forest still exists, but most has been relative length of the two periods: the Sahelian extensively destroyed and replaced by industrial regime with brief sporadic floods during the rainy plantations (coffee, cacao and rubber). In Sierra season, and the transitional tropical regime with a Leone, for instance, traditionally included in the longer flood period and a less severe dry period. The rain-forest area, the savanna now extends almost to tropical regime (and variants) is widespread in the the coast. In Côte dlvoire as well, the rain-forest is so-called Sahelian and Sudanian zones in West more and more confined to areas where access is Africa, and can be observed in most of the large difficult, and the so-called Baoule “Gap” extends rivers: Sénégal, Gambia, upper and middle Niger, much further south. Volta, Chari, Ouémé, Mono and many coastal Further north is the Guinea-Sudanian transition rivers. zone (White, 1983), covered today with secondary The equatorial regime (Fig. 17.41, north of the grassland and wooded grassland. In the Sudanian equator, includes a long dry period from December region, the surviving stands of natural vegetation to April, a first flood period from May to July, a belong to various types of woodland, but in places second and short dry period from August to where agricultural activities are possible, the natural September, and a second flood period from October vegetation has been profoundly modified and to December. Such a regime occurs mainly in woodlands in various stages of destruction or regen- coastal rivers from Côte d’lvoire (Iltis and Lévêque, eration occur.