The essential feature of the floodplain, which distinguishes it from other aquatic systems, is the alternation of flood and dry phases. Flooding may be brought about by two main agencies, (i) local rainfall and (ii) overspill from the river.
Local rainfall and drainage can inundate depressions in the plain independently of any rise in river level. This type of flooding has been noted from the Gambia River by Svensson (1933) and the Kafue by Carey (1971) and is probably common to all systems to a greater or lesser degree. Extensive areas can be flooded in this way and the inundation of the Upper Chari (Bar Salamat and Bar Azoum) appears to be almost entirely of this type (Durand, pers. comm.).
Overspill from the river occurs as a slow motion wave of high water travels down the river channel overflowing the banks and levées to flood the low-lying plain (Fig.5).
Figure 5 Representative hydrological regimes of three African floodplains
A. Kafue River at (a) Namwala and (b) Kasaka
B. Senegal River at (a) Bakel and (b) Dagana
C. Niger River at (a) Kolikoro, (b) Mopti and (c) Diré
The two types of flooding are quite distinct, one resulting in flow from the plain to the river and the other from river to plain. In many regions they may occur at different times of the year. In long rivers, for instance, the arrival of the flood crest may be delayed until the dry season. Such a delay is typical of the Niger River where at the level of Malanville (Dahomey) two floods occur, one in August-November caused by rainfall and drainage of local rivers and a second in December-March due to the arrival of the flood from the headwaters of the river.
The tendency for floods to arrive at different times from the diverse tributaries of large rivers may lead to complex flood patterns or eventually to the effective suppression of the fluctuation in water height.
Figure 6 Flood regimes of some major African floodplains
Typical flood patterns of major African floodplains shown in Fig. 6 indicate that the flooded phase of the annual cycle lasts about six months (range 4.5–8 months). It is however difficult to give absolute figures for flood duration for any one floodplain due to the time taken for the flood crest to pass the length of the area. Fig.5 shows the hydrological regime of three floodplains where the crest takes about a month to travel downstream from one end of the plain to the other. The same phenomenon makes assessment of absolute flooded areas difficult as the water level may be falling in one part of the system while rising in another and in extreme cases, such as the Okavango, one portion of the plain is dry while the other is flooded.
A typical flood cycle for one floodplain pool is shown in Fig.7.
Figure 7 Annual cycle of flooding of a typical floodplain pool of the Senegal River (after Reizer, 1974) Key as for Figure 3
Hydrological Indices: Apart from simple water height as measured at a gauge, a number of other measures of flood intensity have been used to assess the effects of flooding on the dynamics of fish in the system. Chapman et al. (University of Idaho, 1971) and Dudley (1972) have used a flood index (FI) by measuring the area under a curve of water depth for the flooded phase of the water cycle. Kapetsky (1974) uses a modification of this index to obtain three hydrological indices (HI) (Fig.8).
Figure 8 Calculation of the three hydrological Indices
(from Kapetsky, 1974)
| Index 1: | the area below a water level curve at which the floodplain initially becomes inundated until it finally becomes dry (shown in black in Fig.8). This is a measure of the annual extent and duration of the inundation of the flood-plain |
| Index 2: | the area above the water level curve for the time when the floodplain becomes dry until it is once again flooded. This index measures the extent of the aquatic environment when the floodplain is dry and includes the duration of the dry phase |
| Index 3: | the area above the water level curve only until the minimum annual water level is reached (indicated by black line in Fig.8). This is a measure of the extent and duration of the contraction of the aquatic environment after the floodplain has dried. |
Index 3 is similar to the DDF (Draw-down*Factor) used by Lagler et al. (University of Michigan, 1971) which is the sum of the values for flood storage volume of the Kafue flats for the months September through December.
These indices are, however, insensitive to differences in regime type caused by variations in duration and height of floods, i.e., a long shallow flood may give the same index as a high short flood.
From the moderate amount of data existing from various African systems it appears that great similarities of hydrological behaviour exist between them. In fact it is generally true that variations in hydrology along one river system are greater than those between equivalent reaches of different river systems. Table IV gives crude data on the conductivity and pH of several major African river systems.
TABLE IV
Conductivity and pH of some African river systems
(main river underlined)
River | Conductivity (K20 μ mhos) | pH | |
River channel | Floodplain pool (dry season) | ||
| Bandama | 145 | 6.7–7.6 | |
| Chari | 42–73 | 6.9–7.7 | |
| Congo/Zaïre | 44–108 | 5.5–6.5 | |
Ubangi | 19.4–56 | 6.2–6.7 | |
Luapula | 150–180 | ||
Lualaba | 145–255 | 145–255 | |
Ruzizi | 828 | ||
| Lt Scarcies | 35–55 | 7.1–7.4 | |
| Moa | 36 | 6.6 | |
| Niger | 31–70 | 82 | 6.7–7.2 |
Mayo Kebbi | 89 | ||
Sokoto | 6.9–8.1 | ||
| Nile | 150–500 | 116 | |
Kagera | 93–99 | ||
Sobat | 112 | 7.2 | |
Bugurgu stream | 245–395 | 230–350 | 7.1–7.8 |
Bahr-el-Ghazal | 550 | 7.8 | |
Semliki | 400–910 | ||
| Orange | 159 | 7.7 | |
| Ouémé | 60 | 150–160 | |
| Prah | 140 | ||
| Rokel | 40 | 6.9–7.3 | |
| Ruaha | 108–136 | 6.9–7.9 | |
| Senegal | 40–130 | 73–385 | 6.8–7.1 |
| Volta | 41–124 | 6.5–7.3 | |
Black Volta | 62 | 6.4 | |
Red Volta | 62 | 6.5 | |
White Volta | 119 | 7.2 | |
| Zambezi | 50–96 | 57–102 | 7.4 |
Shire | 220–225 | 7.5–8.8 | |
Kafue | 130–320 | 70–280 | |
Physical and chemical changes follow the flood cycle closely.
Dissolved oxygen: Dissolved oxygen levels are generally lower in both river channel and floodplain pools during the dry season than during the floods (e.g., Egborge, 1971; Carey, 1971), although local variations do occur induced by wind action or by photosynthetic activity of aquatic plants (Holden and Green, 1960). Lowered dissolved oxygen levels can be induced in the river early in the floods by the flushing out of stagnant waters from the swamps (Holden and Green, 1960; Tait, 1967, University of Idaho et al., 1971; Scully, 1972). The smaller pools of floodplains may become completely deoxygenated in the dry season (Welcomme, 1971) and fish mortalities ascribed to wind induced overturn of the de-oxygenated lagoon waters, have been described by Tait (1967a).
pH: pH is the most variable of factors between different river systems. In general pH is lower in the flood season and rises in the dry in the river channel (Tait, 1967; Egborge, 1971). Changes in the floodplain pools during the dry season seem to depend on the type of soil; thus in the marshy type 1 swamps of the Gambia system the water was more acid than the river (Johnels, 1954), little difference was noted between pools and river in the Senegal (Centre Technique Forestier Tropical, 1972a) and water in the lagoons of the Sokoto River tended to alkalinity due to the concentration of calcium by evaporation (Holden and Green, 1960). Fairly basic differences appear to exist between Guinean rivers, which tend to acidity and in extreme cases may have a pH as low as 4.5 (Berg, 1961) and Soudanian rivers with a neutral or alkaline pH.
Conductivity: A general tendency for conductivity to be higher in the dry season than the wet is noted by the Centre Technique Forestier Tropical (1972a) for the Senegal, Duerre (FAO, 1969) for the Barotse plain, Egborge (1971) for the Oshun River and Carey (1971) for the Kafue. This does not necessarily mean that the total quantity of salts is less during the floods, rather it is thought by Holden and Green (1960) that the total remains unchanged but is more diluted due to the greater quantity of water in the system. Anomalies in productivity in Lake Chad (Lamoalle, in press) correlated with the failure of the Yaéré floods might suggest that there is an increase in ions coming down the Chari River which would normally remain in some bound form on the floodplain (Daget, pers. comm.). The role of nutrients in the floodplain/river system, however, remains largely uninvestigated, particularly in conjunction with variations in primary productivity (see 4.3). However, Welcomme (in press) has found that edaphic factors, represented by conductivity, account for about 61 percent of the variation between the actual and expected theoretical catch in African river systems. The mean conductivity of the system may therefore influence the productivity relative to other systems.
Current: It is perhaps commonly thought that floodplain waters are largely static. This does not appear to be the case as currents of varying intensity flow across the plains during the whole period of inundation. These currents are probably characteristic for the plains and are regulated by the openings of the levée. In some rivers, for example, in the Yaérés, the water spilling over from one river, the Logone, is voided by a second channel, the El Beid. In other rivers water entering the floodplain system upstream emerges again many kilometres downstream. Evidently such flow patterns will condition the behaviour of the fish inhabiting the system.
Phytoplankton: Blache (1964) wrote of the Yaérés system of the Logone River that the immediate entry into solution of organic and mineral nutrients (faeces, dried vegetation, ash, etc,) from the soil leads to an explosive increase in phyto- and zooplankton. This view is not upheld by subsequent workers in other areas of Africa who are in general agreement that the maximum production of phytoplankton occurs during the dry season, and that during the floods concentrations are low (Centre Technique Forestier Tropical, 1972a; Holden and Green, 1960; Talling and Rzóska, 1966). In fact, Egborge (1974) found phytoplankton production to be strongly correlated with conductivity and transparency, and inversely correlated with water level and current velocity. The main location of phytoplankton production within a river-floodplain system appears to be in the lagoons after separation from the river (Centre Technique Forestier Tropical, 1966, 1972; Holden and Green, 1960) and Egborge (1974) has found that all forms of phytoplankton except diatoms are dominant in the backwaters of the Oshun River in the dry season. Occasional phytoplankton blooms have been recorded from rivers during the dry season (Carey, 1971). Maximum phytoplankton production therefore appears to occur in the lagoons and pools of the floodplain in the dry season. However, the suggestion has been advanced by Holden and Green (1960) that the total amount of phytoplankton in the system changes little and that there is merely a dilution which results in a reduced density per unit area.
Zooplankton: There is little information on zooplankton production although a marked correlation between this and conductivity has been noted (FAO/UN, 1969). High production of zooplankton organisms has been noticed in larger floodplain lakes during low water, but this remains low in the rivers (FAO/UN, 1970). Zooplankton would therefore appear to follow similar patterns as phytoplankton.
Other invertebrates: Insect and other invertebrate populations of floodplains remain relatively poorly studied. Carey (1967) states that the largest concentrations of macroinvertebrates was found in weeds in lagoons, but that they were generally widely distributed and abundant in flooded areas. Personal observations have shown the enormous increase in macro-invertebrates that are possible during the flooded phase, when on three successive floods in the Ouémé River, pulmonate gastropods appeared in great quantities during the second month of flooding and persisted until the end of the floods; these organisms are generally absent from the environment during the dry season.
Higher vegetation: The most notable feature of floodplains during the period of rising water is the rapidity of growth of the higher vegetation. Often this is burnt off completely in the dry season, but rapidly colonizes the inundated zone and growth rates of 75–100 cm in a fortnight have been recorded by Van Rensburg (FAO/UN, 1968). Estimates of production of grasses indicate a mean weight of 16 000 kg/ha after about 5 weeks of flooding. Correlated with the increased amount of vegetation is the periphyton, including filamentous algae, which appears attached to most surfaces (Carey, 1971; personal observation), and it seems likely that during periods of high flow where adequate supports are to be found, periphyton (or “Aufwuchs”) replaces the phytoplankton at least to a certain extent.
The higher vegetation of the Kafue flats has been studied in detail by Van Rensburg (FAO/UN, 1968) and that of several other East African valleys has been described by Vesey-Fitzgerald (1970). Both these authors trace vegetation patterns to the drainage and pH of the soils. Dominance and growth of various elements of the plant community are closely linked to the flood cycle. Major grasses of the floodplain are Oryza barthii, Echinochloa pyramidalis, Leersia sp. with Cynodon dactylum and Chloris gayana in better drained areas. Permanent waters are fringed with Vossia cuspidata, Echinochloa stagnina and Cyperus papyrus. Silt banks support dense growths of Phragmites sp. and in alkaline areas Scirpus spp. Permanent waters of lagoons have floating masses of Pistia stratiotes, and Nymphae and Trappa are also common.
It seems that little of the traditional richness of the floodplain is discernable from either the chemical changes in the water or the plankton production during the flooded part of the cycle. Indeed both these factors would seem to indicate greater productivity at low water. However, the period of rising water is one of intense growth activity for higher vegetation and it is possible that the utilization of salts by the growing plants is so rapid that few free ions enter the aqueous component of the system. Growth of fish, and also aquatic macro-invertebrates is equally rapid and although few species are planktonophageous, an adequate source of nutrients must be available to sustain their growth. This would appear to be located in the detritus and possible in the epiphytic organisms. In general little information exists on food chains and energy inter-relationships of floodplain organisms and the subject deserves considerable further study.
