2. THE MORPHOLOGY OF FLOODPLAINS

2.1 General Morphology

A floodplain is an area of relatively low lying flat land seasonally submerged by overspill from an adjacent river, lake or swamps (Fig.1). Fringing, floodplains (Fig.2) are normal features of the longitudinal evolution of a river and are usually long and narrow directly adjacent to the main river channel. However, other larger areas of flooded plain can arise by geographic accident at any stage in the course of the river. Such areas are often extensive and diffuse and are usually associated with either ‘Central Delta’ systems, as in the Niger and Kafue Flats (Fig. 3), or genuine estuarine deltas as with the Ouémé and lower Senegal.

Figure 1 Cross sections of 3 river/floodplain systems (vertical scale exaggerated)

A = Kafue (UNDP/FAO, 1968)		Main river channel
B = Niger at Onitsha (NEDECO, 1959)		Permanent swamps
C = Ouémé at Zinire		Stream or secondary channel
		lagoon

Figure 2 Reach of the Niger River (Nigeria) showing fringing floodplain with creeks, swamps and lagoons

Figure 3 Portion of the Kafue flats showing creeks, channels, swamps and lagoons

According to Leopold, Wolman and Miller (1964) a typical floodplain includes the following features:

1. the river channel
2. oxbows or oxbow lakes, representing the cut-off portion of meander bends
3. point bars, loci of deposition on the convex side of river curves
4. meander scrolls, depressions and rises on the convex side of bends formed as the channel migrated laterally down valley
5. sloughs, areas of dead water, formed both in meander-scroll depressions and along the valley walls as flood flows move directly down valley, scouring adjacent to the valley walls
6. natural levées, raised berms or crests above the floodplain surface adjacent to the channel, usually containing coarser materials deposited as flood flows over the top of the channel banks. These are most frequently found on the concave banks. Where most of the load in transit is fine-grained, natural levées may be absent or nearly imperceptible
7. backswamp deposits, overbank deposits of finer sediments deposited in slack water ponded between the natural levées and the valley wall or river terrace
8. sand splays, deposits of flood debris usually of coarser sand particles in the form of splays or scattered debris.

In view of these authors floodplains are depositional features of a river valley whereby sediment is temporarily stored in the floodplain. Under equilibrium conditions averaged over a number of years net inflow of sediments equals net outflow, although local changes in the morphology of the floodplain occur through silting of old channels and cutting of new ones.

Most of these major features of floodplains can be seen in larger rivers, although in smaller water courses they may be hard to distinguish due to the rapidity with which change occurs in them.

The features of the floodplain that are ecologically most important for fish are:

The main river channel, which usually retains water at all times of the year and may be either simple or braided. Braided channels (see Fig.4) consist of many anastomosing water courses meandering among sandy and vegetated islands, which are exposed at low and flooded at high water. Such channels are typical of portions of the Zambezi, Niger and Congo Rivers and where they occur the lateral floodplain is often restricted in width. There is some evidence (Svensson, 1933; Gosse, 1963) that ecologically such islands are analogous to the lateral floodplain and fulfil a similar role in the biology of the fish.

Figure 4 Braided channel, Niger River
Black = Permanent water; Hatched = Plateau
Dashed = bank

The floodplain, which is submerged during the flood phase and dries out during the low water phase. The majority of floodplains are on rivers of the “Sudanian” type, i.e., with grass savannah vegetation, although some rivers of the “Guinean” type have forested floodplains which give the flooded forest typical of many Zairian rivers.

Swamps and pools (lagoons) situated in depressions of the floodplain which may be permanent, persisting from one flood phase to the next, or temporary, drying out progressively as the low water phase advances (Figs. 2, 3, and 7). The distinction between swamps and pools depends largely on the amount of vegetation, which can alter throughout a season. There is a tendency for permanent pools to become swamps as the water level in them is lowered. In their most extreme form these features comprise a considerable portion of the floodplain area; systems such as the Luapula and Pongola consist of strings of large permanent lakes, and the Elephant marshes of the Shire River floodplain are extensive persistant swamps. Pools may form in simple depressions in the plain filling as the water extends across the surface and retaining water after the flood has subsided. Such blind pools loose water by evaporation, or if below the water table remain as permanent bodies. More complex types of pools, connected to the main river by channels or creeks are also found on some floodplains. These may form discrete systems which only interconnect at the height of the floods (Fig.7).

Channels and creeks, which connect the depressions of the floodplain with each other and with the river form a network over the plain. Some of these penetrate the levée and represent the main paths of water movement during the earlier periods of rising water and the later phases of falling water (Figs. 2 and 3).

Levées or raised banks bordering the main river channel, which may be significant in the way in which they direct or deflect the water flow over the floodplain and delay both the onset of flooding and the drainage and drying of the plain.

The importance of these features is frequently acknowledged by the peoples living on floodplain systems and is reflected in their language where each is named (Holden, 1963; Hurault, 1965; CTFT, 1972; Rzoska, 1974). For example, in the Senegal the levée is “Fondé” and the depressions of the floodplain “Oualo” are fed by channels “tiangol” and retain in their lowest part a permanent pool or marsh “Vindou”.

2.2 Classification of Floodplains

In addition to the basic classification (2.1) of floodplains into fringing and deltaic types based on their geomorphology further classifications have been proposed.

Svensson (1933) distinguishes three main types of floodplain swamp depending on the manner of their inundation. His type 1 swamps are formed from local precipitation or drainage only and become connected with the river or other lower lying swamps by a single channel or creek. Such swamps are never influenced by variations in river height, but fish may migrate to them by ascending the connecting channel. Type 2 areas are situated immediately alongside the river and are flooded directly by it. Swamps of the third type (type 3) are intermediate between the other two and are located in the shallow depressions of the floodplain separated from the river by levées. Access to them is by channel and flooding results in the first instance from local rainfall and drainage and only later by a rise in river level.

Floodplains may also be separated by the size and permanence of the water body with which they are associated as the dynamics of fish populations of river/floodplain systems connected with large lakes (e.g., the Yaérés of the Chari-Logone Rivers (Durand, 1970) or the floodplain of the Kenyan rivers flowing into lake Victoria) are likely to differ greatly from those of rivers that break into a series of isolated pools in the dry season, if only by virtue of the carrying capacity of the water mass available during low water (Van Someren, 1961).

Rivers, themselves, vary in nature regionally within the African continent. Daget and Iltis (1965) classify West African rivers into (i) Guinean and (ii) Sudanian types.

1. Guinean rivers are those which drain the forested areas of West Africa. They run their course through gallery forest and have floodplains which may extend to inundate large areas of forest floor. These are reputedly less productive than Sudanian rivers.

2. Sudanian rivers drain the open savannah regions and have little or no fringing forest. The floodplain is usually extensive and grass-covered.

Any absolute classification of floodplains into types based on their morphology is, however, difficult as the range of variation encompasses too many intermediate forms. It is, nevertheless, useful to bear these differences in mind when interpreting the behaviour of the different regions.

2.3 Distribution and Area

Fringing floodplains may be regarded as normal developmental features of rivers and they are present to a greater or lesser extent in most African water courses. Major floodplains arising from local geographical anomalies are somewhat rarer and are confined to only a few river systems. The flooded and dry areas of some representative floodplains are given in Table I.

TABLE I

Characteristics of some African floodplains

Floodplain	A1 Area at peak flood (ha)	A2 Area during low water (ha)			Authority
Barotse (Zambezi)	512 000	32 915	6	1	MacDonald
Benue* (Nigeria)	310 000	129 000	42		FAO/UN, 1970
Kafue flats	434 000	145 560	34	1	Kapetsky
Massilli*	1 500	200	13	1	Barry
Niger (Central Delta)	2 000 000	400 000	20		Konare; Raimondo
Niger* (Niger)	90 704	27 000	30		FAO/UN, 1971
Niger* (Dahomey)	27 440	3 200	12		FAO/UN, 1971
Niger* (Nigeria)	480 000	180 000	38		FAO/UN, 1970
Okavango	1 600 000	312 000	20	1	Cross
Ogun*	4 250	2 500	59	1	Dada
Oshun*	3 740	2 000	53	1	Dada
Ouémé	100 000	5 170	5		Pers. obs.
Pongolo*	10 416	2 927	28	1	Coke
Senegal*	1 295 000	78 700	6	1	Lessent, Fall, Sere
Elephant & Ndinde Marshes
(Shire)2	67 300	46 049	68	1	Hastings
Sudd (Nile)	9 200 000	1 000 000	11		Rzóska, 1974
Volta* (White Volta)2	85 324	10 221	12	1	Vanderpuye
Yaérés (Logone)2	700 000			1	Ali Garam

* Fringing floodplains
¹ Replies to CIFA Working Party forms
² Part of a larger system

Less data are available on the composition of the low-water components of the system (Table II) although the figures that are shown indicate the relatively low production of the total area that is contributed by the main channel.

TABLE II

Dry season composition of floodplain systems

- Negligible area
( ) Percentage of total