CHAPTER 1

MORPHOLOGY OF RIVER SYSTEMS

FORM OF RIVER SYSTEMS

TYPES OF RIVER

Rivers are linear systems which serve to evacuate water falling on the continental masses towards the oceans. This transfer involves the dissipation of the kinetic energy inherent in the water and the morphology of river channels evolves so as to even out the loss of this energy along the length of the river. The hydraulic processes arising from this loss act in a predictable manner within the river channel, thus the forms taken by the various rivers of the world are closely comparable where conditions of bed-rock, elevation and rainfall are similar (Leopold et al., 1964). In effect, greater differences exist between the various zones of one river than between homologous zones of different rivers. Thus biological studies on rivers tend to treat sub-sets of river systems, such as “trout streams” or “potamon reaches”, rather than to consider the system as a whole from headwater to mouth. However, such subdivisions are for convenience of study only and any river system should ultimately be viewed as a continuum showing a succession of characters along its length.

Features of the geography of any particular river basin can impose certain characteristics on the river. One useful distinction is between: (i) reservoir rivers, which have extensive lakes, swamps or floodplains near their headwaters resulting in the gradual release of floodwaters and sustained flow with only slight variations in rate; and (ii) flood rivers, where there are extremes of annual fluctuation in water level from severe flood to sometimes complete desiccation in the dry season. The most extreme form of flood rivers, i.e. those which frequently cease to flow or even dry out seasonally, have been termed “sandbank” rivers by Jackson (1961a and 1963).

A second distinction originates from the type of landscape through which the river flows. Here (i) tropical forest rivers have many of the characteristics of reservoir rivers in that variations of flow are evened out by the retention of water in the flooded forest. Such rivers tend to have black waters with low pH, low conductivity and ionic content, low silt load and high humic content; (ii) Savanna rivers may be of either sandbank or flood type, depending on the form of their basins. The pH of their waters is rarely extreme, varying from slightly acid to slightly alkaline, conductivities are often reasonably high as are silt loads; (iii) Desert rivers, which receive no tributaries in their dry land course, tend to conform to the flood type. They show greatly increased conductivity and alkalinity along their lengths as the water becomes concentrated by evaporation, and in their more extreme form end up as salt marsh or lake; (iv) Tundra rivers drain the arctic and sub-arctic regions northwards and tend to have flow regimes that are dependent on winter freezing. Their ionic contents are frequently poor as the lands over which they flow were denuded of topsoil during the glaciations. Mixed systems also occur, and larger rivers, especially, may change their nature several times during their course. Equally, developments within their basin may change what were once forest rivers into savanna rivers, and eventually by erosion, siltation and water use into desert rivers.

Considerable modifications have been carried out in many river systems, particularly in the temperate zone where there are few large water courses which now show all their original features, and the numbers of reservoir rivers, in the form of regulated streams, are steadily increasing throughout the world due to interventions in the aquatic system aimed at controlling river flow.

LONGITUDINAL PROFILE

Rivers tend to have longitudinal profiles which are concave open to the sky (Fig. 1.1). This means that within any one river, there is typically a succession of types of water course with steep slopes near the source to minimal slope near the mouth. This succession is by no means always adhered to, and many major rivers, through accidents of terrain, alternate between fast-flowing, rocky and slow-flowing, muddy stretches. Thus, after torrential upper courses, rivers such as the Niger (Fig. 1.1), the Zaire or the Danube show several successive reaches of floodplain and rapids along their length. The different types of water course plainly support different communities of living organisms and this has formed the basis for several systems of geographical and ecological zoning. Geographically, such zoning is reflected in everyday speech, which distinguishes between a variety of types of water course, including torrents, creeks, brooks, streams, rivers, etc. Ecologically, too, such distinctions have value as they generally correspond to many differing conditions including flow, slope or bottom type, which determine the types of plant and animal community living in them. Because of the variety of types of flowing water that are recognized, it is not possible to produce a comprehensive classification of rivers that is ecologically satisfying, although some authors such as Petts (1984) have recently published such listings. However, a fundamental difference does occur between fast-flowing streams and slow-flowing rivers. This corresponds to a classification proposed by Illies and Botosaneanu (1963) after examination of the many schemes proposed for river zonation which divides the river course into two main classes - the rhithron and the potamon.

Figure 1.1  Logitudinal profiles of river systems:
A. Theoretical ideal profile; B. The Niger River

The rhithron is defined as the region extending from the source to the point where mean monthly temperatures rise to 20°C, where oxygen concentrations are always high, flow is fast and turbulent and the bed is composed of rocks, stones or gravel with occasional sandy or silty patches. The rhithron is subdivided into three zones - the epi, meta- and hypo-rhithron covering a range of water courses from strong streams to small rivers.

The potamon is the region where monthly mean temperatures rise to over 20°C, oxygen deficits may occur, flow is slow and the bed is mainly sand or mud. Three sub-zones are distinguished - the epipotamon, the metapotamon and finally, the hypopotamon, which is that brackishwater zone affected by marine waters.

Because under this system temperature is important in defining the various zones, the change-over from rhithron to potamon tends to be at higher altitudes in the tropics, where at low altitudes a true rhithron zone may be entirely absent. This accounts for the differences in emphasis in studies or river biology in the temperate zone, which have almost completely addressed the rhithron and in the tropics, which are much more preoccupied with the potamon. However, because of the fundamental differences between animal and plant communities of torrential headwaters and slow-flowing lowland rivers regardless of temperature, it is useful to conserve this distinction even for tropical systems. Thus in this work I have adopted the term rhithron to cover the steeper, rocky, torrential upper reaches of rivers and potamon to cover the slow, mature lowland reaches.

TRIBUTARY DEVELOPMENT AND STREAM ORDER

Stream order

The dendritic arrangement of the channels of a river throughout its drainage basin is well known. Several suggestions for ranking streams forming this type of pattern have been proposed. The most widely accepted is that whereby streams are categorized according to order in a hierarchy defined as follows: first-order streams are those having no tributaries, second-order streams are formed by the union of two first order streams, third order streams by the union of second order streams and so on. In its original form the system provided for one stream, usually the longest, of each category to be extended headward in such a way that the main channel of the river extends continuously from source to mouth (Horton, 1945) (see Fig. 1.2). Later modifications of the system suppressed this idea in favour of the more simple classification of all streams of the same order into one class (Strahler, 1957).

For ecological studies of rivers, each system has its advantages. The former is of use when considering the evolution of some characteristic, for example fish catch, along the whole length of the river. The latter is a more natural grouping and is useful in generalized studies in that streams of any particular order tend to form sets, members of which can be considered together. Sudden changes in faunal abundance are not uncommon below the junction of streams, particularly those of similar order, where abrupt differences in flow, sediment load and other hydrological factors produce correspondingly gross changes in the channel of the river. These, in turn, lead to a shift in the ecological factors favouring one species group over another.

Clear relationships emerge between the numbers and lengths of streams of each order, whichever system of ordering is adopted. These show that the number of streams of different order in a watershed increases with decreasing order, according to a logarithmic relationship of the form:

where N_s = Number of streams of any order and s = order.

The length of stream of any order (L_s) decreases with decreasing order (s) in a similar manner:

The factors for a, b, x and y will vary according to continent or climatic zone.

Figure 1.2   Different systems for ordering river systems:
A. Horton (1945); B. Strahler (1957); C. Strahler's system as applied to the Logone River at the height of Moundou

These relationships show that there is a very large number of small tributaries whose combined lengths make up a considerable percentage of the total in any system (Table 1.1). For example, Leopold et al., 1964, estimated that, of the 5 million kilometres of United States' streams with a mean length equal to or greater than 1.6 kilometres over 90% were located in streams of fourth-order or less. The relevant formulae are:

N_s = 7 661 837 (-1.58^s)

and

L_s + 0.697 (0.8320^s)

^a According to Strahler

Low-order streams are often of a torrential, rhithronic nature, whereas potamon reaches tend to be concentrated around higher-order rivers. Low-order tributaries in lowland areas tend to be potamonic in character especially in the tropics.

Relation of river length to drainage basin area

The area of drainage basins and the length of streams within them are also related. Thus for the fifty largest rivers of the world, as ranked by mean annual discharge, a relationship:

L = 1.7084A^0.5418 (r² = .70)

emerges, where L = the length of the main channel in kilometres and A = the area of the drainage basin in km². When analysed by region the coefficient and the exponent of this relationship vary, as shown in Table 1.2.

^a   Derived from Leopold et al., 1964

The coefficient (a) and exponent (b) are themselves related for this data set by the equation:

b = 0.5797a^-0.1492

Similarly, Gregory and Walling (1973) summarized data on variations in drainage density for various parts of the world where total channel lengths are related to area. These, together with the relationships in Table 1.2 indicate that there are between 1.6 and 8 kilometres of channel per square kilometre of land surface, depending on landform and rainfall.

MORPHOLOGY OF THE RHITHRON

The main morphological characteristic of rhithron reaches is the alternation of pools and riffles which arise from changes in gradient (Fig. 1.3). The steeper epi-rhithron is dominated by rapids, waterfalls and cascades, but as the river proceeds downstream, the proportion of pool-like reaches relative to the riffles increases and eventually the hypo-rhithron merges into the potamon. Riffles are steep, shallow zones having coarse bottoms of boulders, rocks or pebbles. Pools are flatter, deeper zones with bottoms of finer material. They may be more complex in form, especially in the hypo-rhithron, with diverticula and backwaters having muddy or detritus bottoms, giving greater ecological diversity. According to Leopold et al., 1964, one pool-riffle sequence always occupies a length of channel equivalent to between five and seven channel widths, irrespective of the form or geographic location of the river. The ratio between the relative lengths of pool or riffle lying within this segment do, however, differ considerably. The productivity of rhithron reaches is frequently expressed in terms of pool-riffle ratios, in which a slight dominance of the riffle component is deemed favourable for salmonids because of the increased invertebrate food to be found in these zones.

Historical evidence presented by Sedell and Luchessa, (1981), and the existing situation in streams in some less settled parts of the world, indicate that the present appearance of upland streams throughout the temperate and much of the tropical world is very different from that pertaining under regimes unmodified by man. A considerable portion of the inhabited world was once forested and streams running through such areas received large quantities of wood, vegetation, boulders and other flow modifying structures such as beaver dams. Thus streams up to 7th order were, and still are in some parts of the world, conditioned by such obstructions which produced much better regimes than are common today, with extensive overback flooding, greater retention of water outside the stream channel, considerable instream floodplain storage of carbon and more diversified habitat structures, (Swanson et al., 1982).

Few attempts have been made to classify the rhithron into morphological types other than pools and riffles. The variability that exists within these two major habitat types is such that further division is needed if the areas are to be adequately described. One attempt at such a division has been made by Bisson and Sedell (1982), who, taking US salmonid streams as an example, produced a typology which seems sufficiently flexible to encompass rhithronic streams in other areas of the world. This classification subdivides the various habitats as follows:

Riffle:   Low gradient riffles - shallow stream reaches with moderate current velocity and turbulence. Substrate usually gravel, pebble and cobblesized particles.

Rapids - gradient greater than 4 percent with swiftly flowing water and considerable turbulence. Substrate generally coarser with larger boulders.

Cascades - steep reaches with variations in gradient consist of alternating small waterfalls and shallow pools. Substrate consists of boulders.

Pools:   Secondary channel pools - often temporary pools remaining in areas flooded by freshets, often associated with gravel bars; sand or silt substrate.

Backwater pools - found along channel margins caused by eddies from large obstructions. Often shallow and dominated by fine-grained substrates.

Trench pools - long, often deep, slots arising downstream of major obstructions. Usually with coarse-grained but stable bottoms.

Figure 1.3   The morphology of the rhithron showing the division of the channel into a sequence of pools and riffles

Plunge pools - arise where a stream passes over a major obstacle and scours out the bottom. Variable in depth and substrate.

Lateral scour pools - occur where flow is diverted by a large obstacle or obstruction.

Dammed pools - consist of water impounded upstream of a complete or nearly complete channel blockage. Tends toward low current and finer substrates.

Glides:  a third general habitat category consisting of moderately shallow reaches with even flow that lack obstructions or pronounced turbulence. Bottoms usually gravel and small cobbles. Glides are particularly common in large rivers passing through mountainous terrain where channels may run relatively straight for many kilometers.

As rhithron reaches are for the most part situated in low-order streams, their flood regimes are apt to be somewhat “flashy” with rapid changes in discharge. The water is usually confined within a well defined bed, although peak spates may flood a narrow strip fringing the channel. During periods of high discharge, the riffle pool complex tends to be completely submerged and from the surface the two components may appear to lose their identity. At intermediate discharges the riffle has turbulent flow while the pool has laminar flow, often with relatively calm areas at the banks. As discharge falls the distinction between riffles and pools increases until eventually, when flow ceases completely, the riffles are left dry and the pools retain water giving the channel the appearance of a string of beads.

At slightly lesser slopes for the same bankfull discharge braided channels arise. These consist of many anastomosing anabranches, which may themselves meander, winding among rocky, sandy or vegetated islands which are exposed at low water and flooded at high water (Fig. 1.4). Pool and riffle systems may appear within the individual anabranches in steeper and stonier reaches.

Figure 1.4   An example of a braided channel; the Niger River at Ayourou

MORPHOLOGY OF THE POTAMON

The potamon of reservoir and regulated rivers consists only of the channel which may be meandrine or braided in form. In flood rivers, however, there are two major components to the potamon: (i) the channel, and (ii) the floodplain which correspond to the French terminology “lit mineur” and “lit majeur” and epresent the beds of the river in its two main phases - low water and flood.

The form of many of the worlds floodplains has undoubtedly been changed considerably by human activities over the last few thousands years. Prior to human occupancy, it seems likely that most rivers were associated with riparian gallery forests, and that, in more arid areas, the plains themselves were covered with scrub of the bush savanna type. These conditions still persist in some Latin American rivers, and give water regimes and channel forms which are more stable than those of the open savanna rivers which have resulted from clearance of the trees for agriculture, grazing and firewood.

According to Leopold et al. (1964) floodplains will typically include the following features:

(a)  the river channel;

(b)  oxbows or oxbow lakes representing the cut-off portion of meander bends;

(c)  point bars - loci of deposition on the convex side of curves in the river channel;

(d)  meander scrolls - depressions and rises on the convex side of bends formed as the channel migrates laterally down valley by the erosion of the concave bank;

(e)  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;

(f)   natural levees - raised berms or crests above the floodplain surface adjacent to the channel, usually containing coarser materials deposited as floods flow over the top of the channel banks. They are most frequently found at the concave bank. Where most of the silt load in transit is fine-grained, natural levees may be absent or nearly imperceptible;

(g)  backswamp deposits - overbank deposits of finer sediments deposited in slack water ponded between the natural levees and the wall or terrace riser;

(h)  sand splay - deposits of flood debris usually of coarser and sand particles in the form of splays or scattered debris.

These features, which are illustrated diagramatically in Fig. 1.5, determine the quantity, distribution and flow of water in the system throughout the year. Most of them are readily distinguisable in moderate sized floodplains, but in smaller valleys are obscured by the rapidity with which changes occur. Individual features of the floodplain are briefly described below (numbers in text refer to Fig. 1.5).

The channels - the lotic component

The main channel (1) or channels of the river and its anabranches (2) usually retain water, but not necessarily flowing water, at all times of the year. As the river enters its alluvial plain it starts to meander forming wide convoluted channels whose curves are proportional to river width (Leopold et al., 1964). In some larger rivers, the Amazon and Zaire, for example, braided channels occur in which the islands form levees with depression lakes at their centre. Where such braided channels occur the lateral floodplain is sometimes limited in width and its whole extent may come to be contained within the main channel. In these instances the islands are analogous to the lateral floodplain and fulfil a similar role in the biology of the fish (Svensson, 1933; Gosse, 1963).

In addition to the main channel or channels of the river, floodplains have a network of channels and creeks which penetrate the levees to connect the main river with the back-swamps and meander scroll lakes. Such channels may or may not retain water at all times of the year, but they represent the main path of water and fish movement during the earlier periods of rising water and later phases of falling water.

On many floodplains naturally occurring channels are supplemented by artificial canals constructed for navigation, irrigation, drainage or even fisheries.

The floodplain

The floodplain is itself divided into two components: (i) the plain itself which is seasonally inundated, but remains dry for at least part of the year, and (ii) the standing waters which remain in the plain during the dry season.

The alluvial plain of a river can be divided into two main zones. Firstly, the levee regions (7), which more or less follow the course of the river channel and its former beds, consist of raised areas that are flooded for the shortest time annually. Secondly, the flats, which extend from the levee to the terrace or plateau delimiting the plain. Exceptionally high levees sometimes occur immediately adjacent to the channel which are only occasionally submerged by the highest of floods. Such areas, together with the raised terraces bordering the plain are used for human habitation, or give island refuges for cattle or wild game during the flood season. Levees may be much reduced or even completely absent in rivers carrying very fine silt. Because coarser material is deposited early after the slowing of the flow as a river enters the plain, there is a tendency for the raised areas bordering the river channel to diminish in height downstream. In river basins with exceptionally high silt loads, for example the Chao Phrya delta in Thailand prior to the installation of flood control structures (see Fig. 1.6), the process of levee building and of deposition within the main channel may raise the river channel and levee high above the surrounding plain.

Figure 1.5   Diagram of the main geomorphological features of a floodplain; numbers refer to description in text

Figure 1.6   Plan and cross-sections of the Chao-Phrya delta, Thailand, befre the installation of flood control structures, showing channels raised withn levees above the level of the surrounding plain. (After Ohya, 1966)

The flats which make up the greater proportion of the plain show slight differences in relief due to old depositional features. More depressed parts are interspersed with standing waters of various types. The edges, adjacent to the terrace or valley wall delimiting the plain, may be more deeply excavated where locally increased flow scours deposits to form lagoon or backswamp complexes (6). Irregularities may also be formed where inflowing tributaries deposit alluvial fans. Both the levee and the terraces may be covered with dense gallery forest (see Figs. 1.7a and 1.7b), but many floodplains are occupied only with sparse scrub or are denuded of trees altogether (savanna plains).

Figure 1.7   Schematic profile of: A. The Amazon (after Sioli, 1964), and B. The Middle Parana (after Bonetto, 1975), showing distribution of vegetation and main floodplain features

Standing waters (the lentic component)

Permanent or semi-permanent standing waters are left by receding floods in the form of sloughs in oxbows (4), meander scroll depressions (5), backswamps (6), or the residual channels left by the former course of the river (3). These water bodies expand and contract according to the annual flood cycle (Fig. 1.8) and during the highest floods tend to merge into a continuous sheet of water covering the whole plain.

Figure 1.8   Annual cycle of flooding of a typical floodplain depression of the Senegal River (Vindou Edi). (After Reizer, 1974)

Distinction is often made between lakes, lagoons or pools on the one hand and swamps on the other. Although the terms lake, lagoon and pool have been used interchangeably in the literature, and refer more or less impartially to bodies of water of some depth and slight to moderate vegetation cover, there is a useful distinction to be made between lakes, as large features of a floodplain system which persist relatively unchanged over a number of years, and lagoons and pools as more transitory open water feature. Lagoons remain connected to the river throughout the year, whereas pools are usually smaller and more ephemeral bodies of water which become isolated and have a tendency to dry out in the dry season. The term “swamp” is applied to those depression wetlands whose soil remains saturated or more or less permanently covered with shallow waters and which support, as a result, characteristic growths of vegetation which dominate the environment.

Permanent standing waters of the floodplain are generally shallow, rarely exceeding 4 m in depth and may be in communication with the river. Alternatively, their deepest point may lie below the water table of the plain enabling them to remain wet throughout the year.

Water bodies on the floodplain lose water by evaporation and to a lesser degree by filtration throughout the dry season. This results in the contraction and eventual drying out of many water bodies with a concomitent concentration of dissolved substances. In most river systems this does not produce any appreciable result, as concentrated solutions are rapidly washed out by the next flood and the average conductivity of the water remains low. However, in some regions of high salinity or one way flow, temporary brackish pools or salt pans can result. The most extreme examples of such areas are perhaps the one million hectare Plain of Reeds of the Mekong Delta, where there are extraordinarily high concentrations of alum and the Ovambo floodplain which terminates in the saline Etosha pan, (see Fig. 1.14) and the Helmand river in Afghanistan with its terminal Sistan Marshes.

Very large lakes or groups of lakes are associated with some floodplains. These are sometimes distinct entities geologically, but ecologically they are usually completely integrated into the river/floodplain system. The greatest of such lakes, the Grand Lac of the Mekong (Fig. 1.9) floods an area of 11 000 km², most of which was forested, but reduces to about 2 500 km² of open water in the dry season. It is connected to the main river by a broad channel with reversible flow, the Tonle Sap.

Figure 1.9   The delta of the Mekong river with the Grand Lac and Tonle Sap

A similar but smaller feature is found in the Senegal River valley where the Lac de Guiers is connected to the main channel by the River Tawey. The lake has an extreme dry season area of 120 km² and expands to twice this when flood waters flow into it through the Tawey in August and September. The lake district of the Niger Central Delta (Fig. 1.10) forms part of the general flood system at high water, but breaks up into 18 major lakes with a combined area of 2 400 km² at low water. The 11 000 km² Kamelondo depression floodplain of the Lualaba contains some 50 permanent and semi-permanent lakes, having a total area of 1 545 km² and of which the largest, Upemba, has a dry season area of 530 km². In the Magdalena River (Fig. 1.11) the waters are confined within some 800 lakes (cienagas) of varying size and permanence at low water. Their total area is about 3 400 km², although some individual water bodies, for example the Cienagas, Zapatosa (119 km²) and Ayapel (123 km²) are of considerable size.

Figure 1.10   The internal delta of the Niger river and its tributary the Bani, in Mali, showing the numerous lakes at the northern end of the plain

The varzea floodplain of the Amazon is similarly interspersed with lakes, some of which are very large. There are 20 lakes of over 50 km² and the Lago Grande do Curuai covers 630 km². While these resemble the floodplain lakes found in other systems a special type of water body is also found in the Amazon. The clear water tributaries to the lower and middle reaches of the river have large widened mouthbays formed originally from drowned valleys. Such “river-lakes” are the site of sedimentation at their upstream ends where new varzea type floodplains are formed from the deposits.

The individual water bodies of the floodplain usually persist over long periods during which they age, although particularly high floods can produce sudden changes through scour and deposition. The ageing process consists of a natural succession from open water lake to dry land via the intermediate stages of shallow lagoon and swamp ecosystems. Botnariuc (1967), in particular, has studied the processes involved in the transformation of the lakes of the Danube floodplain. Here the transition from lake (“Ghiol”) to reed-grown pond (“Japse”) is caused mainly by the growth of emergent vegetation which slows water currents and accelerates siltation thus allowing a further extension of the vegetated area, although the process is partially reversible when higher floods may scour deposits and vegetation from the lake bottom. In the Amazonian floodplain too, the progression from water bodies invaded by aquatic grasses, through shrub vegetation to different stages of floodplain forest, noted by Junk (1983) is frequently prevented from reaching its climax through scour by exceptional floods. Siltation can be detected in the deposition of new varzea in the mouth lakes of the Amazon, or in the change in form of lakes and channels viewed as a time series by remote sensing. Sidelooking radar, and satellite imagery show floodplains to be littered with the remains of old floodplain features now silted over. The succession may also be reconstructed from series of lakes such as those described by Green (1972) for the meander complex of the Suia Missu River in Brazil, or by Castella et al. (1984) for the relic beds (lones) of the Rhone river in France. The disappearance of older features of the floodplain in this way is compensated for by the generation of new bodies of water by the constantly changing course of the main river channel. The speed with which this occurs is related to the discharge of the river and its silt load, but it is to be supposed that, given relative constancy of these paramaters, the ratio of open water bodies on the plain to total area remains relatively unchanged through time.

Figure 1.11   The internal delta of the Magdalena river at its confluence with the Cauca and San Jorge rivers in Colombia showing the numerous floodplain lakes (cienagas)

Proportionality of floodplain features

The proportion of the floodplain which remains permanently under water is generally as difficult to establish as is the total area submerged at the peak of the floods. There is for the most part a lack of reliable maps showing floodplain features and even where they do exist the period of mapping rarely coincides with the period of minimum water. However, the use of satellite imagery presents a possible solution to this problem in areas where cloud cover remains within acceptable limits. Further complications arise from the instability of floodplain features and the changes wrought by man himself. In most populated areas the floodplain and its hydraulic regime have been considerably modified by the digging of canals, the raising of artificial levees and the levelling of depressions. Some information on relative areas is available from the best studied floodplains in Africa (Table 1.3). As flood regimes vary in the intensity of both their flood and dry season components, values for high water and low water areas fluctuate about a mean. The figures quoted in Tables 1.4 and 1.5 should be treated as such averages, being only approximations derived from relatively small-scale maps.

In Africa the area of permanent water ranges from 5 to 59 percent of the total flooded area, with a pronounced mode at between 10 and 20 percent. There is insufficient information to judge whether significant differences exist between the flood ratios of the various categories of floodplain and these may depend more on soil type or local climate. It is difficult to describe the simple ratio between flooded and permanent water areas. Indeed the extent to which the various depositional features of the plain can change from season to season makes such figures of only temporary value, except as indicators. Table 1.5 shows, for some African plains, the proportion of the permanent waters that remain on the floodplain as standing waters and those located in the divers river channels.

There are pronounced differences in this proportion and the smallness of the sample does not permit definite conclusions to be drawn on the possible causes of this. However, it might be expected that on floodplains where agriculture is intensively practised during the dry season, much of the standing water and particularly the swamps will tend to disappear through drainage and fill. This, in fact, appears to be the case for the cultivated Senegal, Oueme and Pongolo plains as compared with the Niger, Lualaba and Kafue plains, which are used primarily for cattle grazing. The main exception to this is the Shire, in which a large amount of the standing water is classified as permanent swamp. Unfortunately, the dividing line between swamp and lagoon is somewhat fine and vegetated areas of lagoons are often placed in this category.

¹   United States Operational Navigation Chart - Scale 1:1 000 000

( ) Percentage of total
N.D. No distinction made

The various floodplain features are sufficiently important in the lives of the peoples inhabiting major rivers for a particular terminology to have arisen in many languages. While by no means exhaustive, Table 1.6 lists the main names that have been used in the literature by workers from various countries to describe individual features.

Types of floodplain

It is perhaps hazardous to attempt any definitive classification of floodplains. However, three general types can be discerned whose characteristics are sufficiently different that they may influence either the behaviour of the fish populations inhabiting them, or the problems faced by a fishery.

Fringing floodplains: Nearly all tropical and sub-tropical rivers and many temperate ones have a lateral flood zone. This takes the form of a relatively narrow strip of floodable land lying between the river valley walls (Fig. 1.12). Fringing floodplains are normal developmental features of a river which follow its course in all areas where the slope is favourable. They tend to increase in width the less the slope of the river, and this generally means a progressively greater elaboration of the floodplain along the river's course to the sea.

Figure 1.12   Floodplain fringing the Niger river at the level of Gao in Mali

Internal deltas: Occasionally, river systems encounter geological features which cause them to spread laterally over very large alluvial plains. Such features may be the site of a former lake filled with alluvium, for instance the Yaeres of Lake Chad; the deltaic discharge of a small river into a larger one, such as is found where the Apure flows into the Orinoco; or the backing up of the aquatic systems by an obstruction downstream, the Kafue Flats for example. Such floodplains may occur at any point along the course of the river. The main stream usually becomes divided into anabranches which rejoin the main channel below the deltaic area, or several rivers flowing into the same plain may interconnect with complex and shifting channels. In certain parts of the world, the Bahr Aouk headwaters of the Chari system, the Gran Pantanal of the Paraguay River or the Apure-Arauca tributaries of the Orinoco, for instance, the terrain is so flat that enormous areas of land are flooded to a very shallow depth by rain-water as well as by overspill from the river. Such “sheet flooding” is often dictated by land form processes other than those created by the rivers themselves, but the network of channels and lagoons that arise by the erosion during drainage give such areas a deltaic character.

Coastal deltaic floodplains: The terminal lateral expansion of the alluvial plain and the break-down of the main river channel into distributaries produces the classic fan-shaped delta. Coastal deltaic floodplains are influenced by the marine environment in that, in the dry season, sea water penetrates the main channels as a saline tongue. Tidal effects are often transmitted far upstream even beyond the limits of the saline tongue, but little invasion by salt water occurs during the floods, and only the coastal fringe is submerged by tidal action.

Certain floodplains are intermediate between the internal delta and the coastal delta. These occur in those rivers which discharge deltaically into either inland freshwater lakes, such as the Yaeres systems located at the place of discharge of the Chari-Logone into Lake Chad, or into rivers much larger than themselves. The principal ecological differences between fringing and internal deltaic floodplains on the one hand, and coastal floodplains on the other are, firstly, the greater extent of the water body available to fish in the dry season in the coastal floodplain and, secondly, in the penetration of sea water and marine species into areas adjacent to the sea.

BRIEF REVIEW OF MAJOR RIVERS

This review is intended to highlight those features of the rivers of the world that are particularly significant to fisheries. Thus less emphasis is placed on the numerucal3ly dominant small order streams or upper torrential reaches, than on features such as the degree of modification or the elaboration of the floodplains with their associated lakes and swamps.

It is difficult to define accurately the total number and lengths of the world's rivers, although by simple extrapolation some estimate can be made. The relationship for river length as a function of basin area for the world as a whole has a coefficient of 1.7, implying that for any one square kilometre of land surface there are an average at least 1.7 km of channel. Taking the drained surface area of the world as 1.1 × 10⁸ km², a minimum length of channel of 1.8 × 10⁸ km results. This is a minimum estimate as the original equation relates only main channel length to basin area and thus omits any secondary channels.

The drainage pattern of most continents is dominated by few very large river systems, the length, basin area and mean annual discharge of the fifty largest of which are listed in Table 1.7.

AFRICA (Fig. 1.13)

Upland and torrential rivers are fairly common in Africa for, although there are few large mountain chains, the elevated nature of the mass of the continent and the existence of individual massifs means that most rivers take their source in highland regions. For example, many of the West African rivers including the Senegal and Niger, take their source in the Fouta Djallon mountains of Guinea; the Benue and Logone rivers arise in the Mondara mountains of Cameroon; the north flowing tributaries of the Zaire in the highlands of Angola; the Blue Nile in the Ethiopian highlands, etc. However, nearly all African rivers have extremely well developed fringing floodplains in their lowland courses of which perhaps the best studied are the savanna plains of the Senegal and Niger rivers. The Senegal flows through a broad valley which, during the dry season, retains about 500 km² of water confined in “sloughs” of various kinds (66 km²), as well as in the Lac de Guiers (150 km²) and the main river channel (281 km²). At peak floods the river covers 5 000 km² of valley. The Niger and its tributary the Benue river, also cover an extensive lateral plain. In the Republics of Niger, Benin and Nigeria, the Niger itself flooded 5 981 km² at high water, shrinking to about 35 percent of this area in the dry season. Much of the Nigerian plain of this river has since been lost due to the flood control effects of the Kainji dam and its reservoir. The Benue river, also in Nigeria, has a very impressive fringing plain, broad for its length which has a flooded area of 3 100 km² and a dry season area of 1 290 km².

Figure 1.13   Location of the major rivers and floodplains of Africa:

The Zambezi has two plains on its upper course which flank the river for 240 and 96 km respectively. They flood laterally for a greater distance than usual, penetrating up to 16 km² inland on either bank. the combined area of these Barotse plains is over 10 000 km², but only 5 percent of this area remains wet in the dry season.

The greatest of the African coastal deltas, that of the Nile whose floods were the basis of the wealth of pharaonic Egypt, is no longer inundated seasonally. The fringing floodplains of the river have also virtually disappeared due to the flood control measures of the series of dams barring he middle course of the river. Savanna deltas occur in the Senegal river where nearly 8 000 km² are flooded annually, and the Ouémé river, whose 1 000 km² delta terminates in the 180 km² brackish water Lac Nokoue. The Niger delta covers 36 260 km² with a coastal fringe of saline mangrove swamps.

Internal deltas are common in Africa. The largest of these, that of the Niger (Figure 1.10), occurs where sand blown from the Sahara has resulted in the deflection of the Niger river eastwards near Timbuktu. A depositional plain has grown up behind this with lakes lying in the depressions between rocky outcrops. It extends over 20 000–30 000 km² during the four to five month flood season, but its area shrinks to 4 000 km² in the dry season, most of which is retained in the permanent lakes. The Kafue river, backed up by a range of hills, which are now the site of the Kafue Gorge dam, forms an alluvial plain of over 6 600 km² which is inundated almost completely during the rains. Only 1 456 km² of permanent waters remained throughout the year prior to the closing of the dam although the duration and extent of inundation has been increased subsequently. The 7 000 km² Yaérés floodplain is the site of the deltaic discharge of the Chari and Logone rivers into Lake Chad through the Logomathia and El Beid rivers. The present lake and the floodplains of the lower Chari occupy the site of a larger Paleo-Chad, now shrunk in size by the progressive desiccation of the Sahara Desert.

The Yaérés are only part of a much larger family of flooplains centred on the Chari and Logone rivers. At least two other groups of plains subject mainly to sheet flooding by rainfall and local run-off are found in the system. The largest of these extends from the Bahr Aouk and Bahr Salamat rivers covering a considerable portion of southeast Chad and some 37 000 km² of northeastern Central African Republic. The second group spreads between the Logone and Chari rivers and eastward along the Bahr Erguig anabranch of the Chari. According to Blache (1964) the Chari/Logone basin floodplains had a combined area of 90 000 km², of which about 70 percent were inundated at peak floods (September-October) and only 7 percent remained wet in the dry months of April and May during periods of normal rainfall.

Many African flood areas are associated with permanent swamp systems. The most famous of these and the most extensive wetland in the continent is the Sudd of the river Nile. Located at the confluence of the Nile and Bahr el Ghazal, the 16 300 km² permanent complex of papyrus swamp and openwater lagoon swells to twice this area when the floods of the Nile arrive from the south and additional areas of variable extent are flooded by rainfall and local run-off. A large waterway, the Jonglei canal, is currently being excavated to bypass the swamps and make more water available downstream. The Elephant and Ndinde marshes of Malawi which cover 673 km² when flooded, shrink to 384 km² of swamps and lagoons in the dry season. They form part of the larger Shire river system which covers a total flooded area of 1 400 km² or 480 km² in the dry season. A third large swamp complex is centred around the internal delta of the Okavango river and the 800 km² Lake Ngami. These swamps have a residual area of 3 120 km² and cover between 16 000 to 20 000 km² at high water. The Kamulondo depression, one of the few true floodplain areas of the Zaire, contains lakes and swamps with a permanent water area of approximately 7 000 km², which nearly doubles during the flood season. The Rufigi/Ruaha river system of Tanzania contains three large floodplains; the 1 451 km² fringing plain of the Rufigi river itself, the 4 400 km² Usungu plain of the Ruaha river which includes the 518 km² Utungele swamp, and the 6 736 km² floodplain of the Kilombero tributary to the upper Rufigi which contains the 89 km² Kibasira swamp. In common with most Africa wetlands the extent of these large plains varies considerably according to the medium term climatic regime. The sahelian drought of the 1970's and 1980's reduced the area of the plains of the Niger, Senegal and Chari/Logone systems whereas the increased pluviosity of the 1960's in the Central Africa plateau resulted in an enlarged Sudd. Indeed it is now suspected that the environment effects of such natural variation exceed the results of such human interventions as the Jonglei canal.

Two smaller African plains, resemble in some ways the cienaga floodplains of Latin America. The Kifakula plain, formed by the deposition of alluvium by the Luapula river between the Johnson Falls and Lake Mweru, covers 1 500 km² and is scattered with permanent lagoons. The Pongolo floodplain is even smaller (130 km²), but is of interest in that its upstream end is blocked by the Pongolapoort dam, which has permitted experiments on the discharge requirements for the maintenance of fisheries in small floodplains.

True forested floodplains similar to the Amazonian Igapó and várzea forests are more or less confined to the Zaire basin and a few smaller river basins in Cameroon and Gabon. The course of the Zaire below Kisangani, broadens to include an increasing floodable area. which culminates in the Bangala swamps, and the vast complex of flood-lands at the confluence of the Zaire, Ubangui and Sangha rivers. These areas are subject to bi-modal flooding and tend to retain their water to a great extent and therefore differ very considerably from the highly seasonal type of plain. The true extent of this area is difficult to assess and it is almost completely unpopulated.

An unusual type of flood wetland is formed by the overspill from the Cunene river in South Angola. In the wet season this river discharges a considerable part of its flow southwards where it is contained within a graben. As it flows to the south through numerous channels and pools the evaporation raises the salt content until finally the system becomes dry at the Etosha pan. Some 10 000 km² of this Ovambo floodplain consist of highly saline soils and water but conditions are less saline towards the North where some 20 000 km² of waterways and floodable land are retained in a humid condition by ground water seepage.

AMERICA (NORTH) (Fig. 1.14)

Many of the northern rivers of the continent consist of relatively short channels connecting the large numbers of lakes of the Northern forest and tundra. The land is mainly flat with heavy flooding of swampy areas during the spring when melting snow forms the major part of the run-off. Because of the severe winters, streams and swamps freeze for up to six months of the year and the floodplain of these are of the tundra type. Several large rivers drain the area, including the Mackenzie and the Yukon. The Peace and Athabasca rivers together formed a 2 560 km² complex of flood lakes and swamps at their entry to Lake Athabasca (Blench, 1972) but this environment has now been superceded by a series of isolated mud flats following the closure of a dam on the Peace river. Within the United States most rivers are highly modified by various flood control works. For instance, the Colorado is now a cascade of reservoirs and abstraction of water from the system for irrigation is on such scale that there is little outflow to the sea. Similarly the rivers of the Mississippi - Missouri basin, which drain most of the United States have also been extensively altered, the Missouri and Mississippi mainstems having been converted into a series of pooled reaches. While many of the rivers draining the Rockies and the Coastal ranges are turbulent and encased in steep gorges, most streams are associated with floodplains and the 100 year flood area of the US is estimated by Sabol (1974) at 543 000 km² or 6 percent of the total land area of the Nation. Most of Mexico is arid and few large rivers drain the area. However the Grijalva, Usumacina, San Antonio and San Pablo rivers of Tabasco state combine near their mouths into a series of anastomosing channels running through a broad flood zone which covers an area of about 8 000 km² at maximum floods. The plain extends up to 150 km inland and abundant floodplain water bodies are located along the main water courses. The other rivers of Central America are short, often steep and swift flowing streams associated with the mountainous terrain.

AMERICA (SOUTH) (Fig. 1.15)

The Chilean, Peruvian, Equadorian and Colombian rivers draining the Andes westwards to the Pacific are all short and torrential. So too are the upper reaches of the Northflowing Atrato, Magdalena and Sinu rivers of Colombia, and the headwaters of those tributaries which drain the Andes easwards to the Orinoco, Amazon and Parana systems. The Marañon and Ucayali tributaries to the Amazon particularly have long courses in the mountains. Other upland rivers are found in the various plateaux highlands of the continent but on the whole Latin America is relatively flat and most rivers are of the lowland type with extensive plains which are flooded by local rainfall as well as by river overspill.

Figure 1.14   Location of the major rivers and floodplains of North America:

Several morphologically similar floodplains clustered around the Carribean consist of internal or coastal deltas which contain enormous numbers of permanent or semi-permanent lakes, locally termed ‘cienagas’. The most important of these plains is that of the Magdalena river (Fig. 1.11), whose internal delta with the San Jorge and Cauca rivers extends over 20 000 km² of savanna. Most of the plain is well inland but extends seaward along the Canal del Dique on one hand and down the main river channel to the coastal Cienaga Grande on the other. The whole area of 20 000 km² can flood for up to a month, 16 000 km² for between 1 to 3 months, 13 000 km² for 3 to 6 months, and some 4 000 km² for periods varying between 6 to 8 months. When the floods recede completely about 800 cienagas, with an area of 3 260 km² remain. The Atrato river, also in Colombia, extends over a forested alluvial plain for the last 100 km of its course. The plain, which covers 5 300 km² is interspersed with numerous cienagas. Of similar size is the 5 000 km² of savanna flooded by the Catatumbo river as it flows into Lake Maracaibo.

Several vast areas subject to sheet flooding are found on the continent. The greatest of these is probably the Gran Pantanal of the Paraguay river whose shallow interconnecting complex of lakes extends over 80–100 000 km² at peak floods. The savanna “llanos” of Colombia and Venezuela are also subjected annually to shallow sheet flooding. This area is drained by the Meta, Arauca, Capanaparo and Apure rivers which combine into a more deeply inundated deltaic floodplain of over 70 000 km² at their confluence with the Orinoco. The Rupununi river annually floods a very variable area of savanna to a depth of 1 to 2 m. In exceptionally wet years the flooded area extends from the headwaters of the Essequibo river to the Tokutu and Ireng rivers, which flow via the Rio Branco into the Amazon. These sheet flooded plains have in common the shallowness and extensiveness of their inundated area, their numerous anastomosing channels and small temporary lagoons.

Figure 1.15   Location of the major rivers and floodplains of South America:

The Amazon river and its major tributaries have densely forested floodplains for much of their length. However, the main river channels are fringed by, and enclose, vast alluvial plains up to 50 km wide in the upper reaches and 100 km wide further downstream. The Amazonian flood zone reaches its greatest extent in the central delta located between the conflence of the Amazon and Tapajos. The heart of this area, the Ilha Tupinamborana covers about 50 000 km². A similar combination of fringing floodplain and internal delta is found along the Paraguay river below the Gran Pantanal, and along the Paraná river after its confluence with the Paraguay.

Coastal deltaic floodplains are uncommon in South America, although both the Orinoco and the Amazon have densely forested terminal flood areas of 20 000 and 25 000 km² respectively.

ASIA (Fig. 1.16)

Asia is by far the largest and most geographically diverse land mass and its rivers cover a correspondingly large number of types. The North flowing rivers of the Soviet Union, the Lena, the Ob and the Yenisei, form a set of very large rivers which, despite having rapids in their upper reaches, flow for most of their courses through plains which at their Northern extent lie within the permafrost zone. These lower reaches remain frozen for 6–7 months of the year, and this produces a damming effect where the still frozen lower reaches block waters released by the earlier break-up of the ice and the snow melt in the upper reaches to cause extensive flooding. These floodplains, which may be termed “tundra Floodplains”, differ from the temperate and tropical floodplains in that their water bodies freeze completely for a considerable period of the year, and during the summer they retain a marshy character.

The Tigris-Euphrates river system is isolated from the rest of Asia by the arid tracts of Iran and Afghanistan. The upper courses of both rivers are torrential arising in the mountainous regions of Turkey and Syria, but an extensive floodplain is present in the mesopotamic region between them. Here despite several thousand years of irrigation, associated with some of the most antique civilizations, which have left many traces, flooding still continues over the 20,000 km² at high water and the river channels are then confluent with the marshes. At low water, the lake area is reduced to about 5 000 km². The largest lake in the system, the 5,200 km² L. Hammar, serves as a terminal drain (Al Hamed, 1966).

Figure 1.16   Location of the major rivers and floodplains of Asia:

The hydrography of most of Eastern and South Eastern Asia is dominated by the central mountain massif of the Himalayas and the Tibetan plateau. Most of the great rivers of China, India, and the South East Asian Penninsula take their source in these highlands to flow out through a series of coastal plains from China to Pakistan. In China, the Hwang Ho, Yangtze and Si rivers, drain the plateau eastwards and flow across vast plains which were once liable to widespread and repetitive flooding. The Yangtze in particular, is associated with an extensive complex of flood lakes. These systems are now highly controlled by dams and other flood control devices so inundations are less common. The Red, Mekong, Salween, Brahnaputra, Ganges and Indus rivers all rise on the plateau and have very long torrential upper courses before debouching onto extensive lowland floodplains. The Indus river, the site of very early civilization, has a wide braided channel for much of its course. This periodically shifts its bed due to excessive silt deposition raising the river channel and its levees high above the level of the surrounding plain. When it overspills its banks the course of the Indus may be displaced laterally for up to 40 km, losing water by seapage and evaporation in the otherwise desrtic region. The river flows into the sea through a large but infertile deltaic plain. In the virtually rainless areas of Pakistan, the Indus and its floods are essential for irrigated agriculture. Both the Ganges and the Brahmaputra rivers also have unstable beds flowing through braided channels. The main fringing floodplain of the Ganges is over 600 km long and between 16 and 80 km wide. The Brahmaputra has an especially broad channel, up to 12 km in places. The two rivers combine to form an immense delta liable to sheet flooding from rainfall as well as river discharge. The types of floodplain, classified by the duration and depth of inundation of the portion of this plain lying in Bangladesh, are shown in Fig. 1.17. In fact most of the active part of this delta lies in this country as the western region, centred on the Hooghly river in India, is silted and rarely floods through the now dead distributaries. In Bangladesh alone, there are 93 000 km² of floodable land including 28 340 km² of paddy fields which are inundated for 3–4 months of the year. In addition, there are an estimated 14,000 km² of permanent open inland waters. The seaward parts of the plain have dense brackishwater mangrove forests know as “Sunderbuns” some 5 000 km² in area.

The 31 000 km² deltaic alluvial plain of the Irawaddy river is flooded both by rainfall and upper river discharge. Submergence by local precipitation precedes the arrival of the river floods by at least a month. Artificial levées constructed for flood control will eventually change the whole pattern of flooding. A similar but smaller deltaic plain flanks the Chao Phrya river and its distributaries in Thailand (Fig. 1.6). Here the main river channels are raised high above the surrounding backswamps which are deeply flooded each year, although recently the construction of an upstream dam seeks to control the timing and extent of the inundation. Another plain that is fast being altered by flood control measures, irrigation and an ambitious programme of upstream dam construction is that of the Mekong (Fig. 1.9) which is over 74 000 km² in total area. About 21 000 km² of its area are now no longer flooded although 8 850 km of river, canals and irrigation channels take water to every part of the plain. The area of permanent water is only 4 000 km², of which the majority is in the 2 500 km² Grand Lac. In fact this body of water is one of the regions most deeply affected by the modification of the hydraulic regime of the river as its floods have been drastically curtailed.

The peninsular rivers of the Indian sub-continent, the Godavari, Krishna and Cauvery, once had extensive floodplains as did the Mahaweli system in Sri Lanka. These systems have now been modified by dams for power generation and irrigation.

Marshy flooded areas are common as the Indonesia island of Sumatra, Borneo and West Irian, and as some of the Philippine islands. Fundamentally similar in nature these are located on the flat coastal alluvial plains and are submerged and drained by numerous small rivers. In Sumatra, there are the eastward flowing, Hari, Kampari, Rokan and Musi rivers; in Borneo the Southward flowing Kahajan, Barito and Mendawa rivers and in New Guinea, the Fly, Diguil and Pulan rivers. The most complete description of the fish and fisheries of such coastal flood rivers is that of Vaas et al. (1953) who make it clear that, on the island of Sumatra at least, such areas are classic floodplains. In the Oga and Komering rivers, which cover over 500 000 km at peak flood, the lateral plains of “Lebaks” are used for both rice culture and fisheries.

Figure 1.17   The delta of the Ganges and Brahmaputra rivers in Bangladesh showing the major land types classified by depth and duration of flooding

More typical seasonal floodplains, such as the 6 555 km² Kapuas lake district of West Borneo, the 7 000 km² lake district of the Mahakan River in East Borneo (Dunn and Otte, 1982) or the 7 500 km² lateral plains of the Sepik River in Papua New Guinea, are also common on the larger islands.

EUROPE (Fig. 1.18)

Western Europe is drained by a number of relatively short streams although the Danube, Rhine, Rhone and Vistula rivers have discharges sufficient to place them among the first 50 in the world. In eastern Europe a number of larger rivers drain into the Black, Azov and Caspian seas including the Dniester, Dnieper, Don and Ural. Torrential streams are present in the Alps, Pyrenees, Carpathian and Caucasus Mountains, but on the whole European rivers ar of the lowland type, particularly in the East where the drainage is dominated by relatively few large rivers. On the whole European rivers have been highly modified by canalization and dam building so that few retain much trace of their original floodplains. For example, the largest river in Europe, the Volga, has been converted into a cascade of reservoirs and only a very short terminal reach retains its original form, although even here the flow from upstream is modified. The Danube and Ural are perhaps the only major rivers in which some original characters survive (Liepolt 1967). The total area of the Danube floodplains was given as 264 500 km² by Liepolt (1972), but recent damming, flood control and land reclamation schemes have only left some 5 000 km² as liable to inundation in Romania. In the Czechoslovak-Hungarian reach of the river 230 km² are regularly flooded from May to August and 30 km² remain as permanent water in side arms and 86 km² in the main river channel. However, even these will disappear when present plans for the hydraulic management of the system are completed.

Figure 1.18   Location of the major rivers and floodplains of Europe:

AUSTRALIA AND NEW ZEALAND (Fig. 1.19)

The hydrology of Australia is dominated by the aridity of its Central regions. Thus the Murray Darling system which drains most of the inland South-Eastern Australia with a basin area of 1 029 000 km² has a remarkably small total annual discharge of 22 km² or 6% of the total discharge for the continent. Its regime is very variable with long periods of low flow and occasional massive floods. During its wetter phases the system is associated with a lowland marsh system which takes the form of an internal delta with some permanent lakes, e.g., Lakes Meninolee and Tandou, although much of the area may reduce considerably in size at times of drought. In contrast to this south flowing river system there are several north flowing rivers which drain the more humid tropical areas of Northern Australia and Queensland. These carry about 40% of the total discharge by Australian rivers and have regular monsoonal regimes with annual floods which inundate the lowland plains associated with the various rivers. New Zealand's rivers are mostly short and torrential particularly in the mountainous South Island where they are often associated with deep narrow glacial lakes.

Figure 1.19   Location of the major rivers and floodplains of the East Indies and Australia: