Rivers are linear systems which show a gradient of characters along their length. Ideally the longitudinal profile of a river is concave with a steep upper portion near the source, giving way to reaches of progressively less gradient as the mouth is approached. Other features of the river are associated with this progression. The bed material becomes finer the shallower the gradient, and because of the increasing amount of water carried by the river channel this usually becomes wider from source to mouth. Many systems of classification based on the physical characteristics of the river channel and the biological composition of the associated fauna have been proposed to describe the various reaches of the system, some of which have been summarized by Illies and Botosaneanu (1963). Such systems may recognize several different sub-divisions of the river course, but usually make a primary distinction between the steep and torrential upper course (or “rhithron”) and the flat, slow-flowing lower course (or “potamon”). There are, however, exceptions to this generalized succession. Lowland rivers often show little variation in character along their lengths and because the rhithron as originally defined should have water temperatures below 20°C, a true rhithron is frequently absent in the tropics. By contrast, temperate rivers are characterized by relatively lengthy rhithron zones. Although the needs of specialized fishery management may call for fine sub-divisions of the river zones, the division of river reaches into two basic types is probably sufficient for more general purposes. These correspond to the rhithron and potamon and have the following characteristics (see Fig. 1):
(a) Rhithron-like zones which tend to show an alternation between (i) steep, narrow and shallow riffles or rapids, and (ii) flatter, wider and deeper reaches, termed pools. Riffles have high, turbulent flow, coarse bottoms of boulders, rocks or pebbles and limited attached vegetation. Pools have lower flow, bottoms of somewhat finer material and some rooted vegetation.
(b) Potamon reaches, with wide, flat, meandering channels, mud bottoms and considerable rooted and floating vegetation. Zonation within the potamon is both longitudinal and lateral. Longitudinally, there is a repetition of differing habitats associated with the meanders of the channel. Laterally, there is the distinction between the main channel and its floodplain. The floodplain is normally an area of relatively flat land flanking the main channel. In exceptional cases larger floodplain areas arise by geographic accident and some of these such as the Central Delta of the Niger or the Gran Pantanal of the Paraguay River are very extensive. The plain is usually higher near the river, where raised levees limit the main channel, and slopes downward toward the foot of the terrace confining the plain. Many bodies of water are found on the plain ranging from small temporary pools to large permanent lagoons and swamps. Detailed descriptions of the physical and chemical (or geomorphic) processes determining river form are presented in Leopold, Wolman and Miller (1964).
Because of the geomorphic processes governing river form, river systems in any one climatic zone tend to resemble each other and, in fact, many features are universal. 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 and any river system should ultimately be viewed as a continuum showing an evolution of characters along its length. Considerable modifications have been carried out in many river systems, particularly in the temperate zone where there are few large rivers which now show all their original features. Nevertheless, features of the geography of any particular river basin can impose certain characteristics on the river. Examples of such include classification of tropical rivers into two different types according to their flood regime. 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 permanent flow; and (ii) sandbank rivers, where there are extremes of annual fluctuation in water level from severe flood to complete desiccation in the dry season. 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 reservoir 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 sandbank 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. Mixed systems also occur, and larger rivers especially may change their nature several times during their length. 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.
Fig. 1 Main characteristics of a river showing A-A Rhithron-like zone, and B-B Potamon zone
A different approach to the classification of stream types arises from the branching pattern of the river channels in any drainage basin. These have been categorized according to the order of streams in a hierarchy which is defined as follows: first-order streams are those having no tributaries, second-order streams are formed by the junction of first-order streams; third-order streams are formed by the junction 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. 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:
Ns = a.bs,
where Ns = Number of streams of any order and s = order.
The length of streams of any order (Ls) decreases with decreasing order(s) in a similar manner:
Ls = x.ys.
The factors for a and b, x and y will vary according to continent or climatic zone. These equations imply that the majority of the channel length in any river basin is located in a very large number of small lower order tributaries.
One of the most important factors determining the distribution of living forms in fluvial systems is the rate of flow. This, in turn, influences a number of physical or chemical factors such as dissolved oxygen concentration or temperature, which act directly on the fish. As described above, rivers may be separated into two main categories, those in which flow is reasonably constant throughout the year and those in which the amount of water in the system varies seasonally. Rivers of the first category are found naturally in certain well watered parts of the temperate zone, in some tropical highlands and in the heavily forested equatorial tropics. The number of such rivers has grown because human activities for river control usually result in an evening out of the flow throughout the year. In this way, many rivers in the temperate zone and an increasing number in the tropics have much more regular flows now than in their previous undisturbed state.
Fig. 2 Alternative systems for ordering river systems as applied to a diagramatic river
Rivers in which seasonal variations in flow are produced by changes in rainfall during the year are however still in the majority. The type of flow regime depends on the area of the drainage basin of the stream concerned. As illustrated in Fig. 3, lower order streams with small basins have regimes which consist of a series of spates, that occur during the rainy season or seasons, which give a characteristically spiky graph when flow is plotted against time. As basin area increases the extreme variations in flow from the smaller component basins are averaged out to give progressively smoother flood curves.
The figure also indicates another feature, that the flood curve moves down river at a finite speed. This means that, in long river systems, the peak flood may arrive long after the rainy season, or that flood regimes in the lower reaches of the river may be complicated by the arrival of floods from two or more large tributaries whose flood peaks differ in time. In rivers with reasonably constant flow, there are few changes in physical features throughout the year, and in such rivers any seasonality arises from climatic variables other than precipitation, such as temperature. By contrast, in those rivers which have a regular annual succession of high and low waters there are corresponding changes in the form of the aquatic system and in the types of habitats available to the living organisms. In the steep, rhithron-like reaches of the river, the distinction between pool and riffle may be lost during the flood. However, as flow falls, the separation between the two zones increases until, during the dry season, the riffles may dry out leaving a series of disconnected pools. In the potamon the changes in the aquatic system are more complex. During the dry season the water is confined within the main river channels and in permanent water bodies of the floodplain. In extreme cases, the main channel may itself break down into a series of pools. With the onset of the local rains, the floodplain becomes saturated and floodplain depressions begin to fill with rain water. When the flood arrives from upstream the water begins to rise in the river channel and eventually spills over through a system of channels to submerge the plain. During the falling flood the waters recede from the plain to occupy the river channels once more. In so doing, the still flooded depressions become isolated and many of these slowly dry out during the dry season. Such a cycle whereby the aquatic system expands and contracts may occur once or twice per year.
The physical and chemical characteristics of any aquatic system, acting together, determine the nature of the aquatic organisms inhabiting it. The characteristics themselves originate from the interplay between land form and climate within the basin, and because such factors as discharge, flow-rate, channel width or silt load are linked by simple relationships, a relatively small number of ecological groupings emerge which have formed the basis for systematic study.
Two aspects of the ecology of river systems are particularly important, as they provide the principal framework into which other considerations fit: longitudinal distribution within the system, or zonation in space, and seasonality, which corresponds to zonation in time.
Most unmodified rivers have sufficient variation in flow during the year to influence the behaviour of the living organisms. However, there are a considerable number of rivers in which flow varies little throughout the year, and their numbers are being added to as further systems come under control. In such systems the resident living aquatic communities remain relatively stable, although periodic influxes of visiting species might occur in response to changing conditions elsewhere in the system. Seasonality in such rivers arises from climatic variables other than flow, the most important of which is undoubtedly temperature, although day length may also play a significant role. Temperature is largely dependent on latitude with an annual variability that increases with increasing distance from the Equator. The significance of temperature as a determining factor in seasonality, therefore, tends to increase with higher latitudes, although in most systems the favourable temperatures of spring and early summer coincide with the flood season. In the systems in which this does not occur, there is an interplay between flow and temperature as dominant determining factors in seasonality, which seems complex and has not been fully studied to date.
The generalized distinction made on the basis of morphology between rhithron and potamon reaches of rivers extends to the living aquatic communities. There are, however, many species, particularly fish, which ply between the two types of reach and must be considered inhabitants of the river system as a whole.
Fig. 3 Flood regimes of rivers of different basin areas within the Chari-Logone River system showing increasing smoothing of the flood curve as the size of the basin increases. The figure also shows the time taken for the flood crest to travel downstream, indicated by arrow.
The rhithron is characterized by turbulent flow and relatively low temperatures. Generally, the water is highly oxygenated, but at low water the pool and riffle system may break up into a series of pools, whose waters may become completely depleted of oxygen. During periods of spate there is no phyto- or zoo-plankton, although during low water transient blooms may occur as flow drops and pools become isolated. Higher vegetation is usually restricted to some resistant forms attached to rocks in the riffles, and to rooted, floating leaved or emergent forms in the pools, especially in the more sheltered slack waters. The main micro-flora and fauna occurs as mats of “aufwuchs” covering the bottom substrate. There are also many insect species adapted to life on the bottom where they shelter among or cling to the rocks. Elements of this benthic fauna become detached and together with any allochthonous matter which may drop onto the water from the surrounding forest or grassland, form a drift which constitutes the basis of one of the principal food chains. Living organisms in this type of habitat must be adapted to resist current, but during the flood phase are not unduly troubled by lack of oxygen. For this reason, when the water level in the stream drops, the riffles begin to desiccate and the dissolved oxygen tensions decline, many forms do not survive. Further adaptations are present, therefore, for recolonization when more favourable conditions are re-established.
The resident fish species in rhithron zones are entirely rheophilic and fall into two main groups. Firstly, there are those species which live on or among the rocks and vegetation of the bottom and are distributed mainly in the riffles. These are of small size and are adapted to grip or cling to the substrate. Such adaptations include mouth suckers, for instance Chiloglanis, ventral friction pads as in Astroblephus or pectoral fin spines adapted as hooks as in Glyptothorax. Other species such as Mastacembelus or Clariallabes have long sinuous shapes that enable them to twine among the holes in the rocky bottom. Secondly, there are those species such as Barbus or Salmo which are adapted to swim sufficiently fast as to resist the current and even move against it. This they cannot do on a sustained basis, however, and frequently take advantage of cover provided by the slack water of the pools and by snags, overhangs and other features which disrupt the current. Because of the severity of the habitat diversity of resident species tends to be low.
The rhithron zones of temperate rivers have been particularly well studied because of the economic and social importance of the sports fisheries for salmonids they support. Although estimate of production and biomass are not wholly reliable, observations in Europe and North America indicate average total community production rates of about 330±215 kg/ha/yr, production/biomass ratios of about 1.6±0.6 and standing stocks of about 200 kg/ha. Equivalent figures for production and biomass of tropical rhithron-like reaches are not available but, because of the small size of the species inhabiting them and the seasonal nature of such streams are probably not greatly in excess of the values for temperate waters. The relatively low production from these types of lower order stream, combined with their small individual area and spatial dispersion means that they are not suited for large-scale fishing activities.
The potamon is environmentally more complex than the rhithron. There is usually a well defined series of river channels flanked by a floodplain. Both running (lotic) and still (lentic) waters may be present. The main river channel, which may divide and recombine to form anabranches, generally consists of a regular succession of meander bends. The channel at each bend is deeper by the outer, concave bank, where the current is fastest, whereas the inner, convex bank consists of a sandy or muddy point bar. At low water, areas of slack current form downstream of the point bars, but during high water these features are submerged. Floating and emergent vegetation usually lines the river banks and floating leaved and emergent vegetation may appear in the slacks below the point bars.
The plain itself contains many types of water body, some of which retain water throughout the inter-flood period. Because of deposition of silt, such features show a succession from open lagoon, through vegetation-lined pools and heavily vegetated swamps to dry land. As old water bodies disappear in this way, new ones are formed by the erosive action of the river flow and scour by the flood waters. During the floods the rising water invades the plain and as this happens organic and inorganic matter lying on the plain enters solution. As a result, conductivity rises and dissolved oxygen concentrations fall in the newly submerged areas, but as the flood persists conductivity falls and dissolved oxygen concentrations rise again.
In the water bodies of the floodplain dissolved oxygen concentrations fall in the dry season, particularly in the smaller pools which may become completely depleted of oxygen. At the same time, the shrinkage in the volume of the water because of evaporation causes both temperature and conductivity to rise. The river channels remain relatively cool and well oxygenated, providing flow persists, but if this should stop the channel breaks down into a chain of pools which behave in a similar manner to the floodplain water bodies.
The occurrence of phytoplankton and zooplankton is closely related to the flow conditions. Thus, during the floods planktonic organisms may be present but are rare, whereas during the dry season blooms form within the standing waters of the plain and also in the river channel. In short rivers these are generally confined to sheltered areas and back waters. In longer rivers the time taken for individual masses of water to travel downstream is sufficient to allow the development of phyto- and zoo-plankton. In rivers whose flow has been slowed by other hydraulic works, plankton also develops to a greater degree. The contribution of the plankton to the primary production is nevertheless slight and the main feature of the floodplain is the rapid proliferation of floating masses of higher vegetation during the floods. This consists mostly of grasses which die back and rot or are burnt in the dry season. Emergent, submersed and free-floating plants are also common in the more sheltered waters throughout the year. The submersed root masses, stems and leaves of the higher vegetation become covered with a complex plant and animal community and another important community lives at the surface in the sheltered waters between the plants. The benthos of the potamon is relatively poor as unstable mud bottoms, heavy siltation and seasonal desiccation do not favour settled bottom-living organisms. Nevertheless, some benthic life does exist, especially in the permanent standing waters, and during the floods ephemeroptera, chironomids and molluscs invade the plain.
Two main factors influence the behaviour of fish communities in the potamon zone, especially in those rivers having extensive flood plains. Firstly, the general upsurge in productivity and the growth of large areas of higher vegetation on the flood plain during the flood provides favourable breeding, feeding and nursery areas for most species of fish. Secondly, there is the increasing harshness of the environment of the water bodies of the flood plain, and often in the river channels themselves during the dry season. Because of this, most fish species in floodplain rivers show a seasonality of behaviour whereby they breed early in the floods, feed and grow on the flood plain and with falling waters retreat to a dry season habitat in which they can weather the severity of the low-water period. So marked is this phenomenon that the most of the growth in any one year is accomplished during the flood period.
There are two main adaptations which enable fish to survive the conditions during low waters. There is a group of species which is specifically adapted to resisting low dissolved oxygen concentration. The adaptations may be in the form of auxiliary respiratory organs for using atmospheric oxygen as in the case of such fishes as Clarias, Erythrinus or Notopterus, or may be physiological as with Carassius or even behavioural as with many cyprinodonts. The same species often have a capacity to support high temperatures. They generally have complex breeding habits with multiple spawnings, a great degree of parental care, and only migrate laterally between the dry season habitat in the main river channel or the standing waters of the flood plain and the flood season habitat in the inundated area. The other group of fish uses the rich habitat provided by the flood plain during the flood but escapes the severe dry season conditions by lateral movement off the plain and longitudinal migration within the main river channel to an alternative habitat. This is usually located in the deeper regions of the main river channel, but may also be in the sea or some other large water body adjacent to the river system. A certain proportion of these species move upriver, even as far as the rhithron zone. Such fishes show few adaptations other than a capacity for fast and sustained swimming. Their breeding strategy is generally simple, relying on a single release of a large number of eggs, either on the flood plain or in the headwater streams. To accomplish this they may undertake migrations for very long distances up-and down-river, which in exceptional cases such as Prochilodus in the Parana River or Pangasius in the Mekong, may exceed 1 000 km.
Because of the size of the system concerned, standing stocks and biological fish production from the potamon reaches of rivers are difficult to calculate. One estimate made in a controlled temperate river, the Thames, gave values as high as 2 000 kg/km²/yr (Mann, 1971), but there is an absence of comparable data for tropical rivers. A further factor which makes estimation of ecological parameters such as ichthyomass and productivity difficult is the flood itself. Obviously, in rhithron reaches, or in modified potamon reaches where there is little lateral expansion of water area, figures expressed in terms of unit area are meaningful. In potamon areas with a seasonal pattern of flooding the area covered by the aquatic system expands and contracts so that per-unit-area figures become relative to the state of flooding of the system. The sum of biological, physical and chemical factors acting on the fish community results in curves for total ichthyomass within a system of the form shown in Fig. 4. Here ichthyomass is minimal just before the flood and reproduction, recruitment and growth during the rising flood leads to a peak in ichthyomass at some point before bankfull in the retreating flood. When transposed to ichthyomass per unit area, even greater variations are observable with a minimum during the rising flood and a maximum at bankfull on the retreating flood. Because of this, figures to be comparable between systems must be expressed in terms of a similar reference area and for this the area at peak flood is perhaps the most appropriate.
Fig. 4 Computer simulation curves representing: