Because water moves throughout a drainage basin either as surface or sub-surface flow to eventually form the network of channels which is the river system, any changes in land or water use within the basin are rapidly reflected in the river. Detailed studies of the processes governing river form, such as those described in Leopold, Wolman and Miller (1964) have shown that there are simple relationships between flow and such factors as channel width and depth or suspended silt load. Which means that any action within a river basin which alters any of these parameters will produce corresponding changes in the morphology of the river. These changes, which are summarized in Fig. 14, in turn affect the aquatic organisms living in the system, sometimes to the advantage of fisheries, in that yields are increased or that certain commercially important species are favoured, but more often to their disadvantage. The most usual changes are in overall water quantity (mean annual flow), in the timing and form of seasonal patterns of discharge and in silt load.
Rivers respond to changes in mean annual flow by a period of adjustment to the new regime after which they stabilize in a form adapted to the altered conditions. Clearly reduction in flow will result in the progressive restriction of the stream to a smaller bed within the original channel with a concomitant loss in habitats for fish and other aquatic organisms. Conversely, overall increases in flow will lead to the enlargement of the river channel by erosion of former banks and other features; these, in time, should lead to an increase in habitat area through the extension of the aquatic system. As well as such changes in overall water quantity, situations arise where the quantity remains the same but the distribution of flow in time is altered. Here, flood peaks may be moved to a different time of year or even be suppressed altogether.
Living aquatic organisms in rivers are usually adapted to the particular pattern of flow found there, consequently changes in flow patterns will produce changes in the biotic component of the ecosystem quite apart from those arising from simple extension or contraction of the aquatic habitat. Fish communities tend to be rheophilic or limnophilic, depending on the type of water regime prevalent in the river's reaches in which they live. Changes in flow will, therefore, tend to favour one or other of these types of community, increasing flows leading to an increasing number of rheophilic species, decreasing flows encouraging colonization by limnophilic species. Adequate flows are also essential for the breeding and migration of many species of fish. Physiologically, fish respond to flood conditions by becoming sexually ripe and by movement to breeding grounds. Also certain flows are needed to maintain such grounds in a suitable condition for spawning and survival of young. For this reason, there has been much concern over determining minimum flow requirements for fish, particularly in areas where high value angling resources such as salmonids coincide with high demands for water. An example of the output from such research is the “Montana method” described by Tennant (1976) (Table 2), whereby the effects of various flow regimes relative to the original regime are described.
The figures in Table 2 have been found to describe the effects of flow reductions on a wide range of North American rivers. Similar series have possible applications elsewhere in the world but, unfortunately, there are insufficient data in the tropical zone to use as a basis for comparison.
In potamon reaches, the problems of maintenance of flow becomes especially critical as the yield of systems with floodplains is closely linked with the extent of flooding. Thus, if at any time the floodplain fails to be inundated recruitment to the stock fails for that year in all those species which spawn on the plain. Such a process may occur naturally in most systems where exceptionally dry years occur as part of the normal year-to-year variability. In recent years an especially prolonged drought in the Sahelian region of Africa has shown how the fish communities of rivers react under such conditions. However, these natural changes are rarely long-term whereas the alterations imposed by man are more or less permanent.
Even if the average flows within the system are unaltered, changes in the timing or the form of the flood may have far-reaching consequences. In many fish species breeding success depends on the coincidence of a range of factors of which flow is but one. Consequently, races or strains of species have become adapted to a particular timing in their breeding and displacement of the flood to the wrong period of the year does not permit them to reproduce successfully. Equally, abrupt changes in flow characteristics can influence breeding success adversely. Overly rapid rises and falls in water level can leave nests or spawning grounds dry at critical periods or can result in eggs and fry being washed away. The precipitous decline of the flood can result in fish being trapped in temporary water bodies for lack of time to find passage to the main channel of the river.
Fig. 14 Diagram of relationship of various factors influencing fish communities
Water has a capacity to transport particles the size and quantity of which are determined by the rate of flow; the faster the flow the larger the particle size supported and the greater the amount of sediment transported. When the particle load is excessive for any level of flow, particles are deposited, causing siltation. Conversely, if the load is less than the transport capacity of the water, particles will be picked up, causing erosion or scour. Siltation is a natural feature along the length of rivers and normally results in a gradation of particle size from lower order streams with the coarsest material to higher order streams with the finest. This natural sedimentation contributes to the development of many of the morphological features of rivers, including levees, point bars, meander bends and, particularly, floodplains. These latter arise from the deposition of sediment as water spreads out laterally from the main channel of the river slowing its flow in the meantime. However, natural sedimentation is counterbalanced by erosion, which leads to an equilibrium state for any particular flood regime. Within this equilibrium, there is a succession or ageing of individual floodplain features, whereby old lagoons and channels gradually fill with silt and vegetation to be replaced by new ones produced by erosion and scour. Siltation is closely related with the occurrence of emergent vegetation. Depositional loci become colonized by higher plants, which further slow the flow, encouraging faster siltation.
Erosion arises where the flow is locally accelerated, for instance, on the outer curve of meander bends or the outer edge of the flood plain adjacent to the terrace wall. Equally, sediment is picked up by water from which sediment has been deposited, for instance, below dams.
Increases in silt load resulting from changes in land or water use accelerate the natural evolution of the river system, but in so doing cause a number of problems. In rhithron reaches the deposition of fine particles of silt on what is normally a coarse substrate suffocates the rheophilic organisms that normally inhabit such reaches, cutting down on the availability of food. It also renders the substrate unsuitable for use as a spawning ground by those species requiring swift well aerated flows and clear pebble or boulder bottoms. The silt provides an anchorage for vegetation, blocking low order streams and even diverting them into new courses. Further down the river deposition of silt on levees and on the river bottom may lead to the progressive elevation of the whole channel until it stands above the level of the surrounding plain. An extreme flood in a channel in this state may cause the levee to be breached and the river to jump its bed to find a new channel, changing the river course often by some kilometres. Excessive siltation on flood plains chokes the standing waters, which disappear faster than new ones are generated by erosion. Similarly, channels and dead arms of the river are filled and new channels are cut to such an extent that the whole delta of a river may shift along the coastline. At the same time, coastal deltaic flood-plains grow rapidly, especially at their seaward end, where new land continuously appears.
Changes in water quality result from the loading of water with a range of organic and inorganic substances. Addition of certain substances, particularly nitrates and phosphates and some organic compounds, at first produces an enrichment of the water, encouraging the growth of favourable food organisms, thus increasing productivity especially in rivers that are initially impoverished. As loads increase, enrichment passes to eutrophication and less favourable organisms flourish. Eventually, with further increases in loading the capacity of the system to satisfy the biological oxygen demand may be exceeded. The conditions in the river deteriorate rapidly with a loss in capacity to support life. Because there is a natural succession of physical and chemical conditions in a river whereby the lower potamon reaches are to a certain extent naturally eutrophicated, fish communities in the potamon tend to be adapted to eutrophic conditions and can resist a certain measure of deoxygenation. In the swifter flowing, highly oxygenated rhithron areas, however, fish species are not so tolerant and even minor reductions in oxygen tensions resulting in the elimination of the more sensitive among them.
Loading with other type of organic and inorganic substances may cause pollution of the aquatic system. The effects of pollution range from lethal toxicity which kills fish at some stage in their life cycle, to sub-lethal effects which are difficult to detect but which alter the fish's behaviour in such a manner as to prevent it completing its normal life cycle. Furthermore, accumulation of otherwise non-toxic substances within the flesh of the fish can render this either unsafe or unpalatable for consumption.
Other forms of pollution include thermal loading, whereby the temperature of the water is raised to a level unsuitable for the presence of the fish, and loading with fine particulate matter which, apart from siltation, directly affects the fish by choking and erosion of the gills.
Levels of pollution and eutrophication are closely linked to water quantity as discharges which may not be serious during peak flows may become serious as flow rates decline and relative concentrations increase. This is particularly noticeable in standing waters of certain flood plains, where pockets of polluted water are isolated from the main current.
Conservation areas such as wildlife parks and nature reserves often rely on water as an essential part of their appeal. Because waters within such areas are protected along with other features of the fauna and landscape conservation areas may be considered beneficial to the fish community or at least a harmonious joint use of the resource. In many parks and reserves where fishing is permitted, this is so. However, in other cases where access to the waters is prohibited to fishermen, the fishery does not benefit despite the presence of a healthy fish community.
Apart from considerations of access, the presence of wildlife within river basins has certain direct effects on the fish community as a result of ecological interactions. Examples of such are the complex effects of beaver dams in North American rivers, which tend to reduce populations of rheophilic species in favour of limnophilic species, or the improvement in productivity of tropical river waters when fertilized by the dung of hippopotami and the herds of ungulates which graze the floodplains during the dry season. Indications that some larger elements of wildlife, such as crocodiles, play an important role in the conservation of productivity by acting as nutrient banks within poor aquatic systems such as the black waters of equatorial forest rivers are given by Fittkau (1973). Another important impact of wildlife on fish populations is that of predation. In many areas fish-eating predators are serious competitors of man for the fish resource. Piscivorous birds alone have been estimated to take more fish than the fishery from places as widely separated as the Parana River in South America and the Senegal River in Africa. Much of this fish is taken from potamon reaches during the period of drawdown when many fish are stranded in temporary pools and would in any case be lost to the stock, although not necessarily to the fishery. In addition to water birds, other significant predators outside the fish community are crocodiles, otters and freshwater dolphins.
Throughout much of the world, cattle are grazed on unmodified flood plains during the dry season. On the extensive flood zones of the potamon the presence of cattle is considered beneficial. Not only do they convert grass into dung which is more readily broken down and dissolved, thereby hastening the recycling of nutrients, but the tradition of burning off the old, moribund stems of the flood season grasses to permit more rapid growth of new vegetation creates quantities of ash which serve the same purpose.
Other advantages may accrue to the fishery where the plains are managed for cattle by the installation of holding ponds for watering. Typical of these are the “modulos” of Venezuela, crescent-shaped dikes which retain the retreating flood forming temporary pools. Fish are trapped within these and may be harvested some time during the dry season or eventually such structures may be used for extensive aquaculture.
Some concern, however, has been expressed in localities where simple fish farming ponds on flood plains are interfered with by cattle. Here, banks of ponds are trampled down and waters are muddied and polluted. As an answer to this, such flood plains are zoned according to use. More serious interactions have been described from the United States by Platts (1978) where high densities of animals degrade bottom lands and their associated water courses. Changes include loss of vegetation cover, instability and erosion of stream banks, increase in silt load with consequent siltation of channels and decrease in water quality. Because of these changes fish populations, usually of salmonids, are much lower in grazed than in ungrazed reaches of lower order streams.
Woodlands play an invaluable role in stabilizing the landscape, preventing erosion of the topsoil and evening out surface flow from rainfall. They shade the waters of lower order streams, and trees and branches which have fallen into the channel of the river provide cover for many species of fish. In addition, the fall of organic matter such as leaves, bark, branches or insects from vegetation overhanging water courses provides an essential source of allochthonous food in otherwise nutrient-poor waters. For example, Blackburn and Petr (1979) summarizing data from several parts of the world, show that between 3.5 and 8 t of plant litter/ha/year fall into low order forested streams. Removal of trees through deforestation negates these beneficial effects and induces a number of changes within the aquatic system, particularly of low order streams. Thus, in areas where large-scale deforestation has occurred, silt loads tend to increase due to the wash-down of top soil and flows become erratic, regimes taking on the more spiky character of flash flooding. Temperatures rise, especially in small streams, and the food available to fish is reduced. Furthermore, river banks are less stable without the binding influence of trees, bank erosion increases and fallen portions of trees eventually disappear, reducing the cover available to fish.
In high order streams the interactions between forested and aquatic systems are less readily understood. High order streams inherit the results of actions in their low order tributaries. Thus, increased silt loads and altered flood regimes arising from deforestation of their low order tributaries are experienced throughout the system. In heavily forested systems such as the Amazon, Zaire or Mekong, deforestation of the floodplain also destabilizes the bank area leading to increased erosion and silting, although flood regimes are by this time less readily influenced. In such forest areas, the trees appear to act as a nutrient sink and their disappearance from what are fundamentally poor systems may well have long-term results on the nutrient balance. In other systems, particularly those of tropical and semi-tropical savannas, the reverse may well be true. The flood plains and levees of such systems were initially, and in some cases still are, covered with gallery forests most of which have now been removed for agriculture and human settlement. Clearly as this process has been historically fairly slow, and so little information is available on the pre-cleared state of such rivers it is difficult to evaluate the impact of these changes on the fish populations. However, despite such possible adverse effects as changes in the flood regime, decreased stability of floodplain features and reduction in allochthonous food supply, it would appear that such cleared savanna plains are more productive now than when they were forested.
Further adverse effects on aquatic systems arise directly from the forestry industry, where the process of logging itself contributes to degradation of stream quality. With bad logging techniques, excessive amounts of waste timber and soil enter the stream, causing increases in stream bed load, suspended sediments and eutrophicating solids with a result that dissolved oxygen concentrations fall and the stream bottom becomes choked with silt (Graynoth, 1979). In contrast, one of the few environmentally detrimental effects of fisheries bears on woodlands. The process of smoking fish to preserve them requires large amounts of wood, of the order of 4–5 kg per kilogramme of fish to be smoked. This has led to a serious depletion of wood particularly in marginal areas such as the Sahelian zone of the Niger River, where wood is in any case uncommon.
At their simplest, agricultural activities in river basins have little effect on fisheries and in certain cases may be regarded as a non-competing joint use of the resource. On flood plains, the early slash and burn technique and later the drawdown agriculture that replaces it use the dry phase of the cycle in complement to the fishery so that the plain is productive throughout the year. However, land clearance and poor agricultural techniques particularly on marginal lands on hill slopes in the upper parts of the river basin rapidly lead to erosion and siltation of the water courses. The exceptionally heavy silt loads of many tropical and sub-tropical rivers originate in this manner. As the need for crops intensifies, especially for grain, agricultural methods become more advanced and pressures are placed upon the aquatic system to the detriment of fisheries. The capacity of the unmodified floodplain for agricultural production is rapidly attained. Further increases are limited by uncertainty of floods and by such irregularities in the flood plain itself as lagoons and swamps. Increasing intensity of agriculture leads to the filling in of such features with a loss in fish habitats and to an increasing control of the hydrological regime through drainage and irrigation, thus affecting the breeding and growth of the fish. Because control of the hydrological regime tends to diminish the time and extent of the inundation two main problems arise. Firstly, silt is no longer deposited on the floodplain and the absence of flooding also means that the supply of nitrate to the soil through blue-green algae during inundation no longer occurs. The alternation of anaerobic-aerobic phases in the soils of the plain also increases availability of phosphates and potassium. Once this natural process of fertilization is stopped, the soils of the plain rapidly become depleted of nutrients, necessitating applications of artificial fertilizer. These find their way into the aquatic system, raising the risk of eutrophication. The application of inorganic substances to the plain, together with a water table near to the surface and lack of flooding, leads to the build-up of salts in the soil. If this process is allowed to continue soils become salinified and the plain becomes useless for agriculture. Secondly, the silt that is no longer deposited on the plain is laid down in the main channels of the river, there producing all the symptoms of siltation. In upstream areas, where water is extracted for irrigation, flows may be reduced below levels consistent with the maintenance of the aquatic environment.
Throughout much of the tropical world, the flood plains of rivers are used for wet agriculture. At its most primitive, this consists merely in planting floating rice on the plain, although attacks by fish on the growing plants quickly force the farmers to construct simple bunds. More intensive rice culture requires a complete control over the hydrological regime and systems of dikes, ditches and flood control dams, which effectively reclaim portions of the floodplain, are a necessary accompaniment to this form of agriculture. Because fish and rice use the same phase of the aquatic system, the potentials for interaction between them are considerable. On one hand the fish may affect the rice adversely, especially during its tender young phase. On the other hand, fish may control many of the pests such as stem borers which attack the rice. In practice, it is difficult to exclude fish from rice fields and traditionally the fish were reared, or let grow, as a second crop. However, with the increase in demand for rice, many farmers have adopted a system of double cropping. This is detrimental to the fishery in that the fish have less time to grow between croppings. In addition, further maximum rice production can only be achieved through the use of insecticides to control insect pests and allegedly this too harms the fish resident in the rice fields.
Many of the world's cities are located on the banks of rivers where they exert a disturbing effect on the aquatic ecosystem. Recent urban growth has caused the cities to spill out onto the floodplains of large rivers, increasing the risk of flooding. Consequently, measures have to be taken to control the flood with all the attendant effects on the fish fauna. For similar reasons, smaller rivers running through urban areas are channelized or even driven underground. Because cities consist of large areas of impermeable surfaces, which are efficiently drained, any precipitation over the urban area is rapidly transferred to the adjacent aquatic system. This may produce local flash floods, especially in small streams, although such effects are slight in the basin as a whole. Water regimes are also affected by the withdrawal of large amounts of water for domestic and industrial uses. This can influence flows considerably, especially in smaller streams.
One of the main effects of urbanization is the great increase in pollution and eutrophication. Even when adequately treated, domestic sewage contains large amounts of eutrophicating phosphorus and nitrogen. Industrial development which is usually closely associated with urban growth leads to the discharge of a wide variety of inorganic and organic chemicals into the water, many of which are directly toxic to fish. Requirements for energy lead to the establishment of power stations which utilize enormous quantities of water for cooling and thus raise the temperature of the river sometimes appreciably. Alternatively, power requirements are met by hydro-electric plants which depend on the damming of rivers for controlled release of water. These factors combine to produce far-reaching degradational changes in the aquatic ecosystem, changes which are compounded by the tendency to construct series of cities along water courses. This means that no sooner has the river recovered from the effects of one urban area, than it is loaded by its successor downstream.
Other problems arise with transportation facilities to urban centres. Navigation within the river often requires regularization of flow as well as some measures to modify the river channel, including channelization and dredging and installation of weirs and locks. In addition, wash from boats erodes banks, accelerates siltation and destroys marginal vegetation. Roads and railways cut across floodplains on embankments raised above the level of the plain. These are not usually equipped with sufficient drains and bridges to allow normal water flow, with the result that considerable portions of the plain become isolated and the movements of the fish are interrupted.
Processes associated with mining disturb the aquatic ecosystem in a number of ways. The first and most serious of these is siltation from material excavated during mining. This is frequently spread in dumps, from which particles are dislodged and washed into rivers, and may also be discharged directly into the aquatic system. Heavy silt loads and disturbed flow patterns also arise where large amounts of water are abstracted from the rivers for washing ores and extraction of heavy metals after which they are returned heavily loaded with tailings. Many mining by-products are themselves toxic especially in the long run, when leachates of such minerals as copper, lead or tin from dumps and tailing ponds can poison whole stretches of river, rendering them unsuitable for any form of life. Equally, pollution by fine particulate matter such as china clay can damage fish or drive them away from affected zones.
A specialized form of mining, the extraction of gravel and sand from river beds, such as for the building industry, also damages streams especially in the rhithron zone. Not only is there a great increase in silt load, with choking of downstream habitats, but the removal of gravel lessens the area available for spawning by fish depending on this type of substrate, for instance salmonids.
The control of flow within a river system is required for a number of reasons touched on in the above description of the effects of individual uses. Such activities as power generation, irrigation, domestic water supply, navigation or flood control all use dams, levees or channels, which alter the morphology of the river system and hence the abundance and composition of the fish community.
Dams: Dams are probably the most common of flow-control devices. The water stored in the reservoir behind the dam during periods of high discharge can be released slowly throughout the rest of the year for generation of electricity, irrigation and industrial or domestic water supply. Reservoirs are valuable sites for fisheries and much work has been devoted to the management of such water bodies for this purpose - see, for instance, Lowe-McConnell (1966) or Ackermann et al. (1973). When a river valley is flooded following the construction of a dam, certain changes occur in the original fish community. Migratory species tend to diminish in abundance and are generally confined to the upstream end of the reservoir. Other species with specialized breeding habits which rely on the floodplain for spawning success disappear, whereas certain species, often among the least common elements of the original fauna, increase in abundance and dominate the new environment.
Downstream of the dam the discharge is controlled often to a point where minimum flow requirements are not met, and flooding no longer occurs on the floodplain. Both these phenomena act to the detriment of the fish stocks reducing abundance and altering species composition in favour of non-migratory species. In the case of large mainstream dams, for example, the Kainji Dam on the Niger River, more fish production may be lost from the downstream fisheries than is gained from the upstream reservoirs. Ironically, just below the dam local increases in fish production may be experienced in the form of tailrace fisheries. These arise where migrating fish blocked from further advance upstream congregate in the area immediately below the dam and attract various predatory species. The blockage of migration itself has many serious implications. Where a dam intervenes in the migratory pathway between the breeding and feeding areas of a species, the species in question tends to diminish in abundance and eventually to disappear from the system. Many species have been eliminated in this manner and yields from the areas where such species are caught upstream and downstream of dams have declined.
Levees: Artificial raising of river banks is used to prevent the river spreading laterally outward from the main river channel. Such structures are most common in the potamon, where broad floodplains flank the river. Levees of this kind prevent fish from moving onto the floodplain to breed or to occupy standing water bodies. Consequently, catch and specific diversity drop in reaches of the river where this type of control is applied. Because the levee prevents the silt load of the river being spread into adjacent flood lands, siltation problems occur within the main river and in unleveed reaches of the river. Also because waters cannot spread laterally at times of high discharge, flow within the channel is also increased often beyond the level tolerated by fish which are then swept out of protected positions into unfavourable environments. This is particularly troublesome where main stream spawners have pelagic or semi-pelagic eggs and fry are present. In these cases the young stages are swept past the floodplain areas necessary for their growth.
Channelization: Simplification of river channels by straightening them and artificially stabilizing the banks is a common means of facilitating and accelerating the passage of water. Many kilometres of river have been lost in this process. Extensive studies, particularly in the United States of America, where a policy of stream channelization has been pursued for some years, shows that both catch and specific diversity are always less in channelized stretches of a river when compared with unchannelized stretches of the same river.
