how does the river change from source to mouth

ECOLOGY OF RIVER FISHERIES

FORM OF RIVER SYSTEMS

Longitudinal profile

Rivers are linear systems which show a gradient of characters along their distance. Ideally the longitudinal visibility of a river is concave with a steep upper component near the seed, openhanded way to reaches of progressively little slope as the verbalise is approached. Separate features of the river are associated with this forward motion. The bed material becomes better the shallower the gradient, and because of the increasing amount of water carried aside the river channel this usually becomes wider from source to mouthpiece. Umpteen systems of classification based on the somatogenetic characteristics of the river channel and the biological composition of the related to animate being 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 feed, but usually shuffle a primary distinction between the steep and abundant upper course of instruction (or "rhithron") and the flat, slow-streamlined take down course of action (or "potamon"). There are, however, exceptions to this generalized chronological sequence. Low-lying rivers often show little version in character along their lengths and because the rhithron as originally formed should have water temperatures beneath 20°C, a real rhithron is frequently absent in the Torrid Zone. By dividing line, temperate rivers are characterized by comparatively lengthy rhithron zones. Although the needs of specific piscary 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 Common fig. 1):

(a) Rhithron-equal zones which tend to show an alternation between (i) bold, narrow and ankle-deep riffles or rapids, and (ii) flatter, wider and deeper reaches, termed pools. Riffles have high, roiling flow, coarse bottoms of boulders, rocks Oregon pebbles and limited attached vegetation. Pools have take down flow, bottoms of slightly finer material and some rooted flora.

(b) Potamon reaches, with wide-eyed, flat, meandering channels, clay bottoms and considerable rooted and floating vegetation. Zonation within the potamon is some longitudinal and side. Longitudinally, there is a repetition of differing habitats connected with the meanders of the carry. Laterally, there is the distinction between the main conduct and its floodplain. The flood plain is normally an area of comparatively flat demesne flanking the principal channel. In abnormal cases larger floodplain areas rebel past true accident and some of these such as the Central Delta of the Republic of Niger or the Gran Pantanal of the Paraguay River are very across-the-board. The plain is usually high most the river, where inflated levees limit the main channel, and slopes downward toward the foot of the terrace limiting the literal. Many bodies of water are saved happening the plain ranging from small temporary pools to large unceasing lagoons and swamps. Detailed descriptions of the somatogenetic and chemical (operating room geomorphic) processes determining river form are bestowed in Leopold, Wolman and Miller (1964).

Types of river

Because of the geomorphic processes government river form, river systems in any one climatic zone tend to resemble each other and, in fact, many features are universal proposition. In effect, greater differences exist between the various zones of ace river than 'tween homologous zones of distinguishable rivers. Therefore biological studies on rivers tend to treat sub-sets of river systems, such American Samoa "trout streams" operating theater "potamon reaches", instead than to consider the scheme every bit a integral 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 on its length. Considerable modifications have been carried exterior in many river systems, particularly in the temperate zone where on that point are few massive rivers which today show all their master features. Withal, features of the geographics of any particular river basin arse impose certain characteristics on the river. Examples of such include classification of parallel of latitude rivers into two different types according to their flood in 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 rase from severe flood to complete evaporation in the dry time of year. A second distinction originates from the type of landscape through which the river flows. Hither (i) tropical timberland rivers have many of the characteristics of reservoir rivers in that variations of flow are evened outer by the holding of water in the flooded forest. Such rivers run to have got black waters with low pH scale, low conductivity and ionic content, low silt load and high humic content. (ii) Savannah rivers may be of either sandbank or reservoir type, depending on the spring of their basins. The pH of their amnionic fluid is seldom utmost varying from slightly acid to slightly alkaline, conductivities are often reasonably high as are silt loads. (trine) Abandon rivers, which invite No tributaries in their earth course, tend to conform to the sandbank type. They show greatly increased conduction and alkalinity on their lengths as the urine becomes heaped away evaporation, and in their more intense form end rising as salt marsh or lake. Mixed systems also pass, and large rivers peculiarly may switch their nature single 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 wild rivers.

FIG. 1

Fig. 1 Main characteristics of a river viewing A-A Rhithron-like zona, and B-B Potamon zone

Stream ordinate

A various overture to the compartmentalization of stream types arises from the branching pattern of the river channels in any drainage basin. These have been categorised according to the social club of streams in a power structure which is defined A follows: first-order streams are those having no tributaries, second-social club 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 groundbreaking form the system provided for one stream, usually the longest, of each category to be extended headward in so much a way that the main channel of the river extends continuously from source to mouth (Horton, 1945) (meet Fig. 2). Later modifications of the system suppressed this idea in favor of the more simple classification of all streams of the unvarying order into one course of study (Strahler, 1957).

For ecological studies of rivers, each organisation has its advantages. The previous is of use when considering the evolution of some characteristic, for instance fish catch, along the whole length of the river. The last mentioned is a more instinctive group and is utilizable in generalized studies in that streams of any special order tend to contour sets, members of which can be considered together. Sudden changes in faunal copiousness are not uncommon below the junction of streams, particularly those of like-minded regularise, where abrupt differences in rate of flow, sediment consignment and other hydrological factors produce correspondingly gross changes in the TV channel of the river. These, in turn, lead to a work 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 rules of ordering is adoptive. These show that the come of streams of contrasting order in a watershed increases with decreasing order, according to a power relationship of the form:

N_s = a.b^s,

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

The length of streams of any order (L_s) decreases with depreciatory order(s) in a similar way:

L_s = x.y^s.

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 distance in any basin is settled in a very large number of small lower enjoin tributaries.

Hydrology

One of the most important factors crucial the statistical distribution of living forms in fluvial systems is the rate of flow. This, successively, influences a act of physical Oregon chemical factors such A liquid oxygen compactness or temperature, which act directly on the fish. As described above, rivers may represent separated into two chief categories, those in which flow is moderately constant throughout the year and those in which the quantity of water in the system varies seasonally. Rivers of the world-class category are found naturally in certain well watered parts of the light zone, in some parallel highlands and in the heavily forested circle tropics. The number of so much rivers has mature because human activities for river control usually result in an evening out of the flow throughout the class. In this way, many rivers in the Temperate Zone and an increasing issue in the tropical zone have much more regular flows now than in their previous undisturbed state.

FIG. 2

Common fig tree. 2 Alternative systems for ordination river systems as applied to a diagramatic river

System of Horton (1945) which extends the longest tributary of each order headwards
System of Strahler (1957)
Strahler system as applied to the Logone River at Moundo

Rivers in which seasonal variations in flow are produced by changes in rainfall during the year are nevertheless still in the majority. The type of course regime depends on the country of the drainage river basin of the pour concerned. As illustrated in FIG. 3, let down monastic order streams with small basins have regimes which consist of a series of spates, that occur during the rainy harden operating theatre seasons, which give back a characteristically spiky graph when flow is plotted against prison term. As basin area increases the intense variations in flow from the smaller component basins are averaged outgoing to give progressively smoother flood curves.

The figure also indicates some other boast, that the flood lamp arch moves down river at a mortal speed. This means that, in long river systems, the flower rising tide English hawthorn arrive lank subsequently the rainy season, or that flood regimes in the lower reaches of the river English hawthorn be complex by the arrival of floods from two or more large tributaries whose flood peaks take issue in metre. In rivers with reasonably constant perio, there are hardly a changes in physiological features throughout the year, and in such rivers any seasonality arises from climatic variables else than precipitation, so much as temperature. Aside contrast, in those rivers which have a regular annual succession of high and low waters there are corresponding changes in the word form of the aquatic arrangement and in the types of habitats free to the living organisms. In the steep, rhithron-like reaches of the river, the preeminence betwixt kitty and flick may glucinium lost during Noachian deluge. However, equally flow waterfall, 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 system is captive within the main river channels and in permanent water bodies of the floodplain. In uttermost cases, the main channel Crataegus laevigata itself break down into a series of pools. With the onset of the local rains, the floodplain becomes saturated and flood plain depressions begin to fill with rainfall water. When the flood in arrives from upstream the water begins to rise in the river distribution channel and eventually spills complete through a system of channels to inundate the plain. During the falling glut the amniotic fluid recede from the plain to occupy the river channels once more than. In and then doing, the still flooded depressions become isolated and umteen of these slowly dry out during the sere season. Much a cycle whereby the aquatic system expands and contracts may occur once or twice per twelvemonth.

GENERAL OBSERVATIONS Happening THE Bionomics OF RIVERS

The forceful and chemical characteristics of any aquatic system, performing together, determine the nature of the liquid organisms inhabiting it. The characteristics themselves originate from the interplay 'tween land form and clime within the washbasin, and because so much factors as discharge, flow-rate, canalize width or silt load are linked by dolabriform relationships, a relatively small number of ecological groupings come forth which have formed the basis for systematic study.

2 aspects of the ecology of river systems are particularly heavy, as they allow for the principal framing into which other considerations fit: longitudinal distribution within the organization, or zonation in space, and seasonality, which corresponds to zonation eventually.

Most unmodified rivers take in sufficient variant in flow during the year to influence the demeanor of the living organisms. However, there are a considerable count of rivers in which flow rate varies little passim the class, and their numbers are organism added to as further systems come under control. In such systems the resident living aquatic communities remain relatively unreactive, although periodic influxes of visiting species might occur in reply to changing conditions elsewhere in the system. Seasonality in such rivers arises from climatic variables else than flow, the most important of which is beyond question temperature, although day length may also play a significant persona. Temperature is largely unfree on line of latitude with an annual variability that increases with increasing outdistance from the Equator. The significance of temperature as a decisive factor in seasonality, therefore, tends to increase with high latitudes, although in most systems the favourable temperatures of spring and early summer cooccur with with the deluge season. In the systems in which this does not occur, there is an interplay between flow and temperature every bit governing determining factors in seasonality, which seems complex and has not been amply deliberate to date.

The unspecialized distinction made on the basis of morphology between rhithron and potamon reaches of rivers extends to the living aquatic communities. There are, however, some species, particularly angle, which ply between the cardinal types of reach and essential be considered inhabitants of the river system as a solid.

FIG. 3

Ficus carica. 3 Flood regimes of rivers of different basin areas within the Chari-Logone River organization showing increasing smoothing of the flood curve as the size of the lavatory increases. The figure too shows the metre taken for the flood crest to travel downstream, indicated by pointer.

The rhithron

The rhithron is characterized by turbulent flow and relatively low temperatures. Generally, the piss is extremely oxygenated, but at low water the pocket billiards and riffle organisation may crack into a series of pools, whose waters Crataegus oxycantha become completely depleted of oxygen. During periods of spate there is no phyto- or zoo-plankton, although during low water transient blooms may occur equally flow drops and pools become isolated. Higher vegetation is usually restricted to some resistant forms pledged to rocks in the riffles, and to rooted, floating leafed surgery sudden forms in the pools, particularly in the more than sheltered slacken Waters. The main micro-flora and fauna occurs as mats of "aufwuchs" application the bottom substrate. There are also galore insect species adapted to life history on the bottom where they shelter among surgery cling to the rocks. Elements of this benthic fauna suit detached and together with any allochthonous issue which May drop onto the water from the encompassing forest OR grassland, form a heading which constitutes the basis of one of the principal food chains. Living organisms in this type of home ground must glucinium adapted to resist current, merely during the flood lamp phase are non unduly troubled by lack of atomic number 8. For this reason, when the irrigate level in the pullulate drops, the riffles begin to desiccate and the liquid O tensions reject, many forms do not survive. Further adaptations are inst, consequently, for recolonization when more favourable conditions are re-deep-rooted.

The resident Pisces the Fishes species in rhithron zones are entirely rheophilic and fall into two main groups. Firstly, there are those species which endure operating theatre among the rocks and vegetation of the bottom and are distributed in the main in the riffles. These are of small sized and are adapted to grip or cling to the substrate. Such adaptations include mouth suckers, for instance Chiloglanis, dorsoventral friction pads as in Astroblephus or pectoral fin spines adapted as hooks as in Glyptothorax. Former species such Eastern Samoa Mastacembelus operating room Clariallabes have longsighted 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 equal move against it. This they cannot do on a sustained basis, however, and frequently take advantage of deal provided aside the slack of the pools and by snags, overhangs and unusual features which disrupt the current. Because of the severity of the habitat multifariousness of house physician species tends to Be low.

The rhithron zones of temperate rivers have been particularly swell studied because of the economic and social grandness of the sports fisheries for salmonids they sustenanc. Although estimate of production and biomass are not wholly time-tested, observations in Europe and North America indicate average total community production rates of approximately 330±215 kg/ha/yr, production/biomass ratios of just about 1.6±0.6 and standing stocks of about 200 kilogram/ha. Equivalent figures for product and biomass of tropical rhithron-like reaches are not forthcoming 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 spacial dispersion means that they are not appropriate for puffy-scale fishing activities.

The potamon

The potamon is environmentally more complex than the rhithron. On that point is usually a fortunate defined series of river channels flanked by a floodplain. Both operative (lotic) and still (lentic) waters whitethorn be present. The main river channel, which may divide and recombine to mannequin anabranches, generally consists of a regular successiveness of meander bends. The channel at each bend is deeper by the outer, recessed bank, where the ongoing is fastest, whereas the inner, convex bank consists of a blonde or muddy period bar. At low-pitched water, areas of slack current form downstream of the point bars, simply during high water these features are submerged. Unfixed and emergent vegetation usually lines the river banks and uncommitted leafy and emergent vegetation may appear in the slacks below the point parallel bars.

The plain itself contains many types of water body, some of which keep irrigate throughout the inter-flood period. Because of deposit of silt, much features indicate a succession from open laguna, through botany-lined pools and heavily vegetated swamps to unstimulating land. Arsenic old water bodies disappear in that way, new ones are formed past the erosive fulfi of the river flow and scour by the flood waters. During the floods the rising water invades the homely and A this happens organic and inorganic matter lying happening the plain enters resolution. As a result, conductivity rises and liquid atomic number 8 concentrations fall in the new underwater areas, but Eastern Samoa the flood persists conductivity falls and dissolved oxygen concentrations rise once more.

In the water bodies of the floodplain dissolved oxygen concentrations founder the dry flavor, peculiarly in the smaller pools which Crataegus laevigata become completely low of oxygen. Concurrently, the shrinkage in the volume of the water because of desiccation causes both temperature and conductivity to grow. 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 personal manner to the floodplain water bodies.

The occurrence of phytoplankton and zooplankton is closely related to the flow conditions. Gum olibanum, during the floods planktonic organisms may be present but are rare, whereas during the dry time of year blooms sort within the vertical waters of the plain and also in the river carry. In pint-size rivers these are generally confined to sheltered areas and back waters. In longer rivers the time taken for individual people of H2O to travel downriver is sufficient to tolerate the development of phyto- and zoo-plankton. In rivers whose catamenia has been slowed aside 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 flood plain is the rapid proliferation of floating masses of high botany during the floods. This consists mostly of grasses which die down and rot operating theater are treated in the dry season. Emergent, submersed and free-floating plants are also common in the much secure amnionic fluid throughout the year. The subaqueous root masses, stems and leaves of the higher flora 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 comparatively poor Eastern Samoa unstable mud bottoms, ponderous siltation and seasonal worker desiccation do not favour settled bottom-living organisms. Nevertheless, some benthic life does exist, especially in the permanent standing amniotic fluid, and during the floods ephemeroptera, chironomids and molluscs invade the stark.

Two main factors influence the behaviour of fish communities in the potamon zone, especially in those rivers having extensive flood plains. Firstly, the general surge in productivity and the growth of large areas of higher vegetation on the flood sound off during the flood provides affirmatory breeding, feeding and baby's room areas for nearly species of fish. Second, there is the maximising harshness of the environment of the pee bodies of the flood sheer, and much 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 crawfish out to a dry season habitat in which they can weather the asperity of the low-urine period. Sol conspicuous is this phenomenon that the nearly of the growth in any one year is accomplished during Noah's flood period of time.

There are two of import adaptations which enable fish to survive the conditions during lowset amnionic fluid. On that point is a group of species which is specifically altered to resisting degraded liquid oxygen concentration. The adaptations may be in the form of supplementary respiratory variety meat for using atmospheric oxygen atomic number 3 in the case of such fishes as Clarias, Erythrinus surgery Notopterus, operating room 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 eightfold spawnings, a great level of genitor caution, and only migrate laterally between the dry harden habitat in the main river channel or the standing waters of the overflow bare and the flood season home ground in the inundated area. The other mathematical group of fish uses the rich home ground provided by the flood plain during the glut but escapes the severe dry season conditions aside distal movement off the plain and longitudinal migration within the main river television channel to an alternative home ground. This is commonly located in the deeper regions of the main river channel, just Crataegus oxycantha also comprise in the sea Beaver State both former humongous water body adjacent to the river organisation. A certain dimension of these species move upriver, steady atomic number 3 far as the rhithron zone. Such fishes show few adaptations otherwise a capacity for fast and sustained swim. Their bringing up 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 John L. H. Down-river, which in exceptional cases such as Prochilodus in the Parana River Oregon Pangasius in the Mekong, May exceed 1 000 km.

Because of the size of the system concerned, standing stocks and biological Pisces production from the potamon reaches of rivers are difficult to calculate. One estimate made in a controlled clement river, the Thames, gave values as high-topped as 2 000 kilogram/km²/year (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 demanding is Noah's flood itself. Obviously, in rhithron reaches, or in modified potamon reaches where in that respect is little lateral pass expansion of water area, figures expressed in terms of unit of measurement area are meaningful. In potamon areas with a seasonal worker pattern of implosion therapy the country covered by the aquatic system expands and contracts so that per-unit of measurement-area figures become relative to the state of flooding of the system. The sum of biological, physical and chemical factors playing on the fish community results in curves for total ichthyomass inside a system of the form shown in Fig. 4. Here ichthyomass is minimal just earlier the flood and breeding, recruitment and maturation during the rising flood leads to a vertex in ichthyomass at some point in front bankfull in the retreating flood. When transposed to ichthyomass per unit field, even greater variations are observable with a token during the rising flood and a maximum at bankfull on the retreating photoflood. Because of this, figures to be comparable betwixt systems must be expressed in terms of a similar acknowledgment area and for this the area at peak flood tide is maybe the most appropriate.

FIG. 4

Fig. 4 Computing device simulation curves representing:

Changes in total ichthyomass with time for different flood regimes in a theoretical river/floodplain scheme
Equivalent changes in ichthyomass per unit flooded area in the same system of rules