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Fluvial geomorphology is the study of various topographic features due to erosion, transportation, and deposition of sediments by a river. There is a close interaction between water, sediments, and catchment boundaries which make distinct features, such as bed forms, cross-sectional geometries, and planforms, in a channel. Continuous changes in the processes and forms in a river basin take place. In any river basin, the geomorphological evolution is a usual and natural process. Streams are dynamic and increasingly important part of the physical environment. Their behaviour is of interest to a broad diversity of concerns, ranging from flood control, navigation, and development of water resources to recreation. Streams represent a potential risk to human beings and property through floods and bank erosion. Rivers are dynamic units whose characteristics differ over time and space with changes in the environmental controls. The character and behaviour of a fluvial system at any particular location reflect the integrated effect of a set of upstream controls, notably climate, geology, land use, and basin physiography, which together determine the hydrologic regime and the quantity and type of sediment supplied (Knighton, 1984).
The physical background of a river basin is a part of the geographical elements which play very significant roles in controlling the geomorphological and hydrological phenomena in the river basin. In the watershed area, stream anatomy and interactions are the reflection of the relief, geology, tectonic characteristics, structural geology, geomorphology, climatic condition, nature of soil texture and structure, and riparian vegetation. Tectonic characteristics generate the relief that drives the erosional mechanisms which provide a gargantuan amount of sediment on a river, causing channel avulsion and change of channel gradient (Schumm, 2005). Geology and climate control the discharge and sediment load, which influence the channel geometry (Knighton, 1984). The overall geometry of a stream channel is controlled by the climate and geology of a river basin. Climate is a controlling factor in determining the stream type and hydraulic geometry (Schumm, 2005). Morphometric and hydraulic characteristics as well as the behaviour of discharge of any river channel are largely dependent on monthly and annual patterns of rainfall. Variation of the discharge character of a stream is influenced by climatic factors and physical characteristics of a basin (Knighton, 1984). The bank erosion rate and its spatio-temporal distribution mostly depend on geology, soil (e.g. size, gradation, cohesivity, and stratification of bank sediments), climate (e.g. amount, intensity, and duration of rainfall), and vegetation (e.g. type, density, and root system) (Knighton, 1984; Maiti, 2016). Bank erosion affects the socio-economic status of the human population in the fluvial environment. At present, the integration between physical and social parameters has become much more significant in the field of geographical research and can help regional planning and development.
Alluvial streams are very dynamic landforms subject to rapid change in channel configuration and flow pattern. Higher stream energy channels and less consolidated riverbank and bed materials are most dynamic and susceptible. Stream channels interact with many geographical factors, like climate, topography, lithology, vegetation, land use, and area of catchment. The river basin is a set of morphological attributes, for example, divides, valley side slopes, stream gradients, channel spacing, and floodplains. In a morphological system, the river basin at different parts is considered as the meaningful arrangements of these morphological attributes (Maiti, 2016). The elements of the morphological structures are statistically related one to other, e.g. valley side slopes and stream gradients are directly related to the abundance and spacing of channels (Schumm, 1977). Changes in fluvial morphology may be due to land use changes and human interventions, for example, engineering constructions, cultivation, deforestation, and flooding. Destructive floods can modify stream-channel morphology nearly instantly.
A brief history of geomorphology and fluvial geomorphology
Geomorphology deals with the study of the earth’s surface, i.e. landforms, and their origin, evolution, and the processes that shape them. The history of geological and geomorphological investigations can serve to illustrate both the progress and pitfalls involved in the scientific understanding of the earth’s surface and recent geological history (Oldroyd and Grapes, 2008). Giovanni Targioni-Tarzetti (1712–1783) in Italy, Jean-Etienne Guettard (1715–1786) in France, Mikhail Lomonosov (1711–1765) in Russia, and James Hutton (1726–1797) in Scotland established the link between geomorphology and geology.
Fluvial geomorphology is the geomorphology of rivers and a significant branch of geomorphology. The history of mankind is closely connected with the history of fluvial geomorphology. The history of fluvial geomorphology started to develop in ancient times with the history of river development. In ancient times, the content of fluvial geomorphology was the normal geomorphologic phenomena of rivers, but at present fluvial geomorphology has developed as the mathematical and engineering geomorphology of rivers. A primary objective of fluvial geomorphology is to explain the relationship among the physical properties of flow in mobile-bed channels, the mechanics of sediment transport driven by the flow, and the alluvial channel forms created by spatially differentiated sediment transport. Graf (1988) opined that geomorphology is the study of earth surface forms and processes and fluvial phenomena related to running water. The science seeks to investigate the complexity of behaviour of river channels as a range of scales from cross-sections to catchments; it seeks to investigate the range of processes and responses over a very long time scale but usually within the most recent climate cycle (Newson and Sear, 1998). Geomorphology is the main branch of physical geography. It is a spatial science dealing with time and space. It is also deals with the origin of the earth’s topographic features caused by various geomorphic processes, e.g. fluvial, glacial, coastal, aeolian, and periglacial.
Fluvial geomorphology is concerned with the creation of landforms by river processes through the removal and transfer of materials on the earth’s surface. Process studies are rooted in a number of disciplines, the earliest identifying ‘processes’ as evolutionary time sequences in landforms. Fluvial systems exist over a range of scales, from centimetre-wide intertidal channels and other drainage networks on a sandy beach to the more than 6 million km2 drainage basin of the Amazon. All such systems are complex, self-organizing, and hierarchical in structure. Modelling complex behaviour in hierarchical systems does commonly incorporate lower level ‘process rules’, and this underlies the need in fluvial geomorphology for improved understanding of process mechanisms at all levels. Catchment factors (varying the gradient, sediment, and discharge at individual sites) affect local processes, which in turn may operate to change the character and extent of catchments.
Since the 4th century BC, many people have studied the formation of the earth. Ancient Greeks and Romans, such as Aristotle, Strabo, Herodotus, Xenophanes, and many others, demonstrated the origin of valleys, formation of deltas, presence of seashells on mountains, etc. based on their field observations. Traditionally, the history of the development of landscapes was carried out by mapping the sedimentary and morphological features. For understanding the evolution of landscapes, the golden rule ‘the present is the key to the past’, which was propounded by Scottish geologist James Hutton in 1785, has been followed. This rule assumes that the processes that are visible in action today must have occurred in the past also and can be used to infer the reasons for formation of the landscape in the past.
Hutton in 1785 provided a meaningful thought in regard to earth surface processes whereby soil and rock are eroded from the land to the sea. Later on, John Playfair (1748–1810) not only rescued Hutton’s ideas from relative obscurity but also contributed to the original ideas of Hutton on the nature and behaviour of river systems. Charles Lyell (1797–1875), in his influential three volume treatise Principles of Geology (1830–1833), emphasized the differential erosive powers of rivers and discussed cases where river systems did not divide simply like the branches of a tree, but cut through higher ground or occupied the eroded axes of anticlines.
The word geomorphology was first coined and used between the 1870s and 1880s. It became popular when William Morris Davis propounded the concept ‘geographical cycle of erosion’ in 1889, also known as the ‘Davis cycle of erosion’ or ‘complete cycle of river life’. A lot of work has been accomplished in the field of functional and historical geomorphology. In the present day, many other fields of geomorphology have emerged, including tectonic geomorphology, submarine geomorphology, planetary geomorphology, climatic geomorphology, and modelling geomorphology. Powel in 1875 explored a genetic classification of river valleys and consequently classified them into antecedent, superimposed, and consequent valleys, and propounded the significant concept of ‘limit of maximum vertical erosion’ by streams and proposed the term ‘base level’. Following the concepts of Powel, Mallot in 1928 opined three types of base levels: ultimate, local, and temporary.
Gilbert (1877, p. 112) wrote:
Let us suppose that a stream endowed with a constant volume of water is at some point continuously supplied with as great a load as it is capable of carrying. For so great a distance as its velocity remains the same, it will neither corrode (downward) nor deposit, but will leave the grade of its bed unchanged. But if in its progress it reaches a place where a less declivity of bed gives a diminished velocity, its capacity for transportation will become less than the load and part of the load will be deposited. Or if in its progress it reaches a place where a greater declivity of bed gives an increased velocity, the capacity for transportation will become greater than the load and there will be corrosion of the bed. In this way a stream which has a supply of debris equal to its capacity, tends to build up the gentler slopes of its bed and cut away the steeper. It tends to establish a single uniform grade.
Gilbert was the first geoscientist who opined the concept of the ‘graded profile of a river’ and found the rela...
