River valleys as one of the relief forms of the earth's surface are the subject of study of geomorphology. The range of issues of interest to this geological and geographical discipline includes the study of the origin, evolution and structure of river valleys, their dynamics and characteristic features.
What is the river valley?
River valleys are among the negative landforms. This is the name of the surface areas characterized by a relative decrease in level. The valleys are characterized by a linear elongated shape, to some extent complicated by sinuosity. Throughout their length, the valleys have a generally uniform slope.
The structure of the river valley depends on a combination of physical and geographical conditions and geology features inherent in the area through which the river flows. The combined action of these factors can change over time, and such changes also affect the morphology of the valley.
Genesis and development of valleys
The origin of river valleys maybe associated with the presence of tectonic conditions conducive to the formation of a river (folds and faults of various types) or with the movement of glaciers. However, the main and necessary factor in the emergence of the valley is the work of flowing waters, their erosive activity.
There are such types of water erosion that determine the structure of the river valley, as:
- Bottom, as a result of which the stream crashes into the surface and builds up a depression. Predominant in the early stages of valley development, when the river is still being laid.
- Lateral, expressed in the washing away of the banks by the water flow, which leads to the expansion of the valley. This type of erosion is more pronounced when the river enters the stage of maturity. At this time, the slope of the river decreases significantly as it approaches the equilibrium profile relative to the level of the basin into which it flows (the so-called erosion basis). Under the influence of lateral erosion, the water flow forms meanders - meanders of the current.
When the river begins to silt, overgrow, form a large number of swampy old women, this means that it has reached old age. The river valley becomes extremely wide, and the current slows down. The profile of such an old river is already as close as possible to the basis of erosion.
Elements of valley morphology
In the process of evolution of the river, the main elements of the structure of the river valley are formed. Let's briefly characterize each of them.
- Current - a section of the valley through which the main flow of water is carried out. It is occupied by the river during interflood periods.seasons. The stable elements of the channel are the bottom and the banks.
- Floodplain - a more elevated section of the valley, flooded during the flood. Sometimes the floodplain is called the meadow terrace of the river. Within its limits, there is a near-channel or alluvial swell formed by sandy and silty deposits.
- Terraces are staggered former floodplains that were filled with water at previous stages of the valley's development, when the river cut into the surface to an even lesser extent. Terraces may be open or buried by subsequent sedimentation.
- Indigenous shores are the boundary edges of the valley. Their level exceeds the upper, earliest river terrace.
The channel and the floodplain are attributed to the bed, or the bottom of the valley, and the terraces, together with the primary banks, to its slopes.
River valley profiles
Depending on the cut under which this form of land relief is considered, the structural features of the longitudinal and transverse profiles of river valleys are distinguished.
Longitudinal profile is a section of a valley drawn along its length along a line called a thalweg connecting the lowest points of the bed, that is, along the greatest depth. The longitudinal profile reflects such parameters of the river valley as the dip - the difference in heights in a particular section and along the entire length - and the slope, understood as the ratio of the dip to the length of the section under consideration.
The cross profile is a section of the valley in a plane perpendicular to its direction. This is an important indicator of the morphological type of the river valley.
Typesprofiles of valleys along the longitudinal section
In the structure of the longitudinal profiles of river valleys, several types are distinguished depending on how the slopes are distributed along the length of the valley:
- A straight profile is formed when the river along its entire length has a slope that is close to uniform. Such a structure of the valley can be found mainly in small rivers.
- The stepped profile is characterized by the difference in slopes in certain parts of the valley. It is inherent in rapids rivers, water streams that form waterfalls, reaches or flow through flowing lakes.
- A gently concave profile has the general appearance of an unevenly concave curve. Near the source, this line is steeper; as it approaches the mouth, it becomes more and more flat. Such a profile of the bottom develops in mature rivers, the course of which is mostly confined to flat, tectonically calm areas.
- Faulting or convex profile, observed quite rarely, has a slight slope in the upper reaches of the river and a significant one in the lower reaches of the valley.
The greatest degree of approximation to the ideal equilibrium profile is characteristic of the smoothly concave shape of the valley bed, however, in reality, due to the combined action of many factors, the profile always has elements of a stepped structure.
An example of a complex profile demonstrates the structural features of the Mississippi river valley - one of the greatest water arteries in the world. The river valley is morphologically divided into the Upper and Lower Mississippi, which differ in structure. The first one hasstepped profile with many thresholds and rifts; the second is a pronounced flat valley, wide and gently sloping. Due to intense silting, the river repeatedly changed its course and the place where it flows into the Gulf of Mexico - this phenomenon is known as "delta wandering".
Complex valleys, as if made up of sections with different structure and origin, are inherent in almost all major rivers: the Amazon, Nile, Danube, Volga, Yenisei and many others.
Classification of valleys by transverse profiles
- V-shaped valley in the section has a triangular shape. Such a profile is also called undeveloped. Valleys of this type are, as a rule, young, with intensive deepening of the bottom and destruction of slopes due to the processes of collapse, talus, etc. These valleys do not have terraces and a pronounced floodplain.
- Valley with parabolic profile. Its bottom is quite rounded, the slopes are long, but do not show a stepped-terraced structure. Their formation is associated with the work of powerful water flows, creating a large amount of various kinds of loose deposits.
- Trapezoid valley has well-developed terraces and thick sediments. The presence of a stepped terraced structure testifies to a complex and long history, during which eras with a predominance of erosion, which expanded and deepened the valley floor, alternatedperiods of sedimentation. The width of the valley can be an order of magnitude greater than the width of the riverbed.
- The valley in the form of a gutter differs from the previous type by an even greater width and gentler slopes. In the history of such valleys, epochs of accumulation of sedimentary deposits prevailed.
- Planimorphic type of valley with indistinct boundaries, a large number of channels and arms is typical for large, very old rivers.
Geology and structure of river valleys
Tectonics of the area plays a very significant role in shaping the features of the river valley. The presence of structures such as faults or faults contribute to its formation, and the crushing zones encountered by the water flow on its way accelerate the erosion process. The nature of tectonic folds and their orientation relative to the axis of the valley affect the symmetry of its transverse profile. Thus, valleys formed along faults are often asymmetric, while those passing along an anticline or synclinal fold, on the contrary, are symmetrical.
The structure of the valley also depends on the composition of the rocks that make up its bed, since rocks of various types are susceptible to erosion to varying degrees. Compliant clayey rocks facilitate erosion, deepening of the bottom and washing away of the banks. If the flow hits rocky outcrops of stable rocks, rapids form in the longitudinal profile of the valley.
Practical significance of the question
Knowing the structure of the valley is necessary when designing hydraulic structures, for example, when calculating the strength characteristics of dams and the power of hydroelectric power plants. It is no less important in the construction of bridges, roads, and in the development of areas adjacent to rivers.
Studying the morphology of valleys is also important for the correct assessment of the resistance to water erosion of land within river valleys. Ancient, buried river valleys are being explored for structure in the exploration of groundwater and alluvial mineral deposits.
Establishing the stratigraphy of Quaternary deposits, carrying out paleogeographic reconstructions and many other scientific issues, in turn, cannot do without taking into account the structure of river valleys. As you can easily see, it is required when solving the widest range of academic and applied problems.
