Everyone saw ledges along the slopes of the valley with horizontal or slightly inclined platforms - these are river terraces. The first, rising above the channel, is called floodplain, and above - floodplain, no matter how many they are: first, second, and so on. On calm flat rivers there are usually three, four or five floodplain terraces, and mountain rivers have swept the banks on eight or even ten such ledges. This is usually associated with tectonic mobility, that is, earthquakes in young mountains, then river terraces grow.
Origin
According to the geological structure and origin, river terraces are divided into socle, accumulative and erosive. When you need to build a bridge over a river, dam or any other structure that will be affected by the river system, just the geological assessment of the coast is of great importance. It is necessary to precisely establish the intensity and nature of the development of river erosion and sediment accumulation.
Erosion appears in cases when the river erodes the channel and washed the banks. This occurs at different scales throughout the river valley. At the same time, where the banks are eroded, accumulation (accumulation) of sediment occurs, which the river also brings with it. The structure of the valley consists of three main geomorphological elements. This channel, floodplain and river terraces. The channel is the deepest place throughout the valley; it is occupied by a stream of water. Floodplain - part of the valley flooded during the flood. Sometimes floodplains are huge, as, for example, on the Volga - up to sixty kilometers. Elements of the river valley also include river terraces.
What are the terraces on the river and why
Erosion terraces are most often formed on mountain rivers; there are almost no river deposits on them. All types of river terraces are beautiful, but erosive ones are real sculptures. Accumulative are also called nested, leaned, because they consist almost entirely of alluvial material (alluvial deposits). The bedrock of the bedrock is not visible on them.
Such are the accumulative river terraces, for example, on the rivers Don, Volga and many others. Basement terraces at the base necessarily show bedrock, alluvial deposits on them are present only partially. Those traveling by motor ships along our rivers claim that they have not seen anything more beautiful than the long river terrace. Determining the species, in principle, is a simple task.
Sediment accumulation
The river brings the main deposits to the mouth, to the lower reaches, the so-called delta, which is a cone of this stem with numerous arms and ducts. A significant part of the life-giving silt brought by the river also remains on the floodplains, it is there that the grass grows best and agriculture brings the greatest yield. From what is the structure of floodplains, and river terraces change their appearance. They seem to smooth out on the plains closer to the mouth.
Accumulation (accumulation) of the main part of river sediments occurs in the lower reaches of the rivers - deltas, which are a discharge cone with an extensive network of branches and channels. A significant part of alluvial (river) sediments accumulates in riverbeds and floodplains. In different areas, sediments are called differently: delta, old, floodplain, channel.
Types of River Terraces
Here, the leading role in the determination is played by the characteristic of alluvium. The riverbed, for example, on flat rivers, mainly consists of sand and gravel. But the mountain rivers are strong and swift. They carry large fragments of rock (gravel, pebble, boulders), and, of course, all the grooves between the stones are filled with sand and clay. This is the formation of the river valley and the formation of river terraces.
Alluvium on floodplains is always formed during a flood or flood, and therefore consists of loam, sandy loam, clay, sand. And silt from the bottom of the river gives it life-giving. The composition of the floodplain alluvium is heterogeneous, not sustained by properties. These layers are very plastic and compress differently.
The most favorable for any construction are deposits of high terraces and very low, although the latter are weaker. However, old deposits are not suitable for bridges at all. It is there that there is a huge water saturation and the largest amount of silt.
River erosion
In the formation of valleys of absolutely any kind and type, river erosion plays a paramount role. It is deep (bottom) and lateral. The latter leads to erosion of the coast. The level of the basin where the river flows into is called the erosion basis. It is he who shows the depth of penetration into the shore of a water stream.
The development of the river valley goes through several stages. First, water crashes into the rock and forms a steep narrow valley with steep slopes, bottom erosion always sharply dominates here. Further, the profile has already been formed, and lateral erosion is intensified, washing the shore until it collapses. In such places the rivers flow sinuous, looping a lot, forming bends - meanders. Here, the geological activity of the river is extremely variable.
River Valley Formation
The concave section of the valley (usually in our hemisphere is the right bank) is washed away, and the rocks removed are deposited on the opposite - left - bank. So islands are formed, shallows. Meandering among the sediments that she herself applied, the river is forced to form old women - lakes that are filled with silt, other sediments, and this area becomes marshy. At this stage, the equilibrium profile appears at the river.
Our economic activities, especially engineering structures, intensify river erosion. For example, a huge amount of water is discharged into rivers from areas where artificial irrigation has been established, work is carried out to navigate to deepen the bottom, and so on. Another example, when erosion is almost completely weakened, which also has a detrimental effect (especially for spawning fish) on the state of the river valley, when dam overlapping streams are built and reservoirs are created.
River and time
Each river terrace consists of a platform (this is its surface), a cliff (this is its ledge), an edge and a rear seam (this is the edge of the terrace). The river does not always flow in the same way; from time to time it seems to rejuvenate, the energy of its flow is being revived. Then a new cycle of bottom erosion begins, the bottom deepens, the river straightens and new terraces grow on its banks. The most interesting thing is that the new alluvial deposits in the floodplain are lower than the old ones.
The ancient stepped ledges of the floodplain that resist erosion are higher than the new precipitation brought by the river. They are called floodplain terraces, because they hang over the new floodplain. And the number of terraces available shows how many cycles of erosion the river survived, how many times it rejuvenated during its existence. Then the ancient terraces fancifully erode.
However, in the relief, young terraces are always much better visible. They can be embedded, leaned, nested, superimposed and buried. And each terrace is the remainder of the former bottom, which is increasingly being destroyed and eroded in depth. Such terraces in the Alps look strikingly clear when looking at the Inna Valley and the side branches of this river. Below the city of Innsbruck, both steep, wooded shores rise 350 meters to the once formed site.
What the terraces of a mountain river look like
Not always river deposits form the terrace, very often they consist of hard rocks with a small layer of sediment on the surface. In such cases, most often the ledges are piled one above the other, and all of them are the former bottom, as old as the river itself, deepened into stone. These shifts occurred several times — in terms of the number of terraces, although the shifts are characterized by just a ledge, and during periods of weakening of their erosive activity, the river for a long time slowly formed a site.
Mountain rivers always have pronounced terraces compared to the plain, where the terraces are much lower and their ledges are flattened. However, in any case, it is impossible not to see the presence of terraces and the conditions for their appearance are very easily determined. The river terraces in the mountains are all the more characteristic: they are much more developed. When examining such a valley, you need to climb an almost steep ledge to a flat platform, which also has the same ledge. We got up and saw another platform with its ledge. And she will not be the last. So you can trace the entire system of terraces that rise one above the other.
The rivers were wider and deeper
Terraces can be seen not only along the profile of the river, they are most often located precisely along the banks. Each such step breaks off at the bottom of the past valley, the year before last, the pose before last ... In the lower reaches of the rivers, this is especially clearly expressed. Such observations give rise to the understanding that each platform was the bottom in the previous life of the river, before rejuvenation. For many centuries, the river worked to level this terrace, then abruptly went deep and began to level the next level.
In all communicating valleys (near the river and its tributaries), the same number of terraces, as well as the same height. However, other rivers and the number of ledges, and their height will be completely different. Scientists have not yet fully worked out these issues, and many provisions regarding the formation of river terraces are too early to lead to a common denominator. However, numerous studies and observations justify the above conclusions.