Everyone saw ledges with horizontal or slightly inclined platforms along the slopes of the valley - these are river terraces. The first, rising above the channel, is called the floodplain, and above - the floodplain, no matter how many there are: the first, the second, and so on. Calm lowland rivers usually have three, four or five floodplain terraces, and mountain rivers have thrown their banks up to eight or even ten such ledges. This is usually associated with tectonic mobility, that is, with earthquakes in young mountains, then river terraces also grow.
Origin
According to the geological structure and origin, river terraces are divided into basement, accumulative and erosion. When it comes to building a bridge over a river, a dam or any other structure that will be affected by the river system, it is the geological assessment of the banks that is of great importance. It is necessary to accurately establish the intensity and nature of the development of river erosion and sediment accumulation.
Erosion appears when the river erodes the channel and washes away the banks. This happens at different scales throughout the river valley. At the same time, where the banks are eroded, there is an accumulation (accumulation) of sediments, 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 entire valley, it is occupied by the flow of water. A floodplain is a part of a valley that is flooded during a flood. Sometimes floodplains are huge, as, for example, on the Volga - up to sixty kilometers. River terraces also belong to the elements of the river valley.
What are the terraces on the river and why
Erosion terraces are most often formed on mountain rivers, there are almost no river sediments on them. All types of river terraces are beautiful, but erosional ones are real sculptures. Accumulative are also called nested, leaning, because they consist almost entirely of alluvial material (alluvial deposits). The basement of the bedrock is not visible on them.
These are accumulative river terraces, for example, on the rivers Don, Volga and many others. Socle terraces at their base necessarily show bedrock, alluvial deposits are only partially present on them. Travelers on motor ships on our rivers claim that they have never seen anything more beautiful than a long river terrace. Determining the species is, in principle, a simple task.
Sediment accumulation
The river brings the main sediment to the mouth, tolower reaches, the so-called delta, which is a cone of this removal with numerous branches and channels. A significant part of the life-giving silt brought by the river also remains on the floodplains, it is there that grass grows best and agriculture brings the greatest harvest. From what the structure of the 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 sediment occurs in the lower reaches of rivers - deltas, which are a fan with an extensive network of branches and channels. A significant part of alluvial (river) deposits accumulate in river beds and floodplains. In different areas, sediments are called differently: deltaic, oxbow, floodplain, channel.
Views of river terraces
Here, the characteristic of alluvium plays a leading role in determining. Run-of-river, for example, on flat rivers, mainly consists of sand and gravel. But the mountain rivers are strong and swift. They carry large rock fragments (gravel, gravel, boulders), and, of course, all the grooves between the stones are filled with sand and clay. This is how the formation of the river valley and the formation of river terraces.
Alluvium on floodplains is always formed during high water or high water, and therefore consists of loam, sandy loam, clay, sand. And the silt from the bottom of the river gives it vitality. The composition of floodplain alluvium is heterogeneous, not consistent in properties. These layers are very flexible and compress differently.
Deposits are considered the most favorable for any constructionhigh terraces and very low ones, although the latter are weaker. However, oxbow deposits are not at all suitable for bridges. It is there that there is a huge water saturation and the largest amount of silt.
River erosion
River erosion plays a primary role in the formation of valleys of absolutely any kind and type. It is deep (bottom) and lateral. The latter leads to erosion of the coast. The level of the basin where the river flows is called the erosion base. It is he who shows the depth of cutting into the bank of the water stream.
The development of the river valley goes through several stages. First, the water cuts into the rock and forms a steep narrow valley with steep slopes, where bottom erosion always dominates sharply. Further, the profile has already been formed, and lateral erosion intensifies, washing away the coast before its collapse. In such places, the rivers flow meandering, winding 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 it is the right bank) is washed away, and demolished rocks are deposited on the opposite - left - bank. This is how islands and shoals are formed. Wriggling among the sediments, which she herself caused, the river is forced to form oxbow lakes, which are filled with silt and other sediments, and this area becomes swampy. At this stage, an equilibrium profile appears near the river.
Our economic activities, especially engineering structures, increase river erosion. For example, a huge amount of water is discharged into rivers from those areas where artificial irrigation has been established, work is being done to deepen the bottom for navigation, and so on. Another example is when erosion weakens almost completely, which also has a detrimental effect (especially for spawning fish) on the state of the river valley, when dams blocking the flow 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 be rejuvenated, the energy of its flow is 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 here is that the new alluvial deposits in the floodplain are lower than the old ones.
Ancient stepped ledges of the floodplain, resisting erosion, are higher than the new sediments brought by the river. They are called terraces above the floodplain, because they hang over the new floodplain. And the number of existing terraces shows how many cycles of erosion the river has experienced, how many times it has rejuvenated during its existence. Then the ancient terraces bizarrely weathered.
However, young terraces are always much better visible in the relief. They can be embedded, leaning, nested, superimposed and buried. And each terrace is a remnant of the former bottom, which more and more collapsed and eroded into the depths. They look amazingly clearterraces in the Alps, if we consider the valley of the Inn and the side branches of this river. Below the city of Innsbruck, both steep wooded banks rise 350 meters to the once formed platform.
What the terraces of a mountain river look like
River sediments do not always form a 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, ancient, like the river itself, deepened into stone. These shifts occurred several times - according to the number of terraces, although it is the ledge that characterizes the shift, and during periods of weakening of its eroding activity, the river formed a platform for a long time and slowly.
Mountain rivers always have pronounced terraces compared to the plains, where the terraces are much lower and their ledges are smoothed. However, in any case, it is impossible not to see the presence of terraces and it is quite easy to determine the conditions for their appearance. 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 sheer ledge to a flat area, which also has the same ledge. We got up - and saw another platform with its own ledge. And she won't be the last. So you can trace the entire system of terraces that rise one above the other.
The rivers were wider, but they are deeper
Terraces can be seen not only along the profile of the river, they are most often located along the banks. Each such step breaks off at the bottom of the last valley, before last, behindthe year before last … In the lower reaches of the rivers, this is especially pronounced. 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 deeper and began to level the next level.
All connected valleys (near the river and its tributaries) have the same number of terraces and the same height. However, for other rivers, both the number of ledges and their height will be completely different. Scientists have not yet fully worked out these issues, and it is too early to bring many provisions regarding the formation of river terraces to a common denominator. However, numerous studies and observations fully justify the above conclusions.
