Inland waters are an erosive agent of the first magnitude. In the form of rivers that run over the surface, or underground currents, water wears away the materials that pass through and drags the remains in the direction of the sea, leaving them deposited in various places, forming new soils and, ultimately, modeling the landscape.
Water creates waterfalls, grottos, gorges, meanders and deltas. Sometimes it floods certain regions, more or less wide, of the territory. Life has developed more prolifically, since always, on the banks of rivers.
The erosive action of the rivers
Erosion due to running water follows the same stages in which the course of a river is naturally divided. There is a first stage in which the mechanical erosion caused by the water and the materials that it drags is very intense in the high course of the river. In the second stage of transport, mechanical erosion is still active, but chemical erosion begins to act. This takes place in the middle course. Finally, in the low course the sedimentation of the transported materials predominates, the mechanical action is greatly reduced and practically only chemical erosion acts.
The erosive action of a river is due to water energy. It is able to tear out pieces of rock that, being dragged by the current, act like a hammer on the riverbed, releasing new fragments. As the channel is not regular, there are usually swirls that drag sand and gravel, polishing the bottom of the river and creating cavities.
Other times, the steep slope makes the water form jumps, waterfalls or waterfalls, some of which reach up to 1000 meters high. The jump zone gradually recedes upstream as it wears out. In other cases, when the course encounters major obstacles, the water "searches" for the most fragile areas, wears them out and forms gorges or canyons.
In calcareous soils, the appearance of underground caves caused by chemical erosion of water is frequent, which transforms insoluble carbonate into soluble bicarbonate and then dissolves it.
During the heavy rainy seasons or when the thaw occurs, the flow of a river can increase so much that it does not fit within its channel. Then the water overflows along the banks. This phenomenon sometimes occurs gradually, but others do so violently, causing great erosion throughout the territory.
The result of erosion consists of more or less fine materials that water drags along the course of the river. In the middle course they begin to deposit when the force of the current is not able to keep these particles in suspension.
But the erosive force then acts on these deposits and wears them out more in the area where the water velocity is higher, while depositing new materials where it is weaker. The end result is sinuous deposits that we call meanders. With time and floods, the river can once again open its way in a straight line, leaving in its margins lagoons in the shape of a crescent that, over time, usually dry out.
The end of the river erosion process takes place at the mouth of the river, although in some cases the force of the current is able to continue eroding the bottom of the continental shelf and form an underwater valley.
In many cases, especially in large rivers with a lot of erosion, the finest materials are deposited at the mouth forming a delta. The deltas are, therefore, extensive sedimentary lands in which there is a constant balance between the destructive force of the current and the deposit of new materials.
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