River erosion
Introduction
Fluvial waters (or rivers) constitute an erosive agent of the first magnitude. Continental water flows, to a large extent, in the form of rivers that run on the surface, or underground currents, wearing away the materials through which they pass and dragging the remains or sediments in the direction of the lower parts of the relief, leaving them deposited in various places. forming terraces, waste cones and, ultimately, modeling the landscape.
The water from river currents can create waterfalls, grottoes, gorges, meanders, canyons, deltas, estuaries, among others. Sometimes it floods certain more or less large regions of the territory, causing economic disasters and victims, despite which, human beings have almost always settled on the banks of rivers, lakes or springs, in order to
to ensure an adequate supply of water.
The erosive action of rivers
Erosion is due to the flow of water and is mostly proportional to the slopes of the relief and, in the case of rivers, to its longitudinal profile, which is why it is usually divided into the three parts or sections into which the course of a river is naturally divided (upper, middle and lower course). As the book by Mª José Aguilera Arilla and others ([1]) points out, the waters of a river transport their load in two ways:
• - Superficial, by dissolution (clays) and suspension of solid materials.
• - At the bottom of the channel due to dragging (sands) and saltation (blocks and boulders).
Thus, there is a first stage in the upper course of the river where the mechanical erosion caused by the water and the materials it carries is very intense. In the second stage, transportation, mechanical erosion is still active but sedimentation begins to act. Finally, in the lower course the sedimentation of the transported materials predominates, mechanical erosion is greatly reduced and practically only sedimentation acts.
The erosive action of a river is due to the energy of the water. It is capable of breaking off pieces of rock that, when dragged by the current, act like a hammer on the riverbed, breaking off new fragments. As the channel is not regular, eddies usually occur that drag sand and gravel, polishing the river bottom and creating cavities. These eddies magnify the erosive power of water which is due, almost exclusively, to turbulence.