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River dynamics

The river is a dynamic system which is constantly changing. It shapes its course as well as the land it crosses, leaving its typical marks. The valleys, where a river erodes and digs its course, are shaped like a “V”. The sides of valleys, which are composed of hard rocks, are not subject to widespread erosion but to linear erosion (that is the erosion of the course itself), which produces deep valleys, gorges and canyons with vertical walls.

In others areas, the river does not erode, but rather transports and deposits fine material. This way it produces alluvial plains and terraces, levelling the V-shaped valleys and filling them with sediments.

If we observe the mechanisms of erosion – transportation – sedimentation, we can better understand the way a river shapes its course and the territory it crosses and the way any intervention on the river-bed changes the profile of the river.

This profile, which starts from a spring and flows toward the sea or a confluence, is dynamic. It is determined by the balance between erosion and deposit of material, especially in the mountain stretches with steep slopes, and is changed with external disturbance. A flat river is a good example of sedimentation where the slope is gentler, the flow of the river is slower and the course is winding.

The meanderings of a river (question 11) result from the combined action of sedimentation and erosion: the external concave bank is quickly eroded and becomes steep. The eroded material from the concave bank deposits on the internal side of the meander, because the current cannot bring into suspension all of the materials in this area. A bank with a gentle slope is formed in this internal convex side. This way, the meanders tend to become more and more pronounced, because the combined action of erosion and sedimentation becomes a constant process once it is started. When a curve is too marked, the river can skip a meander and go back to a linear course, creating a dead branch.