When we think about the rivers it is really difficult to visualize a perfectly straight stream cutting across a plain from one side to the other. On the contrary, what our brain will probably figure is a sinuous watercourse with wide curves that repeat one after the other, calls anse or meandersrecalling the image of a large snake. Sometimes, then, the so-called river bars are found in correspondence with the meanders. But why do rivers always have this shape instead of proceeding in a straight line? It’s a question of geography… and physics! It all depends on morphology of the territoryfrom the water speed and by the combined phenomena of erosion And sedimentation.
A premise: how the course of a river proceeds
Before proceeding with the scientific explanation of what happens along a river it is good to review quickly what is a river. A river is a watercourse which, due to the Earth’s gravitational forceflows from a position of higher altitude to one of lower altitude, until it flows into a lake, a sea or another watercourse.
There water speed it can therefore vary considerably in the long journey from the source to the mouth: where the river will flow along surfaces characterized by a height difference greatertherefore steeper, the water will have a speed and, consequently, a “force” larger. Where, however, the river will flow along surfaces characterized by differences in height minorsso less sloping, the water will slow down and move at speed minors.
Depending on the speed at which it moves, the water in a river is capable of carrying more or less heavy materials: when the water flows very quickly, as for example in steep mountain gorges at high altitudes, its strength is capable of moving stones And pebbles. As the speed of the water decreases, it will abandon these heavy materials along its course and will transport only the lighter ones, such as sand el’clay.
This is the main reason why many plains alluvialthat is, those plains created by the accumulation of debris transported by rivers, are made up of very materials coarse in the parts further upstream, such as at the foot of the hills, and from much more materials ends in the parts further downstream, for example along the coastal strips near the mouths, where the river water flows at lower speeds.
How river bends are formed
Along the waterways, the sinuous “S” shape formed by a succession of individual curves, called anse or meandersoriginates mainly in the heart of alluvial plains, where rivers flow at low speeds along minimal slopes. At higher altitudes, the higher slopes will in fact force the river water to maintain a more sustained speed, making it progress more easily along a trajectory straight.
The formation of curves along a river originates from the dynamic and combined action of two phenomena that act simultaneously: theerosion and the sedimentation. Various factors can lead to the formation of a first curve along the watercourse: alterations to the riverbed, morphology of the terrain, characteristics of the soil, natural events… But once a first curve tends to form, for a physical reason, this it will only be able to continue to increase its breadth.
Because of the Force centrifugethe greater speed of the water along the outermost part of the curve, the part concavewill determine a erosion zone which will cause the river to erode the bank and move further and further outwards. On the contrary, along the internal part of the curve, the part convexthe force of the water is less and the loss of energy causes the release of eroded sediments resulting in a storage area.
The progressive accentuation of the phenomenon will lead to the formation of a curve increasingly markedwhose continuous expansion may lead to “meander cut“: the watercourse digs a new riverbed between the two increasingly closer points which determine the beginning and end of the curvature, causing a definitive separation of the “former bend” from the main course of the river.
