How does a meandering river develop
Why do rivers flow in loops?
The Moselle winds through the country like a snake. Their bends and loops made them famous. It is not that unusual for a river to meander along its way. In their middle and lower reaches, rivers often form loops, so-called meanders. Incidentally, the term comes from the Greek name “Maiandros” for the Menderes River in what is now western Turkey.
In these loops, the water flows at different speeds: on the outside, the water has a longer path, so it flows faster and transports more material. Therefore, the outside of the curve will be more abraded. Over time, a steep slope develops on this side, the impact slope. On the inside of the bend, on the other hand, the water flows more slowly, so that the material carried along, for example mud and gravel, settles. A flat sliding slope forms here. The loops of the meander grow further and further outwards due to erosion on the outside and deposition on the inside.
This process intensifies itself more and more: the more the river is curved, the more the flow velocities differ on the outside and inside, the more material is removed and deposited, and the more the loop grows outwards. Even small, random deviations from the straight river bed can develop into strong meanders over time.
As the meanders grow, the individual loops come closer and closer to each other. At some point there may be a meander break. The water then flows back on the direct route. What remains of the old course of the river is a sickle-shaped oxbow lake that "loops" around a circulating mountain. Well-known circulating mountains are, for example, on the Saar or the Moselle. Oxygen arms can also be found in the Rhine Valley, in the Upper Rhine Rift. However, some of the meanders of the Rhine were artificially pierced in order to straighten the Rhine and develop it as a waterway.
What the Karlsruhe engineer Johann Gottfried Tulla began in 1817 was completed this year: the bends in the Upper Rhine are pierced, the Rhine has been straightened and shortened. With the correction, the bed of the river was also deepened. The Rhine is now navigable as far as Basel.
For decades, construction workers had tackled the river with shovels, wheelbarrows and horse-drawn carts. The bed of the Rhine north of Karlsruhe had to be partially re-excavated: a total of 18 punctures now shorten the original path of the water through its numerous meanders. The old river snares were filled up at their inlet. The result: The Rhine no longer flows in many branches, but concentrates in a dead straight main bed. In addition, dams were built along the river. These are intended to protect the residents from flooding. It is also hoped that the dreaded marsh fever, malaria, will be pushed back by draining the swamps.
The straightening of the Rhine initially met with fierce resistance from farmers and fishermen, some of which was broken by force of arms. However, when the straightened areas were spared from the floods after a flood, the project found more supporters and the work was accelerated.
Since the Rhine has been straightened, its course between Basel and Mannheim is 90 kilometers shorter than before. Because the river bed was also deepened, heavy cargo ships can now penetrate up to Basel.
On risks and side effects ...
Tulla had only wanted the best: "... In cultivated countries brooks, rivers and streams should be canals and the management of the waters should be in the power of the inhabitants." The advantages of river correction were clear: They wanted to make shipping easier, the land for agriculture and livestock farming while reducing the risk of epidemics and floods.
But Tulla's project was controversial from the start. Farmers and fishermen feared economic disadvantages. In addition, the further development of the Upper Rhine showed the unforeseen consequences of this encroachment on nature: the Rhine no longer flowed slowly through the loops, but much faster on a straight path. As a result, it dug deeper into the ground, the water table sank and species-rich wetlands dried up.
In addition, the excess water masses could no longer spread in the Rhine floodplains during floods. Instead, they flowed rapidly downstream and threatened the cities and regions there. So Tulla did not really remove the risk of flooding, only moved it downstream.
From trickle to stream - flowing waters
Gushing groundwater emerges from a spring and flows down the slope as a thin trickle or as a small stream: a flowing body of water has emerged. All rivers start out small. As they run towards the estuary, they unite with other rivers and continue to grow until they become a river or even a broad stream. At its lower end, the flowing water flows into another river, into a lake or into the sea.
Streams, rivers or streams - names that come from our lips are precisely distinguished from one another by scientists (geographers). They can be divided according to their amount of water, their length or their width: If the flowing water is less than half a meter wide, it is called a trickle, if it is more than 2 meters wide, it is a stream. If the water swells up to 10 meters wide, it is a river. And if it gets even wider, the river can be called a stream. For example, one speaks of a river near the Amazon or the Nile, but the Rhine and Danube are also rivers.
The amount of water in the running water increases from the source to the mouth. Still, it flows slower and slower downwards. This is because the slope it flows down is steeper at the top than at the bottom. And because the water flows faster at the top and slower down the valley, it can drag more sand and debris along the upper course than at the lower. More sand and rubble is removed from the upper reaches of a river and more is deposited on the lower reaches.
The water cycle
The water on earth is always on the move. Huge amounts of it are constantly moving - between sea, air and land - in an eternal cycle in which not a single drop is lost.
The motor of the water cycle is the sun: It heats the water of the seas, lakes and rivers so much that it evaporates. Plants also release water vapor into the atmosphere through tiny openings. The humid air rises, tiny water droplets gather in the air and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls on the ground, it seeps into the ground, supplies plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.
The water cycle has been around for almost as long as the earth has existed. He ensures that living beings on our planet are supplied with fresh water. And not only that: Without the water cycle, the weather as we know it would not exist.
constant dripping wears away the stone
Deep gorges in the mountains, wide sandy beaches by the sea and wide rivers that meander through meadows and fields - all of these are landscapes that we know well. Because they are so varied, we find them impressive and beautiful.
The sculptor of all these landscapes is the water cycle. Sooner or later, water shapes the surface of the earth more strongly than any other force. It washes away soil after a downpour. It digs into the ground and loosens parts of the rock. It carries earth and weathered rock debris with it down into the valley. Where the water drains off more slowly, it lets go of its burden of silt, sand and rubble. When there is high water, it floods the flat areas of a valley, the river floodplains. Here, too, it deposits fine mud. When the water finally flows into the sea, it works the coasts and forms very different landscapes, for example cliffs or long sandy beaches.
Water also shapes the landscape in the form of ice. If water freezes in cracks in the stone, it bursts the stone. As a glacier, it planes out notch-shaped river valleys to form round trough valleys. And the moraine landscape in the foothills of the Alps with its boulders and boulders is the result of glaciers that formed the subsoil a long time ago.
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