Why is river basin important

Catchment area and watershed

If a raindrop falls to the ground in Furtwangen in the Black Forest, it flows over the river Breg into the Danube. After a long journey, he finally ends up in Romania in the Black Sea. If, on the other hand, the drop falls only a few kilometers north of Furtwangen, it will wash into the Atlantic at some point - if it does not evaporate or seep away. Because this area is drained over the river Elz into the Rhine and into the North Sea.

Every river has an area of ​​land that - like a funnel - leads the water towards it. The region from which a river feeds is called the catchment area. Because the water always flows from higher elevations to lower elevations, the catchment area is limited by mountains and mountain ridges. These separate the catchment area from other river systems. It can happen that raindrops that fall only a few centimeters apart from the sky are washed in a different direction and land in seas that are far apart.

The boundary that separates one river basin from another is the watershed. The catchment areas of the Danube and Rhine, for example, are separated from each other by the European watershed. This watershed, which is thousands of kilometers long, also limits many other catchment areas: In France, it separates the Rhône, which flows into the Mediterranean, from the Loire, which flows into the Atlantic.

A watershed doesn't have to be on the same line forever. If a mountain range rises or is eroded by glaciers and weathering, this changes the landscape and the gradient. A river that digs deep into the terrain can also "dig" the water away from another river. In the southern Black Forest, for example, the Wutach, a tributary of the Rhine, has tapped some of the source rivers of the Danube over time, thereby shifting the watershed between the Danube and the Rhine. Or as a result of the plate movement, a landscape can even tilt in a different direction in some places, so that the water eventually drains into another river.


The Nile is the longest river on earth. Or is it not? Some researchers doubt this record, claiming that the Amazon is longer. But it is not that easy to measure a flow precisely.

The front runner so far was the African Nile with a length of 6,671 kilometers. It rises from two source rivers: the blue Nile from the Ethiopian highlands and the white Nile from the mountains of Rwanda and Burundi. On its way it flows through Tanzania, Uganda and Sudan and flows into the Mediterranean in Egypt. So long so good. But some researchers now claim that the Amazon is longer. So far, the most gigantic river in South America had an official length of 6,437 kilometers. If you were to add its tiniest source river, however, you would get around 7,000 kilometers. This would clearly make the Amazon longer than its African competitor. And now? The scholars cannot agree. In textbooks and reference works, the Nile is still number one of the longest rivers, while the Amazon is only second. But there is one thing you cannot dispute about the Amazon: it is by far the most water-rich river on earth: two thirds of all river water on earth flows through its bed. At its mouth, 200,000 cubic meters of water flow into the Atlantic every second - that's around a million full bathtubs!

The Nilometer

The life of the ancient Egyptians, who lived around 5,000 years ago, was determined by the Nile. They probably didn't care if their river was longer than any other. What was important, however, was that they could farm on its banks - and that in the middle of Egypt, a land of the desert.

Once a year the Nile burst its banks and spread black and fertile mud on meadows and fields - a perfect fertilizer. Because the river and its water level were so crucial to their lives, the ancient Egyptians built nilometers: from these markings they could see whether the water level was falling or rising. Even taxes were measured according to the nilometer: a high water level promised good harvests and meant high taxes. If the Nile carried little water, the taxes that the farmers had to pay also fell.

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, over 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.


It doesn't matter whether it's raining, hailing or snowing - it's “to blame” for the clouds. Because without clouds there would be no precipitation. However, it depends above all on the temperature, whether there is a downpour or a wild snowstorm.

Most of the precipitation on earth falls as rain. When small water droplets collide in a cloud, they combine to form larger and heavier droplets. Are they too heavy to float; if the temperature is above 0 ° Celsius, they fall on the earth as rain.

When the air temperature is very low, precipitation no longer falls as rain, but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds with water droplets. If the ice formations are big and heavy enough, they dance down from the sky like snowflakes.

If, on the other hand, strong updrafts pull through a towering cloud, there can be hail. Small drops from the lower part of the cloud are swirled upwards, where it is colder than below. There they freeze to form small ice balls, about the size of the heads of a pin. These ice balls are called sleet. If in a very high thundercloud with a strong wind the globules in the cloud are flung up and down several times, more and more raindrops freeze onto the globules. The more the ice balls are driven around in the cloud, the bigger and harder they become. From half a centimeter in diameter, these ice balls are called hail. Hailstones can grow larger than tennis balls and have often already done a lot of damage.

In contrast to precipitation that falls from clouds, there is also precipitation that occurs close to the surface of the earth. If the temperature on the ground drops overnight, the air can absorb less moisture. The excess water then settles on the ground, on plants or on objects: the moisture is clearly visible as dew. If the temperature falls below 0 ° Celsius at night, the water freezes on the objects and forms a whitish layer. Then one no longer speaks of dew, but of frost.

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. On their course 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.

How are valleys formed?

River and valley are inseparable. But why? How do these elongated hollows, called valleys, come about at all? A valley forms wherever water runs off in small streams or large rivers. This is because flowing water digs deeper and deeper into the subsoil. The soil on the sides slides down towards the river bed. A slope forms to the right and left of the watercourse; this creates a valley along the river.

Valleys can look very different: steep walls or gentle slopes, wide valley floors or just enough space for the river. The shape depends on how strongly the water attacks the bottom and the side walls and how stable the rock is.

It's steep in the mountains, at the headwaters of a river. The water shoots down the mountain with force. Because of its high speed it transports a lot of sand and debris there. With this rubble, it grinds the ground heavily and can dig itself deep. This creates rather narrow, deep valleys.

Towards the mouth, the river widens and carries more and more water. As the terrain becomes flatter, the water flows more and more slowly. For this reason, the lower reaches of the river gradually deposit the cargo it has carried along with it on the ground again. Here erosion takes place more on the side walls, so that broad, flat valleys are created.

The rock through which the river flows is also responsible for the different valley shapes: water and rubble dig into solid rock without a lot of rock sliding down the sides. This creates valleys with steep or even almost vertical walls. Soft rock layers, on the other hand, slide quickly and lead to flat slopes.

Valleys are divided into different types based on their shape: Narrow valleys with steep walls are called canyon, with vertical walls one speaks of one Klamm. Narrow valleys with gentler slopes are called Kerbtal or V valley designated. If, on the other hand, the valley floor is significantly wider than the river, it is a Sohlental, or - with steep walls - by one Kastental.

A special form of valleys are Canyons. Here the water has dug its way through different layers of rock that lie on top of each other like several layers of cake. Some layers were easily removed by the river, they were washed out wide and round, the more resistant layers broke off steeply and angularly. The result is a valley, the side walls of which slope down like steps towards the river. A famous example of such a valley is the Grand Canyon in the US state of Arizona.

Mountains in motion

Mountains rise up mighty and rigid. It seems as if nothing and no one can move them from the spot. But that's not true: mountains are constantly in motion - albeit so slowly that we cannot see the change with the naked eye.

The reason for this: the plates of the earth's crust move. And when two of these plates collide, the rock is compressed, pushed and piled up. Similar to a car accident, mountains fold up at the edge of the slab on impact. Mountains and valleys are thus a “crumple zone” of the slabs colliding. However, this does not happen suddenly like in a car accident, but much more slowly than in slow motion. The result is fold mountains like the Andes in South America. There the oceanic Nazca plate slides under the South American plate and squeezes the rock with incredible force. The elongated mountains of the Andes piles up, stretching over a distance of 7,500 kilometers. The Andes are the longest unearthly mountain range in the world.

However, there are also huge mountains below sea level. They run through the middle of the oceans. They, too, owe their existence to the movable plates. Where two plates move away from each other on the ocean floor, magma penetrates from the mantle through the oceanic crust. The hot rock slurry cools on the sea floor and piles up to form mountains that are thousands of meters long: the mid-ocean ridges. Where the lava reaches sea level and swells beyond it, islands like Iceland arise. These mountains, which are born in the sea, are the longest on earth. The Mid-Atlantic Ridge stretches from north to south through the entire Atlantic - about 20,000 kilometers long.