The waters that run through the surface of the emerged lands are very important for living beings, although they represent a very small part of the total water on the planet. Its importance lies in the proportion of salts that have dissolved, very small compared to marine waters. That is why we say that it is fresh water.
In general they come directly from the precipitation that falls from the clouds or from the deposits that they form. Following the force of gravity, the rivers run until they flow into the sea or in no-reach areas that we call lakes.
|Top 15 Rivers||Continent||Length (km)||Cuenca (km2)|
The course of the rivers
Rivers are born in springs from groundwater that surface or in places where glaciers melt. From their birth they follow the slope of the land until they reach the sea. A river with its tributaries drains an area called "river basin".
From its birth in a high and mountainous area to its mouth in the sea, the river tends to decrease its slope. Normally the slope is strong in the first section of the river (high course), and very soft when approaching the mouth (low course). Between the two there is usually a moderate slope (medium course).
The rivers suffer variations in their flow, which increases in the rainy or thawing seasons and decreases in the dry ones. Floods can be gradual or very sharp, leading to catastrophic flooding.
Flow variations define the hydrological regime of a river. Temporary variations occur during or after storms. In extreme cases, flooding can occur when the water supply is greater than the river's ability to evacuate it, overflowing and covering nearby flat areas. The water that circulates underground (basal flow) takes much longer to feed the river flow and can reach it days, weeks or months after the rain generated by the runoff.
If it does not rain at all or the average rainfall is less than normal for long periods of time, the river may become dry when the amount of rainwater accumulated in the soil and the subsoil reduces the basal flow to zero. This can have disastrous consequences for the life of the river and its banks and for people who depend on it for their water supply.
The spatial variation occurs because the flow of the river increases downstream, as the waters of the drainage basin and the contributions of the basins of other rivers that join it as tributaries are collected. Because of this, the river is usually small in the mountains, near its birth, and much larger in the lowlands, near its mouth.
The exception is deserts, in which the amount of water lost by filtration or evaporation in the atmosphere exceeds the amount contributed by surface currents. For example, the flow of the Nile, which is the longest river in the world, decreases markedly when it descends from the mountains of the Sudan and Ethiopia, through the Nubian and Sahara desert, to the Mediterranean Sea.
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