Flooding from a river takes place after torrential rains. The rain swells up the river and the water overflows, flooding land may be far away from the storm.
In cold regions flooding takes place when the snow beings to thaw. The snow melts and flows down the mountain sides and into the river. The river may not be able to contain so much volume of water and it overflows causing floods. In some cases, huge areas flood because the rivers flow north towards the Arctic Ocean. The snow melts first in the warm south and the rivers fill up. The north, however, is still cold and the river mouths are blocked by ice. This results in flooding.
Sometimes floods are caused by sudden landslides that may block a river resulting in the overflowing of the river. There have been several causes where man-made dams collapse and flood the whole area. A fractured huge pipeline can also result in minor floods. Some rivers like the Huang He in china are clogged with mud. This river, also known as the yellow river is said to be the muddiest river of the world. Each year it washes about 2000 million tones of mud down the valley.
It first floods due to the melting snow and then again after a few methods due to the heavy rains. Since the river is full of mud the river cannot all the extra water resulting in floods. Today, huge sluice gates control the floods to some extent. Flooding can also occur at the coast in stormy conditions when there are exceptionally high tides. Low-lying coastal areas risk being flooded by the sea due to a combination of high tides and strong winds. The Thames flood barrier was built to prevent the flooding of London from the sea.
Rocks are also responsible for floods. During heavy rains, permeable rocks like chalk soak in the water. On the other hand, impermeable rocks like clay do not allow water to seep through and the water drains off into streams and rivers. Cities of concrete and tarmac work in the same way. The water drains off into man made drains and channels.
The area that is periodically flooded by a river is called its flood plain. When the water spills out of the river some alluvium is deposited on the banks to form levees, which are, raised banks. This water then slowly seeps into the flood plain depositing a new layer of rich fertile soil. The Egyptians welcome the floods every year as the river Nile deposits its rich alluvium on its banks. Similarly the Ganga and the Niger in mail have spread fertile mud over their flood plains thus increasing the agricultural productivity and helping the local economy.
As the river nears the sea it reaches the flat lowlands and its meanders become wider and wider. After a flood the river sometimes has to change its course, leaving its earlier bend in its route. The water body that is left behind is called on oxbow lake, as it is shaped that way.
Floods can be very disastrous and can wipe out whole villages. Several flood controlling methods have been employed in different places. Dikes, levees and huge sluice gates try to control the onrush of the water. Emergency channels and flood ways have been built to remove the excess water. Huge reservoirs and dams have been constructed to hold back the water. Dredging of the muddy rivers is undertaken frequently. Even with all these measures, we still fear floods, which can have such catastrophic results.
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