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Question

(a) How does deforestation cause soil erosion? 
(b) Explain how deforestation leads to frequent flooding of rivers.


Solution

(a) The roots of trees bind the particles of topsoil together. Due to this binding of soil particles, the wind and water are not able to carry away the topsoil due to which the topsoil does not become loose quickly. When the forests are cut down during deforestation, there are no roots of trees which can bind the soil particle together and prevent them from being carried away by strong winds or flowing rainwater. Since there is no tree cover on the soil to soften the effect of heavy rain, the bare topsoil becomes loose quickly by the force of falling rainwater and erode rapidly. This is called erosion. ​​​​​​

(b) The roots of forest trees help in absorbing some of the rainwater and make it percolate into the ground. This reduces the amount of rainwater which rushes quickly into rivers and flooding does not occur. When the forest trees are cut down, the percolation of rainwater into the soil is reduced. A lot of rainwater from deforested soil rushes into the rivers quickly, causing floods. Deforestation decreases the water holding capacity of soil which leads to floods.

The eroded soil is carried by flowing rainwater into rivers. The eroded soil keeps on collecting on the river bed and decreases depth of the river gradually. Due to decreased depth, the water carrying capacity of the river is reduced. When heavy rain occurs, the river is not able to carry away all the rainwater quickly. The excess water overflows from the banks of the river into the adjoining areas causing the flood. These flood damage standing crops, houses and even drown people living in nearby areas.


Biology
Science for Eighth Class
Standard VIII

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