Soil Erosion – A Vicious Cycle
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In The Land Deterioration
By Professor Nils Nykvist
The impact of soil erosion
One example is the so-called Pontine Marshes in Italy, which were drained completely in the 1920s.
Originally the area was a fertile plain, but even before Roman times, soil erosion and sedimentation had changed it into a swamp, with major health problems because of the mosquitoes that spread malaria .
Highly visible evidence of the large amounts of soil eroding away from too heavily exploited slopes is the reddish-brown water in the rivers (Fig. 16). Where rivers flow into the sea, the soil particles are deposited as sediments, which can cover and destroy coral reefs
, and thus decrease the protection against erosion caused by sea waves.
Figure 16. Rivers transport soil particles that erode from the slopes into the sea, where coral reefs can be destroyed. Ruaha River, Tanzania.
Dams built for power plants or for irrigation purposes reduce the sedimentation rate in the ocean, but cause instead big problems through siltation of dams, which limits their life span considerably.
The depletion of vegetation and the resulting erosion of soil have primarily affected areas with the most beneficial climate for humans. The northernmost parts of America, Asia, and Europe were less attractive to farmers because of the colder climate and are therefore still mostly covered with forests. Another reason for this is that the soil largely consists of moraine, with large boulders and stones that impede cultivation and also provide good protection against erosion.
The tropical rainforests have also been less attractive to humans because of the warm and wet climate, until the last century when they have been more and more destroyed by increased cultivation and deforestation.
In other parts of the globe where dense vegetation previously existed, people’s land use has mostly led to a transport of soil particles from the mountains and foothills into valleys, lakes, or seas. As the soil, with its large water-holding capacity, disappears from the heights, the flow of water down the slopes increases, which worsens both erosion on the slopes and flooding in the valleys.
As the thickness of the soil layers has been reduced, crop production has declined further to an almost complete stop, when most of the soil has eroded away
from the underlying rock. The final stage of soil erosion is bare rocks and soil that fill the valleys and lakes, unless it runs into the sea (Fig. 17).
Figure 17. Bare rocks and thick layers of sediments in the valleys are common in the tropics, due to soil erosion when land has been cultivated too much in relation to its carrying capacity. The rocks in the tropics are often rounded because of physical weathering, especially on protruding edges and corners, by differences in temperature between days and nights. India.
The sparse vegetation on the hills due to erosion means lower consumption of water by the plants and more rain running down the slopes directly into the valleys. The relatively rich harvests of grains and fruits which can be obtained in the valleys of dry lands in mountain areas is often due to the supply of water coming from the mountains.
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