Why are shivaiks prone to erosion and landsides
Answers
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To the Himalayan enthusiast the Siwaliks are often little more than a name, an 'insignificant range of foot-hills', to be crossed impatiently in answer to the call of the grander ranges beyond. On the other hand, modern geography is closely concerned with the economic values of geographical features, and to-day people take much more interest than formerly in their surroundings. It is hoped, therefore, that this paper may stimulate some further interest in the important, and perhaps notorious, part that this low range is playing in Punjab economics.
The damage caused by the great floods which periodically sweep down the Punjab rivers from the high mountains attract large headlines in the newspapers; and, though they may bring great havoc, they are, in the main, caused by unusual phenomena, such as the breaking down of glacier dams and cloudbursts, which are beyond the control of man. Yet nearer at hand, under the very eyes of the public, the Siwaliks and outer ranges of the Himalaya are being slowly converted into a source of destruction, the more dangerous owing to the insidious nature of its growth.
The object of this paper is to show how men, by misguided efforts to better their condition or under the stress of economic pressure, have contravened the laws of nature, thus meriting a punishment which only too often falls upon the heads of those innocent of the offence. The paper begins with a description of the role of mountains and forests as agents for the supply and distribution of water. A brief discussion of the geology of the Siwalik range follows, and the rest of the paper deals specifically with erosion in the Siwaliks of the Hoshiarpur district in the Punjab.
Mountains as a source of water-supply, and the role of mountain-forests.
Throughout the world mountain ranges play an important physical part in the water-supply of man. They act as barriers to the passage of winds laden with moisture received by evaporation from seas, from inland lakes, and from the land itself. The warm air currents, impinging against the mountain slopes, are deflected upwards; they are cooled, and they precipitate their moisture in the form of snow or rain.
In northern India, during the winter and early spring, depressions from the west penetrate far into the Himalayan barrier; heavy snow falls on the inner hills, but little rain is experienced on the outer slopes at this season. Snow melts during the late spring and early- summer, causing a gradual rise in the rivers and streams. At midsummer the rains of the South-west Monsoon, mainly from the Bay of Bengal branch, strike the mountain barrier, and bring rain to the successive ridges south of the Great Himalayan range, where the remaining moisture is given up. The rain is partly absorbed by the soil, but the greater part, as surface run-off, finds its way down through countless streams and rivulets to the main rivers, and thence down to the plains of India where it serves the many requirements of man.
Winter snowfall on bare alpine land melts and drains away slowly; but it does not require much imagination to realize what happens to rain-water falling on bare mountain-sides. The water dashes down the slopes with ever-increasing speed, carrying away the thin soil and discharging itself into the low-lying country with uncontrollable violence. Naked rock is left behind and the land receiving the discharge of such torrents is subject to constant floods. Fortunately for man, it is a provision of nature that certain regions of the globe are covered with forest, and mountainous country is one of them. Below the snow-line, the forest clothes the hill-sides down to the plains and controls the flow of the water from the catchment areas in the hills.
A forest consists of trees or scrub, growing together with bushes, herbage or grass and humus on a soil, of varying depth, derived from the disintegration of the underlying rock. The types of vegetation found in mountainous regions depend mainly on the amount of rainfall and elevation. They vary between wide limits, from dense fir forest to little more than grass, but all play their part as agents of protection. The chief roles of the forest are-
(1) reduction and control of surface run-off,
(2) conservation of moisture, and
(3) protection of the soil from erosion.