Brief Essay on the Formation of Lakes

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Lakes are inland bodies of water found in natural depressions surrounded by higher ground. They vary in size from a pond to larger ones of hundred square kilometres in area and may contain either fresh or salt water. Lakes occupy about 2.7 million square kilometres or nearly 1.8% of the earth’s surface.

In the case of lakes, they mostly occur above the mean sea level whereas the swamps are marshy lands where the water-table has just reached the land surface. Basins, even though are inland water bodies, have their bottom below the water-table always.

Formation of Lakes

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Majority of the lakes are formed due to the action of various natural agencies and some are of tectonic origin. The modes of formation of the lakes are as follows-

1. By River Action

(i) At the foot hill of a waterfall due to the impact of water or due to the whirling action of hard stones carried in river eddies hollows are formed below the general slope of the river bed. These hollows are small and are not of much depth.

These hollows sometimes contain water to form lakes which gene­rally come into existence after the river dies away.

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(ii) As a result of rock-falls or landslides which commonly occurs in mountainous regions, the river valley may be completely blocked. Thus the flow of the river is checked giving rise to a lake in the upstream part of the valley.

(iii) Sometimes the sediments brought by the tributary river forms a bar across the main-river, thereby block its flow and gives rise to a lake.

(iv) If the bedrock is soluble, the solvent action of the river may produce a hollow of considerable dimensions and forms a lake.

(v) The depressed areas in dried up river beds, sometimes contain a sheet of water to the extent of producing a lake.

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(vi) The surface of the flood-plain is uneven due to irregular depo­sition of silt and clay etc. in the form of ridges and the depres­sions left in between may contain water, even after the flood waters recede, to form lakes.

(vii) In the deltaic regions of big rivers, due to extensive branching of rivers and due to subsequent deposition many of the branches are transformed in to lakes.

(viii)Due to intensive meandering of the river, sometimes sections of the river are cut off from the main channel and form ox-bow, horse-shoe or cut-off lakes.

2. By Action of Wind

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(i) As we know, the erosive action of wind is more pronounced in the absence of vegetation. Due to intense degree of deflation, hollows are excavated on the land surface, which may be filled with water during a storm of rain giving rise to a lake.

As it occurs mostly in desert regions, permanent lakes are not formed.

(ii) Sometimes lakes are formed between sand dunes.

(iii) In desert region, basins surrounded by mountain ranges may become the site where the drainage lines converge to form lakes. Such lakes are broad, shallow and ephemeral in nature and are called playa lake.

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3. By Glacial Action

Lakes of glacial origin are mainly due to-

(i) the existence of pre-glacial topography;

(ii) due to glacial erosion;

(iii) due to damming or blocking by glacial or glacio-fluvial deposits;

Basins hollowed in solid rocks by glacial erosion and cirques are often occupied by small lakes (called tarns). The major glacial troughs usually contain large, elongated, trough lakes are referred to as finger lakes.

Piling up of morainic matter across their valleys cause the forma­tion of lakes. The terminal moraine, as we already know, acts as a barrier and when the glacier retreats may hold water to form a lake.

The kettle-holes left by melting of masses of stagnant ice often produce lakes. Material brought by the glaciers are dropped and form ridge-like deposits, across the valley, when the ice disappears and water collects behind the ridges to form lakes.

4. By Marine Action

In coastal regions, the formation of spits and bars due to deposition of rock fragments and coarser sand particles by the wave action often confine a portion of marine water to form a lake called lagoon.

5. By Action of Underground Water

The solvent action of ground water often leads to the formation of large depressions due to the roof collapse over great karst chambers. These depressions are characterized by their extensive size, flat bottom and basin-like shape with steep sides. They are often filled with water forming what is known as Poljee lakes.

6. Due to Tectonic Movements

(i) Folding and faulting of sections of the earth’s crust are mainly due to tectonic movements. Folding and differential faulting like tear faults or thrusts across the pre-existing river valleys very often block river to form a lake.

(ii) Blocks of the earth’s crust downcast along the faults due to the stretching of the earth’s crust give rise to grabens which when occupied by water form lakes. Such are the lakes Nyasa, Tanganyika, Baikal etc.

(iii) Lakes may also result due to earthquake.

7. Due to Volcanic Activity

(i) Lakes are sometimes formed on the craters and calderas of extinct or dormant volcanoes (E.g. Lonar lake in Maharashtra).

(ii) As a result of damming of the pre-existing valleys by lava flows, lakes are formed.

(iii) Circular hollows occurring on the surface of a lava-flow, after it had congealed, may contain water to form lakes.

8. Due to Organic Activity

Growth of coral reefs very often leads to the development of lagoons with the emergence of atolls.

9. Due to Meteoritic Impact

Depression formed due to the impact of large mateorites may contain water to form lakes.

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