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Lakes are classified into ten major groups according to the mode of origin: 1. Glacial Lakes 2. River Lakes 3. Wind-Made Lakes 4. Volcanic Lakes 5. Chemical Action Based Lakes 6. Tectonic Lakes 7. Meteorite Lakes 8. Man-Made Lakes 9. Abandoned Mine Lakes 10. Oceanic Lakes.
Type # 1. Glacial Lakes:
The erosive actions of valley glaciers create cirque, U-shaped valley and the irregular surface of glaciated lowland provides depressions for the development of lake.
Raindeer Lake in Canada, L. de Loux of Switzerland, L di coma, Lago Maggiore and Lago di Garda in Italy, the Great Lakes of North America and Ladoga and Onega Lakes of USSR are some of the examples of the lakes of glacial origin.
Type # 2. River Lakes:
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An Ox-bow lake is a river lake. As a river meanders in its plain course, the flow undercuts the outside banks of the bends and silts are deposited on the inside narrowing the channel. In course of time, a break through may then cut off the meander and ultimately an ox-bow lake is formed by the “Cut off of a loop”.
At the initial stage this cut off appears to be a deserted channel of the main stream. As it is silted up by later floods it degenerates into a swamp soon. Ox-bow lakes are found in the middle and lower Ganga plains and in the Brahmaputra Valley.
Type # 3. Wind-Made Lakes:
Salt marshes, Oases, Playas and Salinas are the examples of wind-made lakes. In arid regions, flood water may pour into inland depressions caused by wind deflation tapping the water table. The Qattara Depression westward from Cairo is 125 m below sea-level. It has salt marshes and the sand excavated from it forms dunes on the sea side. Many smaller basins have become fertile oases.
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A few well-known oases of Egypt namely, Baharia, Farafra, Dakhla and Khargo are above sea-level. They have also originated in the same way. These depressions are the result of wind deflation and are not crustal down warps, like the shotts of Tunisia and Algeria or Death Valley in California in USA. Moreover, they have not been hollowed out by water of the sheet floods due to rare desert rainstorms.
In course of time, the water of the temporary lake are dried up by evaporation. The enclosed lake then forms salt-muds or glistering white sheets of rock salt, gypsum and other salts. On the western part of USA in California, the alluvial plains of arid or semi-arid enclosed basins are known as Playas. The more saline tracts of all playas are distinguished as Salars or Salinas in USA and Mexico, as shotts (chotts in French language) in North Africa.
Type # 4. Volcanic Lakes:
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Volcanoes which have been dormant for a long time may become active with a sudden violent eruption. Then they blow out previously solidified materials and very often destroy much of the top of the cone. The entire cone may crumple and sink back to form a broad basin surrounded by an ash and lava rim. This is a caldera which, in course of time, may come to contain water with an appearance of a lake.
Later, new small cones may form within the caldera. One of the greatest known Calderas is located at the crest of the Barisan Highland in north-west Sumatra and is now occupied by Lake Toba. It lies in a Caldera 1,900 sq. km. in area. Steep walls 6000 metres high encircle the lake except for its river outlet in the south-east.
Crater Lake, of Oregon, USA occupies a Caldera about 10 km diametre, formed by the collapse of what was once a lofty composite cone. This pre-historic caldera with the cone of Wizard Island rising above the lake is a beautiful spot of the world.
Type # 5. Chemical Action Based Lakes:
Karst Lakes are the examples of chemical action based lakes. The solvent action of rainwater on limestone region may produce hollows which form small lakes. Such lakes are formed when the solution in hollows becomes clogged by residual clays.
The collapse of the limestone roof may produce long narrow lakes that were once underground. The Lac de chaillexon in the Jura Mountains near the boundary between France and Switzerland is an example.
In the limestone land of the Yugoslavian Karst, many large depressions known as poljes are found. They do not have surface outlets and contain water. The floor of the polje may carry seasonal lake. It degenerates into salt marsh and totally disappears in summer. An example is Lake Scutari in Yugoslavia.
Type # 6. Tectonic Lakes:
Hollows in the earth’s crust are developed on a large scale by warping or fracturing. Caspian Sea in USSR, Victoria Lake in East Africa, Lough Neagh in North Ireland and Titicaca Lake in Peru/Bolivia are major examples of tectonic lakes. Lakes that are formed by the back tilting of valley systems such as Lake Kyoga in Uganda are also in similar type.
Rift Valley Lake is a tectonic lake. Rift valleys are the usual sites for lakes in their steep-sided linear depressions. The Jordan-East Africa rift valley is a striking example. The Dead Sea, Lake Tanganika, Lake Nyasa (Malawi), etc., are similarly formed. These lakes are elongated in appearance. They illustrate the elongated character of a rift valley.
Differential faulting has also developed lakes in the Great Basin in the western United States. The basin comprising the Lake Baikal in USSR was caused by faults. The Lake Baikal (1,620 metres) is the deepest lake of the world. The Tarim Basin, enclosed by the inward facing faulting scarps of the Altyn Tagh on the south and the Tien Shan on the north once occupied by a vast lake, is now covered with sands, gravels and Lop Nor Marshes.
Type # 7. Meteorite Lakes:
The scars in the earth’s crust caused by the impact of giant meteorites are called astroblems (star wounds). The resulting crater so formed is almost circular in striking contrast with the irregular outlines of the ice-croded depressions occupied by the other lakes. These craters have mostly become lakes.
In the extreme north of Quebec between Hudson Bay and Hudson Strait a circular crater with a diameter of about 3 km has been created by a meteorite. It is now a lake (frozen over). The largest known is the Ashanti Crater (Lake Bosumtwi) in Ghana. It is a crater with a diameter of over 9.5 km.
Type # 8. Man-Made Lakes:
Besides the natural lakes man has now created artificial lakes by erecting a concrete dam across a river valley.
In India, these artificial lakes are known as Sagar, such as Govind Ballav Pant Sagar, Govind Sagar, and so on.
A few examples of Man-made Lake are given below:
Mead lake above the Hoover (Boulder) dam on the Colorado river USA.
Powell lake above the Glen Canyon Dam on the Colorado river USA.
Kariba lake above the Kariba Gorge Dam on the Zambezi river, on the border between Zambia and Zimbabwe.
Govind Ballav Pant Sagar above the Rihand Dam on the Rihand river (a tributary of the Son), U.P. India.
Govind Sagar above the Bhakra Dam on the Sutlej river, Himachal Pradesh, India.
Type # 9. Abandoned-Mine Lakes:
Some of the pits in the Norfolk Broads in Netherlands were flooded and abandoned by the peat cutters. As they could not be drained in course of time they become lakes. Lakes have been formed in the abandoned diamond mines in Kimberley, South Africa.
Type # 10. Oceanic Lakes:
Lagoon is an example of oceanic lake. All waves approach the shore and pass into shallow water, topple over or break. The line of breakers may be several hundred metres from the shore line. As the waves break, they wash forward loose bottom materials only to drop them just landward of the line of breakers forming a submarine ridge almost parallel to the coast.
In course of time, those ridges grow in height. They then appear first above sea-level as a series of narrow islands. The process of deposition by waves fills up the gaps between some of the islands and connects them to form what is called off-shore bars. Between the off-shore bars and the mainland, there remains shallow bodies of water known as lagoons.
Sands largely composed of minute fragments of marine shells migrate from the sea floor to beach where the prevailing winds are on shore. Drifting sand is blown landwards and piled up to dunes. Such sand dunes found along Kerala coast are locally called Teris. These dunes of Pleistocine and current times have helped to form a large number of shallow lagoons and backwater which are locally called Kayals in Kerala.
Drainages from the mainland may discharge into the lagoons. Narrow openings developed in the off-shore bars are known as tidal inlets. Through these inlets the saline water flows land-ward during high tide and comes out seaward during the ebb tide. Generally, salt water vegetation spreads in the lagoons.
Importance of Lakes:
Lakes provide a number of benefits.
The benefits of lakes may be enumerated as follows:
(i) Reservoirs of Water:
Lakes provide water for drinking, factory, irrigation and generating hydel-power.
(ii) Equitable Climate:
Lakes cool the air in summer and warm it during winter. They also enhance the humidity.
(iii) Flood Control:
Rivers passing through the lakes in their course seldom cause disastrous floods. The Wular lake and the Dal lake do not allow the Jhelum river to be flooded and due to lack of such lakes, the Brahmaputra is subjected to very great floods every year.
(iv) Fisheries:
Fishes abound in the lake. Lakes help for the growth of fishing industry.
(v) Common Salt:
The salt lakes yield common salt. Sambar lake, Didwanalake, Pachpadra lake and Kuchman lake have contributed for the growth of salt works in Rajasthan.
(vi) Transport:
Lakes are used for navigation. The Great Lakes-St. Lawrence waterway of USA and Canada is an example.
(vii) Tourists’ Centre:
Lakes add to the scenic grandeur. The Valley of Kashmir in India has become the paradise on earth for the tourists because of its lakes.