Lakes may be classified on the basis of the mode of origin of the basins which contain their water. The most important types of lakes are formed by the action of erosion, deposition, earth movements and volcanic activity.
By the way of exception there may be more than one factor responsible for producing a lake. A sheet of water may collect in a basin carved out by erosion which may be deepened by an embankment formed by deposition. In such cases, more than one factor work is conjunction.
Lakes may by classified is many ways. But the following classification of lakes is based on the mode of their origin:
(i) Lakes due to crustal movements (diastrophism);
(ii) Lakes due to river processes; erosion and deposition;
(iii) Lakes due to glacial action;
(iv) Lakes due to wind action;
(v) Lakes due to volcanic action;
(vi) Lakes due to sea waves and tides;
(vii) Man-made lakes.
(i) Lakes due to crustal movements:
Lake basins may be formed by the down-warping of a portion of the land surface or by the upliftment of a region. By faulting or bending of the earth’s crust, trough-like basins or natural hollows form which get filled with water.
Warping of the earth’s crust through differential movement may also produce lake basins. Dead Sea in the Jordan Valley of Palestine is a remarkable example of a trough-fault basin.
The Wular Lake in the Kashmir Valley also occupies a structural depression. In case, the part of a river valley is up-warped, it will work as dam causing pounding of water, Lakes Nyasa and Tanganyika and many smaller lakes lie in a rift valley formed by faulting.
(ii) Lakes due to erosion and deposition (river processes):
Lakes are formed by the margins of rivers which are in their mature or old stage. The shallow basins in abandoned meanders on flood plains form oxbow lakes.
In delta regions there is irregular deposition of sediments by a large number of distributaries, so some shallow basins surrounded by deposits are formed.
These shallow depressions are filled with water and they are known as delta lakes. Such lakes may also be formed by the presence of two nearby deltas which sometimes enclose a hollow lying between them.
Pot-holes or plunge pools are of common occurrence at the foot of the big waterfalls. Gradually such basins grow in size and finally they become large enough to be called lakes.
(iii) Lakes due to glacial action:
Remember that most of the glacial lakes occupy such basins which have been eroded by the direct action of moving masses of ice called glaciers. Such glacial lakes are formed by the scouring of rock surfaces by glaciations.
After glaciers disappear (melt away) a large number of depressions appear on the surface. These small basins when filled with water are known as glacial lakes. The lakes in a glaciated region are also due to deposition.
In North America as well as in Western Europe glacial lakes are far more numerous than all other types put together. States such as Wisconsin, Manitoba, Minnesota, the plateau of Quebec are dotted with glacial lakes.
As you are aware, these regions were covered by glaciers during the Pleistocene Age. Finland in Europe is the home of glacial lakes.
Sometimes the floor of a growing eirque is subjected to strong glacial abrasion with the result that a shallow rock basin would be etched out by moving ice. After the ice has melted away, such basins on the floor of a cirque are filled with melt-water, and are called tarn or lake.
Ground moraines are often characterized by lake basins which become ground morainal lakes after they are filled with water. Mostly such lakes are relatively small with shallow depth.
But there are also some such lakes which are very large. Ground moraines are characterized for having more lake basins than all other glacial formations combined.
Lakes are also found behind the terminal or marginal moraines. Where the moraines are very high, the lake basins are very deep.
The Grand Lake, Colorado, is a terminal moraine lake. It is true that thousands of such lake basins are scattered over numerous glaciated regions of the world.
There are certain lakes which came into existence because of the damming by the ice-sheets as they advance into an ice-free area.
The ice-front will form one of the lake margins but elsewhere the waters will be impounded by drainage divides such as ridges, scarps, etc. such lakes are called pro-glacial lakes.
There are many examples of former pro-glacial lakes. In Britain there are many pro-glacial lakes impounded by Pleistocene ice-sheets.
Kettle-hole-lake represents a special type of lake due to glacial deposition. It is formed where a big ice-block was buried in glacial deposits.
Finally, the retreating ice-sheet caused ice to melt and leave the hole. When filled with water, this small hole is called a Kettle-hole lake which is a small lake in size and volume.
There are two distinct types of lake basins associated with lateral moraines.
(i) Valleys dammed by lateral moraines, and
(ii) Lakes formed between lateral moraines and valley walls. There are also certain ice dammed basins which originated when valley glacier completely blocked tributary valleys making lake basins. These lakes are generally shallow.
Even though majority of such glacial lakes originate due to the damming effect of river valleys by glacial deposits, there are many lakes in glaciated regions of the world which owe their origin to basins excavated is solid rock. Loch Coruisk is a typical example of this kind.
The Great Lakes of the U.S.A. and Canada represent the world’s largest lake-network. Lakes Superior, Michigan, Huron, Erie, and Ontario occupy former river valleys, which in the Pleistocene Age were covered by the great continental ice-sheets.
Such pre-existing river valleys were enlarged and deepened by glacial erosion. The Great Lakes have a very complex and chequered history, but this is not the appropriate place to go deep into it.
(iv) Lakes due to Wind action:
As we are aware that wind erosion is performed in three different ways i.e. deflation, abrasion and attrition.
Of all these processes, deflation plays dominant role in the creation of a large number of deflation hollows known as blow-outs. Sometimes these hollows reach ground water. Deflation as such occurs mostly in arid and semi- arid regions.
Wind-formed basins through erosion and deposition are converted into lake basins when filled with water.
Such lakes are rarely permanent, since they are found exclusively in arid and semi-arid regions. This is so because there is scanty rainfall, rapid percolation, rapid evaporation, and absence of vegetative cover to check the run-off.
There are certain lakes located between dunes. These lake basins are filled with water from some ephemeral streams that flow down the nearby mountain slopes. Some of these lakes are permanent.
In the dune area of San Luis Valley, Colorado, such lakes are many is number. However, there are numerous valleys which are dammed by wind-blown sand forming small temporary lakes in arid regions.
Because of deflation by the wind action, shallow salt-lakes and swamps are formed. The best examples of such lakes are found is the Quattara depression in Egypt, the Shott el Jerid and Shott el Melrir on both the sides of the Algerian-Tunisian border.
The lowest part of a closed basin is commonly occupied by a dry lake bed called playa. However, occasional floods transform such a playa into Playa Lake. Such a lake is ephemeral in nature. It may dry up within twenty-four hours or may persist for several days or weeks.
(v) Lakes due to volcanic action:
The craters of dormant or extinct volcanoes are the most suitable basins, which form crater lakes when filled with rainwater. One of the largest crater lakes in the world was formed in a caldera which is about 750 square miles in a real extent among the Batak High lands of northern Sumatra.
This extensive crater-lake is surrounded by steep rocky walls. In the south-east it is drained by its river outlet.
In North Island of New Zealand there is a small crater lake on the volcanic peak of Ruapehu.
Crater Lakes formed in the craters of dormant or extinct volcanoes are found is Italy, France, the Philippine Islands, the East Indies, New Zealand, Alaska, Mexico, Nevada, Arizona, etc. Remember that such lakes are fed by rain water and snowfall.
The underground springs and run-off from the rim also contribute water to these lakes.
Sometimes a valley is blocked by a lava- flow resulting in a barrier across it. In Jordan Valley, in the East African rift-valley, and in eastern Turkey such lakes were formed by the outpourings of Lava.
Lake Kivu, Lake Van and Lac d Aydat offer the fine examples of such lakes as were formed by damming effect of lava.
(vi) Lakes due to sea waves and tides:
Littoral currents are able to block the openings or mouths of bays which are turned into lakes. Sea waves may also build offshore bars is such a way that water behind them forms lagoons. Under certain suitable conditions lagoons are cut off from the adjacent bodies of water forming small lakes behind the sand bars.
Lakes are sometimes formed by marine deposits. The sea waves deposit shingles about high- water mark, and sometimes there is a small lagoon or salt-water lake between the bank and the sea-cliff. Sand-hills also contribute in the formation of lagoons.
In Holland and in other low-lying areas many lakes and marshes behind the line of sand-dunes on the coast have been formed in this way.
Chilka Lake and the Pulicat Lake of Nellore are formed by the deposits of sand carried up the coast by sea currents in the Bay of Bengal. These lakes were formed by the deposition of bars and spits across the mouths of small bays and inlets.
(vii) Man-made Lakes and Reservoirs:
Prompted by the benefits of lakes man has built tens of thousands of them by the construction of dams which represent man’s most ambitious and costly construction projects.
Artificial lakes are made where none existed before. Besides, he enlarges such basins as existed before. For building dams across big rivers high technical and engineering skills are required.
Materials used for the construction of dams are earth, rocks, sheet iron and particularly reinforced concrete. Such dams check the flow of a river and create a reservoir behind them which are of varying size and capacity.
The main purpose behind the construction of artificial lakes or large water reservoirs is to harness water for different purposes, such as generation of hydroelectricity, irrigation and supply of drinking water to urban areas. That is why almost all countries of the world have constructed water reservoirs or artificial lakes.
In our own country a large number of such artificial lakes have been constructed at huge costs. Govindsagar on the river Sutlej, Govind Ballabh Pant Sagar on Rihand, and Gandhi Sagar Dam and Jawahar Sagar Dam under Chambal Valley Project are some of the artificial reservoirs.