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Email: firstname.lastname@example.org Lakes Lakes occupy the hollows of the land surface in which water accumulates & vary tremendously in size, shape, depth & mode of formation. The tiny ones are no bigger than ponds or pools, but the large ones are so extensive that they merit the name of the seas e.g. Caspian Sea. It must be noted that the lakes are only temporary feature of the earth crust & eventually be eliminated by draining & silting up. Most of the lakes in the world are fresh water lakes fed by the rivers But in regions where evaporation is greater than precipitation & only few streams filling up the lake, form saline water lakes such as Dead sea & Great Salt Lake of Utah. Lakes formed by earth movement Tectonic lakes Due to warping, bending & fracturing of earth crust, tectonic depressions occur which give rise to lakes of immense sizes & depths Examples → Lake Titicaca (at Andes) – Highest lake of the world, Caspian Sea (Largest lake of the world & 5 times larger than its nearest rival i.e. lake superior) Rift Valley Lakes Due to faulting, a rift valley is formed by sinking of land between two parallel faults which is deep, narrow & elongated in character. Water collect in these troughs & their floors are often below sea level Eg. Lake Tanganyika (World’s deepest lake) & Dead Sea (World’s lowest lake) Lakes formed by Glaciation Cirque lakes / Tarns A glacier on its way down the valley leaves behind circular hollows in the heads of the valleys up in the mountains known as corries or cirque Their over deepened floors may be filled with water to form cirque lakes Those that occupy long & deep glacial troughs down the valley are termed as Ribbon lakes Kettle Lakes They are basically depressions in the outwash plain left by the melting of masses of stagnant ice They are irregular because of the uneven moraine surface & are never of any great size or depth Rock Hollow Lakes Formed by ice scouring when valley glaciers or ice sheets scoop out hollows on the rock surface (lakes enclosed within a rock hollow) Such lakes are abundant in Finland (Land of lakes) Lakes formed due to Moraine damming of Valleys Valley glaciers often deposit moraine debris across a valley so that lakes are formed when water accumulates behind the barrier. Both lateral & terminal moraines are capable of damming valleys. Lakes formed due to deposition of glacial drifts In glaciated lowlands with a predominant drumlin landscape, where drainage is poor There are intervening depressions which are often waterlogged, forming small lakes Lakes formed by volcanic activity Crater & Caldera Lakes During a volcanic explosion, top of the cone may be blown off, leaving behind a natural hollow called a crater, which may be enlarged by subsidence into a caldera In dormant & extinct volcanoes, rain falls straight into the crater or caldera which has no superficial outlet & forms a crater or caldera lake Lava Blocked Lakes In volcanic regions a stream of lava may flow across a valley which may solidify Solidifying of lava may dam the river, leading to the formation of lava blocked lakes. Lakes formed due to subsidence of volcanic land surface The crust of hollow lava flow may collapse Subsidence leaves behind a wide & hollow depression in which a lake may form. Lakes formed by Erosion Karst lakes The solvent action of rain water on limestone carves out solution hollow. When these become clogged with debris, lakes may form in them. The collapse of limestone roofs of underground caverns may result in the exposure of long, narrow lakes that were once underground. The large depressions called Polje, which normally do not have any outlet, may contain lakes. Wind deflated lakes The deflating action of winds in deserts creates deep hollows which may reach water table via which water seeps out forming small shallow lakes. Excessive evaporation causes these to become salt lakes or Playas. Lakes formed by Deposition Lakes formed due to river deposits A river may shorten its course during a flood by cutting its meandering loops leaving behind a horseshoe shaped channel called ox – bow lake. Lakes formed due to marine deposits The action of wind & waves may isolate lagoons along the coasts, enclosed by narrow spit of land known as lagoon lakes. Lagoonis a shallow body of water separated from a larger body of water by barrier islands or reefs. In East Germany & Poland lagoons are called Haffs. Lakes formed due to landslides, screes & avalanches Landslides or screes may block valleys so that rivers are dammed, leading to formation of temporary lakes. Lakes formed by these processes are also known as barrier lakes. Such lakes are short lived because the loose fragments that pile up across the valleys will soon rupture under pressure & will give way to water. When they suddenly give way, the dammed water rushes down causing floods Lakes formed by Human & Biological activities Man-made lakes Besides the natural lakes, man has created artificial lakes by erecting a concrete dam across a river valley This is done so that the river water can be kept in check to form reservoirs. Lakes made by animals Animals like Beavers are particularly interesting. They live in communities & construct dams across the rivers with timber, mud & soil. Such Beaver dams are quite permanent & modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species. Other type of man-made lakes Ornamental lakes → Especially made to attract tourists Lakes made by men mining activities Inland fishing lakes to develop inland fish culture
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