GC Leong: Summary of Lakes and Its Formation - Geography for UPSC CSE

• 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. Example: 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.

THE FORMATION AND ORIGIN OF LAKES

Lakes formed by Earth Movement

1. 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)

2. 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.
  Examples: Lake Tanganyika (World’s deepest lake) & Dead Sea (World’s lowest lake)

Lakes formed by Glaciation

1. 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.

2. 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.

3. 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 by 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

1. 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.

2. 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.

3. 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

1. 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.

2. 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

1. 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.

3. 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

1. 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.

2. 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.

3. 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.