THE FORMATION AND ORIGIN OF LAKES

Lakes are amongst the most varied features of the earth’s surface. They occupy the hollows of the land surface in which water accumulates. They vary tremendously in size, shape, depth and 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 seas, e.g. the Caspian Sea which is 1223 km ( 760 miles) long, as much as 980 m (3215 feet) deep, with a total area of 373 230 sq km (143 550 square miles) and which is more than one – and -a – half times larger than the whole of Ghana!

Lakes may exist temporarily filling up the small depression of undulating ground after a heavy shower. But those which are deep and carry more water than could ever be evaporated remain permanent. Most of the lakes in the world are fresh-water lakes fed by rivers and with outflowing streams e.g. Lake Geneva, Lake Poyang and the Great Lakes of North America. In regions of low precipitation and intense evaporation where there are few rivers strong enough to reach the sea, streams drain into a lake forming a basin of inland drainage. Because of the intense evaporation, these lakes are saline. For example, the Dead sea has a salinity (salt content) of 250 parts per thousand and the Great Salt Lake of Utah, U.S.A ., has a salinity of 220 parts per thousand. But, the Black Sea, into which drain many large rivers, has a salinity of less than 17 parts per thousand! Playas, or salt lakes, are a common feature of deserts.

Lake Chad, which lies to the north-east of Nigeria, is an example of a lake of inland drainage. It receives many rivers, including the Yobe, the Yedseram, the Ngadda and the Shari (1120 km long) and not one flows out of it. This is because the amount of water the lake receives from rain and rivers is not nearly enough to make up for that lost through evaporation. The size of the lake varies, always depending on whether it is dry season or wet season. The shrinking of Lake Chad is so great that some geographers have even described it as a mere big swamp.

It must be pointed out that lakes are only temporary features of the earth’s surface; they will eventually be eliminated by the double process of draining and silting up. In regions of unreliable rainfall, lakes dry up completely during the dry season. In the hot deserts, lakes disappear altogether by the combined processes of evaporation, percolation and outflow. Though the process of lake elimination may not be completed within our span of life, it takes place relatively quickly in terms of geological time.

The following are the various ways in which lakes can be formed. Each of them is placed in a specific category, though in a few cases the lakes could have been formed by more than one single factor.

 

1) Lakes Formed by Earth Movement

a) Tectonic lakes: Due to the warping, sagging, bending and fracturing of the earth’s crust, tectonic depression occurs. Such depressions give rise to lakes of immense sizes and depths.They include Lake Titicaca, occupying a huge depression in the intermont plateau of the Andes, 3810 m(12 500 feet) above sea- level, the highest lake in the world; and the Caspian Sea, the largest lake, almost five times larger than its nearest rival Lake Superior. In West Africa, Lake Chad is an example of a lake occupying a depression. Lake Victoria of East Africa is also formed in the depression between the eastern and western arms of the Rift Valley; it is not a rift valley lake.

 

b) Rift Valley Lakes: Due to faulting, a rift valley is formed by the sinking of land between two parallel faults, deep, narrow and elongated in character. Water collects in these troughs and their floors are often below sea-level (Fig.9.1). The best known example is the East African Rift Valley, which runs through Zambia, Malawi, Tanzania, Kenya and Ethiopia, and extends along the Red Sea to Israel and Jordan over a total distance of 4800 km (3000 miles). It includes such lakes as Tanganyika (1430 m / 4700 feet deep, the world’s deepest lake), Malawi, Rudolf, Edward, Albert, as well as the Dead Sea, 392m (1286feet) below mean sea-level, the world’s lowest lake.

 

2) Lakes Formed by Glaciation

a) Cirque lakes or tarns: A glacier on its way down the valley leaves behind circular hollows in the heads of the valleys up in the mountains. Such hollows are the arm-chair-shaped cirques or corries (Fig. 9.2).

Their over-deepened floors may be filled with water to become cirque lakes, e.g. Red Tarn in the English Lake District (Fig. 9.3). Those that occupy glacial troughs are long and deep and are termed ribbon lakes, e.g. Lake Ullswater.

 

b) Kettle lakes: These are depressions in the out- wash plain left by the melting of masses of stagnant ice. They are irregular because of the uneven morainic surface and are never of any great size or depth, e.g. the meres of Shropshire in England and the kettle-lakes of Orkney in Scotland.

 

c) Rock-hollow lakes: These are formed by ice-scouring when valley glaciers or ice sheets scoop out hollows on the surface. Such lakes of glacial origin are abundant in Finland, indeed the Finns call their country Suomi-the Land of Lakes. It is said that there are over 35 000 glacial lakes in Finland!

 

d) Lakes due to morainic damming of valleys: Valley glaciers often deposit morainic debris across a valley so that lakes are formed when water accumulates behind the barrier.

Both lateral and terminal moraines are capable of damming valleys e.g. Lake Windermere of the Lake District, England.

 

e) Lakes due to the deposition of glacial drifts: In glaciated lowlands with a predominant drumlin landscape, where drainage is poor, there are intervening depressions. These depressions are often water-logged, forming small lakes like those of County Down in Northern Ireland.

 

3) Lakes Formed by Volcanic Activity

a) Crater and caldera lakes: During a volcanic explosion the top of the cone may be blown off, leaving behind a natural hollow called a crater. These depressions are normally dry, bounded by steep cliffs and roughly circular in shape. In dormant or extinct volcanoes, rain falls straight into the crater or caldera, which has no superficial outlet and forms a crater or caldera lake.

The outstanding ones are the Crater Lake in Oregon, U.S.A ., which in fact occupies a caldera; Lake Toba in northern Sumatra and lake Avernus near Naples. Geographers are not yet certain of the cause of Lake Bosumtwi in Ghana, some think it was formed when a meteorite hit the earth surface, while others say it is a caldera.

 

b) Lava-blocked lakes: In volcanic regions a stream of lava may flow across a valley, become solidified and thus dam the river forming a lake e.g. a lava flow blocked the Jordan Valley, forming the Sea of Galilee, which is an inland lake, rather elongated in shape (Fig.9.5)

 

c) Lakes due to subsidence of a volcanic land surface: The crust of a hollow lava flow may collapse. The subsidence leaves behind a wide and shallow depression in which a lake may form, e.g. Myvatn of Iceland.

 

4) Lakes Formed by Erosion

a) Karst lakes: The solvent action of rain -water on lime stone carves out solution hollows. When these become clogged, 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,e.g. the Lac de Chalillexon in the Jura Mountains.

The large depressions called poljes, which normally do not have surface outlets, may contain lakes. During the wet period, these may cover most of the polje floor but they shrink during dry periods due to seepage (Fig. 9.6 ).

An example is Lake Scutari in Yugoslavia.

Solution is important in other rocks such as rock salt. Local subsidence may occur when the underlying beds of rock salt are gradually removed in solution. Many of the meres of Cheshire, England, were probably caused by this, and are also the result of salt-mining operations.

 

b) Wind-deflated lakes: The deflated action of winds in deserts creates hollows. These may reach ground water which seeps out forming small, shallow lakes. Excessive evaporation causes these to become salt lakes and playas. There are found in the Qattara Depression in Egypt and the Great Basin of Utah, U.S.A. (Fig. 9.7).

 

5) Lakes Formed by Deposition

a) Lakes due to river deposits: A river may shorten its course during a flood by cutting across its meandering loops, leaving behind a horseshoe-shaped channel as an ox-bow lake, e.g. those that occur on the flood plains of Lower Mississippi, U.S.A. and Rio Grande, Mexico.

 

b) Lakes due to marine deposits: The action of winds and waves may isolate lagoons along coasts by building spits or bars. As these lagoons of shallow water are enclosed only by a narrowsplit of land, comprising mud, sand and shingle, they may drain away at low tide. They are commonly found off the deltas of large rivers such as the Nile and the Ganges. In East Germany and Poland, lagoons are called haffs. Strong on -shore winds are capable of pushing coastal sand dunes landwards, and these may enclose marshy lagoons. This type of lagoon is well developed in the Landes of south -west France.

 

c) Lakes due to landslides, screes and avalanches: Lakes formed by these processes are also known as barrier lakes. Landslides or screes may block valleys so that rivers are dammed. Such lakes are short-lived, because the loose fragments that pile across the valleys will soon give way under the pressure of water.

When they suddenly give way, the dammed water rushes down, causing floods. Examples of lakes of this type are, Lake Gormire in Yorkshire, blocked by landslides; Ffynnon Frech on Snowdon, blocked by screes (Fig. 9.8).

 

6) Lakes Formed By Human and Biological Activities

a) Man-made lakes: Besides the natural lakes, man has now created artificial lakes by erecting concrete dams across river valleys so that the river water can be kept back to form reservoirs (Fig. 9.9).

Amongst such man-made lakes, the most imposing is Lake Mead above the Hoover Dam on the Colorado River, U.S.A. There are two good examples in West Africa: the Volta Lake above the Akosombo Dam in Ghana, and the Kainji Lake on the River Niger above the Kainji Dam, Nigeria.

 

b) Lakes made by animals: Animals like beavers are particularly interesting. They live in communities and construct dams across the rivers with timber. Such beaver dams are quite permanent and are found in North America, e.g Beaver Lake in Yellowstone National Park, U.S.A.

 

c) Other types of lakes: These include ornamental lakes, especially made to attract tourists, e.g Lake Gardens, Kuala Lumpur, Taiping Lakes. Man’s mining activities, e.g tin mining in West Malaysia, have created numerous lakes. Inland fish culture has necessitated the creation of many fishing-lakes.