Introduction to lakes general

INTRODUCTION TO LAKES GENERAL.

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Lakes are amongst 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 fremation. The tiny ones are no bigger than bonds or pools, but the large ones are no bigger than ponds or pools, but the large ones are so expensive that they merit the name of seas, e.g. the Caspian Sea which is 760 miles long as much as 3,215 feet deep, with a total area of 143, 550 square miles, and is bigger than the whole of Malaysia!
Lakes may exist temporarily filling up the small depressions of undulating ground after a heavy shower than could ever be avaporated remain per- manent. Most of the lakes in the world are fresh- water lakes fed by rivers and with out- flowing steams e.g. lake Geneva, lake Poyang and the Great Lakes of North America In regions of low precipitation and intense evaporation Whare there are few rivers strong enough to reach the aea, 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.

It must be pointed out that lakes are only temporary features of the earth's crust; 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 FORMATION AND ORIGIN OF LAKES
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 depressions occur. Such depressions give rise to lakes of immense size and depths. They include lake Titicaca, occupying a huge depression in the intermont plateau of the Andes, 12,500 feet above sea level the highest lake in the world; and the Caspian Sea, 143,550 square miles, the largest lake, almost 5 times larger than its nearest rival, Lake superior.
   (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. The best known example is the East Africa 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 3,000 miles. It includes such. lakes as Lakes Tanganyika (4,700 feet deep, the world's deepest lake), Malawi, Rudolf, Edward, Albert, as well as the Dead Sea 1,286 feet below mean sea level, the world's lowest lake (Fig. 67).

2. LAKES FORMED BY GLACIATION
   (a) CIRQUE LAKES OR TARNS. A glacier on its way down the valley leaves behind circular hollows. Such hollows are the arm- chair- shaped cirques or corries.
   Their over- deepened floors may be filled with water to become cirque lakes e.g. Red Tarn in the English Lake District (Fig. 68). 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 outwash plain left by themlting 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 35,0000 glacial lakes in Finland!
(d) LAKES DUE TO MORAINIC DAMMING OF VALLEYS. Valley so that lakes are formed when water accumu- lates behind the barrier. Both lateral and terminal moraines are capable of damning valleys e.g. Lake Windermere of the Lake District England.
(e) Lakes due to 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 Northern Ireland.

1. 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. This may be enlarged by subsidence into a caldera. 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 (Fig. 69).
   (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 blocks the Jordan Valley forming the sea of Galilee which is an inland lake, rather elongated in shape.
   (a) 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.

2. LAKES FORMED BY EROSION
   (a) LARST LAKES. The solvent action of rain- water on limestone carves out solution hollows. When these become clogged with debris lakes may form in them. The collapse of limestone roofs of under- ground caverns may result in the exposure of long, narrow lakes that were once underground e.g. the Lac de chillexon in the Jura Mountains.
   The large depressions called poljes, which normally do not have surface outlets, may contain lakes. During wet periods these may cover most of the polje floor but they shrink during dry periods due to seepage (Fig. 70). 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 the meres of Cheshire, England, were probably caused by this, and are also the result of salt- mining operations.
   (a) WIND- DEFIATED LAKES. The deflating 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. These are found in the Qattara Depression in Egypt, and the Great Basin of Utah, U.S.A.

Reference

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