“A lake may be defined as an enclosed body of water, usually but not necessarily fresh water, from which the sea is excluded”.
According to Monk house, “The Lake may be defined as a hollow, more or less extensive, in the earth’s surface which contains water”. Lakes occupy basins or hollows on the earth’s surface. The lake basins are formed in various ways, to be discussed later on. Lake water may be fresh, brackish, or salt, but fresh water lakes are far more abundant than other types.
All lakes occupy natural basins that are undoubtedly associated with other distinctive types of topography. Lakes are among the most extensively distributed of all topographical features for they are found in all parts of the world and at all altitudes.
There is wide variation in the size, depth and altitude of lakes. Lake Titicaca (area 8,300 sq. km) in South America is located at an altitude of 4,300 meters. On the contrary, the Dead Sea is 435 meters below sea level.
As regards depth of lakes, there is a wide variation-from a few meters to a maximum of 1870 meters in Lake Baikal of Siberia, which is the deepest lake in the world. However, it is to be noted that most of the lakes are comparatively shallow with depths of less than 30 meters.
Variation is Lake Level:
The levels of lakes are closely related to the changes in the amount of rainfall. In the small lakes the rise and fall in the levels is the direct result of run-off. But in large lakes it is more noticeable as an effect of seasonal variation. A dry season is followed by a lowering of the lake level, and wet season by a rise.
This is partly because of variations in the amount of underground water supply. There are also changes in the levels of large lakes as a result of wind direction, because when the wind blows constantly for a long enough time, there is drift of water from one end of the lake to the other. Naturally, there is a rise in the water level at the end towards which the water drifts.
Generally lakes lie below the zone of permanent saturation or the water table:
The movement of underground water toward the lake augments its volume. On the contrary, some small lakes lie above the permanent water table. Such lakes suffer leakage, and in case their catchments area is small and the surrounding rocks porous such as, sand or gravel, they may completely disappear.
Another cause for variation in the level is evaporation from the surface of the lake. In dry regions evaporation exceeds precipitation which results in the fluctuation of the lake level.
It is due to these reasons that Lake Chad is greatly expanded during the rainy seasons and shrinks during the dry season. In the desert regions, there are only dry basins or basins with water only for a few months of the year. Desert lakes are mostly ephemeral in nature.
Lakes with no outlets:
It is more common for lakes to have outlet streams. But there are lakes in arid regions, where evaporation is far in excess of the supply of water or where seepage exceeds the supply, which do not have any outlet stream.
Some lakes have inlet stream usually at their heads. But such steams may also be absent is small-size lakes or lakes with a very small drainage basin. Such small lakes get their entire supply of water from rainfall or a negligible contribution of run-off from their narrow rim.
Remember that a lake without outlet suffers so much concentration of dissolved mineral matter that in due course of time it would become a salt lake.
Besides rainfall, underground water supply and an inlet stream, a large number of lakes receive water from tributary streams each bringing with it sediment as well as water.
The outlet streams from lakes have well-regulated volume. There are no floods is such streams. The St. Lawrence flowing from the Great Lakes has a steady volume with no floods, while the rivers which do not flow through lakes have floods that bring disaster with them.
Lakes and Man:
Lakes are of great value to man for more than one reason. Lake’s appeal to man is varied. To many people lakes are important for their scenic beauty.
To quote Worcester, “From an esthetic point of view our clear blue glacial lakes at the foot of great mountains or surrounded by green meadows or primordial forests are unsurpassed by any other object of nature’s handiwork.”
Lakes are matchless for their recreational as well as fishing activity. We visit lakes to enjoy their natural beauty and the greenery of the natural vegetation surrounding them. Lakes give us food in the form of fish.
Lakes provide cheap transportation, and they are natural reservoirs that yield fresh water for the teeming millions living is great cities. Lakes provide us water for irrigating our thirsty fields and orchards by constructing canals from them.
For the generation of hydroelectric power we depend on lakes-natural as well as man-made. Like oceans lakes exercise moderating effect on climate of adjacent regions. Lake water plays important role is reducing daily and seasonal temperature ranges.
The Florida citrus zone bears ample testimony to the significance of lakes for horticulture. Lakes are effective in reducing the fall of nocturnal temperature, which otherwise produces a frost hazard to agriculture during the growing season.
To take advantage of the climatic significance of lakes, the Florida citrus zone is concentrated in the central lake area of the state. Besides, the orchards and vineyards of New York State are concentrated downward from Lake Ontario and on the slopes rising above the Finger Lakes.
If there are lakes in a stream system, they stabilize the stream flow, absorbing excess water in floods. Lakes provide a continued flow of water during dry periods and droughts.
Allured by the benefits of lakes man has produced artificial manmade-lakes which are tens of thousands in number. In the United States alone there are about 65000 artificial lakes and reservoirs made by construction of dams across rivers.
In most of the warm countries, artificial lakes are growing in number to feed various multipurpose projects costing huge amount of money.