Origin of Lake and Thermal Stratification of Lake

Introduction

Lakes

·         Lakes are naturally formed large, deep inland standing water on the surface of the earth. They may range in depth and size.

·         ‘Ford’ define lakes as a body of standing water occupying a distinct basin and lacking continuity with sea.

·         Muttouskii (1918) refers to lake as a standing water body of large expends and deep enough to stratify thermally.

·         Belch (1952), regards all large bodies of standing water as lakes.

  Thermal Stratification

·         Differences in temperature of at different depths in deep lakes are called thermal stratification.

 

Origin of Lakes

Lakes arises in many ways. Naturally lakes are of diverse lake origin. The chief mode of lake formation as follows:

1.       Glacial action-

Glaciers form basin of lakes by substrate excavation and appropriate basin will form. Example- Lakes of Pongong Valley in Kashmir.

North America lakes will form by glacial erosion and deposition. Glacier aberration of slopes in high mountain valley form the basin of lake which later on filled with rain water and melting snow to produce a mountain lake. Besides valley, glaciers left behind a crested shaped ridges of rock debris which form a barrier of water behind them. Lakes are also formed by the deposition of morans and debris. So, as to form a closed basin. Example- Wular lake, Dul lake and Anchor lake of Kashmir and Kumao hills.

 

2.        Land slide-

It also creates lake. Land slide generally obstruct valley and forming natural dams and thereby creates lake basin. Most of the Kumao lakes are created due to land slides.

 

3.       Volcanic action-

Craters of extinct volcanoes filled with water and land slide that block the streams and valleys to form a lake known as crater lakes.

 

4.       Dissolution of substratum rocks-

Dissolution of underlying rocks with subsequent sinking creates a basin which, when filled with water are called dissolution lakes. The Nainital lake which has its outlet over solid limestone may have been produced by the collapse of the surface caused by the removal of lime stone due to solvent action of underground water. Many small lakes in the limestone tracks of the Khasi and Jaintia hills of Assam are believed to be dissolution lakes.

 

5.       Crustal movement of earth-

Movement of the earth crust such as up wrapping form dam or down wrapping form basin. Lakes having such basin are called tectonic lakes.

 

6.       Wind action-

In arid zone, the movement of sand particles and fine loose material may form a lake basin. The removal of soil and sand form an area that may leave behind a depression which form a lake basin. The salt lake of Rajasthan known as Shambhar lake, form due to wind action. The source of water in these lands due to inland drainage.

 

7.       River meanderings-

Various activities of river such as channel producing Oxbow lake, obstruction at mouth of tributaries forming lakes in lower ridges of Ganga and the Brahmaputra are certain common lakes. Further man- made lakes are formed by the construction of urban dam across the river and stream exclusively for irrigation and water storage for fisheries purpose. Naturally formed lakes constitute a great potential of fishing resource in India. Natural lakes are estimated to have about 2.92 million hectare in the country. 

  

  

Thermal stratification of lake

• Lake stratification is separation of lakes into three layers:

-          Epilimnion: the top of the lake

-          Metalimnion: the middle layer, which may change depth throughout the day

-          Hypolimnion: the bottom layer

• The thermal stratification of lakes refers to a change in the water temperature at different depths in the lake, and is due to change in water’s density with temperature.
• Cold water is denser than warm water and the epilimnion generally consists of water that is not as dense as the water in the hypolimnion. However, the temperature of maximum density for freshwater is 4°C.
• In temperate regions where lake water warms up and cools through the seasons, a cyclical pattern of overturn occurs that is repeated from year to year as the cold dense water at the top of the lake sinks. For example, in dimictic lakes, the lake water turns over during the spring and the fall. This process occurs more slowly in deeper water and as a result, a thermal bar may form. If stratification of water lasts for extended periods, the lake is meromictic.
• Photosynthetic biological activity tends to deplete the epilimnion of nutrients; while breakdown of detritus in the hypolimnion depletes it of oxygen.
• On cooling in the autumn, the temperature of the epilimnion falls to that of the hypolimnion. At this point, the water is the same density throughout and wind-induced vertical mixing is possible, a process is called turnover. This facilitates the oxygenation of the deeper parts of the lake, while bringing nutrients to the upper waters.
• In the winter, stratification may also occur with water at less that 4°C floating on water at 4°C in the depths.
• If the water freezes, the lower density of ice ensures that it does so from the top down. The insulting properties of the ice layer retard cooling so inhibiting the complete solidification of the lake.
• In the spring, the upper layers warm until the waters of the lake are all at the same temperature and density, therefore turnover can once again occur. Continued warming of the upper layers leads to the reintroduction of the summer stratification.
• In contrast, lakes in the polar and subpolar regions that are frozen for most of the year do not show stratification in the summer, but exhibit turnover for the duration of this season.
• Stratification occurs in tropical and subtropical lakes, in comparison with temperate lakes, the temperature of the waters of the epilimnion and hypolimnion is higher, while the temperature difference between these strata is less marked. Turnover on cooling is relatively easily induced in these warm water lakes.
• As a result, those in subtropical zones tend to have a winter circulation period, while those n tropical climates with little seasonality may either turnover frequently or have rare or irregular periods of turnover.
• Thermal stratification in lake influence the vertical profile of other physico-chemical parameter like DO₂  ( Clinograde condition), free CO₂, pH, bicarbonate and nutrients.

Typical thermal stratification of a lake into three strata