Earth Science: Earth – Lakes – Origins
Lakes are not simply inland water-filled depressions. They are complex ecosystems with different origins, as varied as the forces of nature. Although the water they contain covers only a small portion of the landmass, lakes can be of great importance to the local environment and climate.
Origins
Lakes cover barely 2 percent of the Earth’s land surface. They vary greatly in size and distribution. Together, they store a quarter of all the freshwater on Earth.
A lake can be formed nearly anywhere on Earth, provided there is an adequate surface hollow and sufficient water. Some lakes are also referred to as a sea, such as the Caspian Sea and the Dead Sea; however, neither of these is as old or as deep as Lake Baikal. The largest continuous freshwater area on Earth is made up by the Great Lakes in North America.
Young and old lakes
Most of the recently formed lakes are remnants from the last ice age, when mighty ice shields and glaciers covered vast parts of the Northern Hemisphere. As the ice masses withdrew, they left valleys, hollows, and depressions behind, where meltwater accumulated. In lowlands, debris blocked off the water and formed giant lake areas.
Lakes of older origin were formed by tectonic changes along the Earth’s crust, when deep depressions, cracks, and rift valleys filled up with water. Examples are the Tanganyika and Malawi Lakes in the East African Rift Valley, the Dead Sea, and Lake Baikal. Other lakes are formed by water accumulating in craters of extinct volcanoes. Cave collapses in karst regions can give rise to doline (sinkhole) lakes.
Habitats
Lakes can be divided up into various habitats. The littoral zone is the sunlight flooded shore region, the profundal zone receives limited light, the pelagial zone is the open water area, and the benthos is the bottom of the lake. Due to the relationship between water temperature and density, lakes form layers called thermoclines.
There is a constant temperature of 39°F (4°C) in the lower region (hypolimnion) of a lake, as this is the temperature at which water is densest. The surface water (epilimnion) has a larger temperature spectrum, and it can even freeze over in winter. Life continues under the ice until the spring weather warms up the surface once again.
ARTIFICIAL LAKES
Some artificial lakes are the result of the removal of raw materials such as gravel or sand Reservoirs exist primarily along the upper reaches of rivers.
They are used for drinking and commercial water storage, flood mitigation, and hydropower The largest reservoir on Earth is Volta Lake in Ghana.
One of the largest storage dams is the Three Gorges Dam in China.