A lake is a body of water, surrounded by land, which is not completely covered by vegetation. The majority of lakes are fresh water, and most lie in the northern hemisphere at higher latitudes. Large lakes are sometimes referred to as "inland seas" and small seas are sometimes referred to as lakes.
The term lake is also used to describe a feature such as Lake Eyre, which is dry most of the time but becomes filled under seasonal conditions of heavy rainfall. Many lakes are artificial and are constructed for hydro-electric power supply, recreation (swimming, wind surfing,...), water supply, etc.
Finland is known as The Land of the Thousand Lakes and Minnesota is known as The Land of Ten Thousand Lakes. The Great Lakes of North America also have ice age origins. Over 60% of the world's lakes are in Canada; this is because of the deranged drainage system that dominates the country.
There are dark basaltic plains on the Moon, similar to lunar maria but smaller, that are called lacus (singular lacus, Latin for "lake"). They were once thought by early astronomers to be literal lakes.
- The largest lake in the world is the Caspian Sea. With a surface area of 394,299 sq. km., it has a surface area greater than the next six largest lakes combined.
- The largest freshwater lake, and second largest lake altogether is Lake Superior with a surface area of 82,414 sq. km.
- The deepest lake is Lake Baikal in Siberia, with a bottom at 1,741 m (5,712 ft.).
- The highest navigable lake is lake Titicaca, at 3821 m above sea level. It is also the second largest lake in South America.
- The world's lowest lake is the Dead Sea, at 396 m (1,302 ft.) below sea level. It is also the lake with the highest salt concentration.
- The largest freshwater-lake island is Manitoulin Island on Lake Huron, with a surface area of 2,766 square km.
- The largest lake located on an island is Nettilling Lake on Baffin Island.
Lake Toba on the island of Sumatra is located in what is probably the largest resurgent caldera on Earth.
Origin of natural lakes
Most lakes are young, as the natural results of erosion will tend to wear away one of the basin sides containing the lake. There are a number of natural processes that can form lakes. A recent tectonic uplift of a mountain range can create bowl-shaped depressions that accumulate water and form lakes. The advance and retreat of glaciers can scrape depressions in the surface where lakes accumulate. Such lakes are common in Scandinavia, Siberia and Canada.
Lakes can also form by means of landslides or by glacial blockages. An example of the later occurred during the last ice age in the state of Washington, when a huge lake formed behind a glacial flow. When the ice retreated, the result was an immense flood that created the Dry Falls monument at Sun Lakes, Washington.
Saline lakes can form where there is no natural outlet or the water evaporates rapidly, and the drainage surface of the water table has a higher than normal salt content. Examples of salt lakes include the Great Salt Lake, the Caspian Sea and the Dead Sea.
Small, crescent-shaped lakes called Oxbow Lakes can form in river valleys as the result of meandering. The slow-moving river forms a sinuous shape as the outer side of bends are worn away more rapidly than the inner side. Eventually a horseshoe bend is formed and the river cuts through the narrow neck. This gap now forms the main passage for the river and the ends of the bend become silted up.
Lake Vostok is an under-ice lake in Antarctica, possibly the largest in the world. The pressure from ice and the internal chemical composition means that if the lake were drilled into, it may result in a fissure and spraying in the same manner as a shaken can of soda.
Some lakes, like Lake Baikal and Lake Tanganyika are volcanic in origin, and lie on geological fault lines. The Crater Lake in Oregon is a lake located within the caldera of an extinct volcano.
A reservoir (French: réservoir) is an artificial lake created by flooding land behind a dam. Some of the world's largest lakes are reservoirs. Artificial lakes can also be made deliberately by digging one or by flooding an open-pit mine.
To build dams, surveyors have to find river valleys which are deep and narrow; the valley sides can then act as natural walls. The best place for building a dam has to be determined. If necessary, humans have to be rehoused and/or historic sites have to be moved, e.g. the temples of Abu Simbel before the construction of the Aswan Dam, creating Lake Nasser.
Lake Mead is North America's largest artificial lake. Lokka is Northern Europe's largest artificial lake, 417 km2 in size.
See also: List of reservoirs and dams
The change in level of a lake is controlled by the difference between the sources of inflow and outflow, compared to the total volume of the lake. The significant input sources are precipitation onto the lake; runoff carried by streams and channels from the lake's catchment area; groundwater channels and aquifers, and man-made sources from outside the catchment area. Output sources are evaporation from the lake; surface and groundwater flows, and any extraction of lakewater by humans. As climate conditions and human water requirements vary, these will create fluctuations in the lake level.
Lakes can be categorized on the basis of their richness of nutrients, which typically effects plant growth. A nutrient poor lake is called oligotrophic, and are generally clear and have a low concentration of plant life. Mesotropic lakes have good clarity and an average level of nutrients. Eutrophic lakes are enriched with nutrients, resulting in good plant growth and possible algal blooms. A hypertrophic lake is a water body that has been highly enriched with nutrients. These lakes typically have poor clarity and are subject to algal blooms. Lakes typically reach this condition due to human activities, such as heavy use of fertilizers in the lake catchment area. Such lakes are of little use, and have a poor ecosystem.
Abiotic and biotic limnology
Lake Billy Chinook, Deschutes National Forest, Oregon
Limnology divides lakes in three zones: littoral zone, which is a sloped area that is close to land; open-water zone , where sunlight is abundant; and deep-water zone , where little sunlight can reach. The depth which light can reach in lakes depends on the density and motion of particles. These particles can be sedimentary or biological in origin and are responsible for the color of the water. Decaying plant matter for instance is responsible for a yellow or brown color, while algae result in greenish water. In very shallow water bodies, iron oxides make water reddish brown. Biological particles are algae and detritus. A sediment particle is in suspension if its weight is less than the random turbidity forces acting upon it. The turbidity is a decisive factor in the transparency of the water. Bottom-dwelling detritivorous fish are responsible for turbid waters, because they stir the mud in search for food. Piscivorous fish eat plant-eating (planktonivorous) fish, thus increasing the number of algae (see aquatic trophic cascade ). The light depth or transparency is measured by using a Secchi disk. This is a 20 cm disk with alternating white and black quadrants. The depth at which the disk is no longer visible, is the Secchi depth, and is a measure for transparency. It is commonly used to test eutrophication.
A lake moderates the surrounding region's temperature and climate because water has a very high specific heat capacity (4186).
How lakes disappear
A lake may be deposited with sediment, and gradually, the lake becomes a wetland, such as a swamp or marsh. An important difference exists between lowland and highland lakes: lowland lakes are more placid, are less rocky/more sedimentary, have a less sloping bottom, and generally contain more plant life. Large waterplants (typically reeds) accelerate thi closing process significantly because they trap sediment. Turbid lakes, and lakes with much plant-eating fish, tend to disappear slower. A "disappearing" lake (barely noticeable on a human timescale) typically has a water's edge with extensive plant mats. They become a new habitat for other plants (like peat moss, when conditions are right) and animals, many of which are very rare. Gradually, the lake closes, and young peat may form, forming a fen. In lowland river valleys (allowing the river to meander), the presence of peat is explained by the closing of historical oxbow lakes. In the very last stages of sucession , more trees would grow in, eventually turning the wetland into a forest.