Lake

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A lake (from Latin lacus) is a body of water or other liquid of considerable size contained on a body of land. A vast majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes. In ecology the environment of a lake is referred to as lacustrine. Large lakes are occasionally referred to as "inland seas" and small seas are occasionally referred to as lakes. Smaller lakes tend to put the word "lake" after the name, as in Green Lake, while larger lakes often invert the word order, as in Lake Ontario, at least in North America. In some places, the word "lake" does not correctly appear in the name at all (eg Windermere in Cumbria).

Most lakes have a natural outflow in the form of a river or stream, but some do not, and lose water solely by evaporation and/or underground seepage. They are termed endorheic lakes (see below).

The term lake is also used to describe a feature such as Lake Eyre, which is a dry basin most of the time but may become filled under seasonal conditions of heavy rainfall.

Many lakes are artificial and are constructed for hydro-electric power supply, recreational purposes, industrial use, agricultural use, or domestic water supply.

Evidence of extra-terrestrial lakes exists; "definitive evidence of lakes filled with methane" was announced by NASA as returned by the Cassini Probe observing the moon Titan, which orbits the planet Saturn.

[edit] Origin of natural lakes

Geologically speaking, most lakes are young. The natural results of erosion will tend to wear away one of the basin sides containing the lake, such as the shores of Lake Baikal in Russia which is estimated to be 25 – 30 million years old. 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, Patagonia, Siberia and Canada. Lakes can also form by means of landslides or by glacial blockages. An example of the latter occurred during the last ice age in the US 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 at Sun Lakes, Washington.

Salt lakes (also called saline lakes) can form where there is no natural outlet or where the water evaporates rapidly, and the drainage surface of the water table has a higher than normal salt content. Examples of salt lakes include Great Salt Lake, the Caspian Sea, the Aral 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 eroded away more rapidly than the inner side. Eventually a horseshoe bend is formed and the river cuts through the narrow neck. This new passage then forms the main passage for the river and the ends of the bend become silted up, thus forming a bow-shaped lake.

Lake Vostok is a subglacial 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 which would spray in a similar fashion to a geyser.

Some lakes, such as Lake Baikal and Lake Tanganyika lie along continental rift zones, and are created by the crust's subsidence as two plates are pulled apart. These lakes are the oldest and deepest in the world, and may be destined over millions of years to become oceans. The Red Sea is thought to have originated as a rift valley lake.

Crater Lake in Oregon, USA is a lake located within the caldera of Mount Mazama. The caldera was created in a massive volcanic eruption that lead to the subsidence of Mount Mazama around 4860 BC. Since that time, all eruptions on Mazama have been confined to the caldera.

Some lakes, such as Lake Jackson, USA come into existence as a result of sinkhole activity.

[edit] Types of lakes

• Periglacial: Part of the lake's margin is formed by an ice sheet, ice cap or glacier, the ice having obstructed the natural drainage of the land.
• Subglacial: A lake which is permanently covered by ice. They can occur under glaciers and ice caps or ice sheets. There are many such lakes, but Lake Vostok in Antarctica is by far the largest. They are kept liquid because the overlying ice acts as a thermal insulator retaining energy introduced to its underside by friction, water percolating through crevasses, by the pressure from the mass of the ice sheet above or by geothermal heating below.

• Artificial, also called a reservoir: A lake created by flooding land behind a dam, by human excavation, or by the flooding of an open pit mine (sometimes called a quarry). Some of the world's largest lakes are reservoirs. Husain Sagar is a reservoir in India built in 1562.
• Endorheic, also called terminal or closed: A lake which has no significant outflow, either through rivers, or underground diffusion. Any water within an endorheic basin leaves the system only through evaporation. These lakes are most common in desert locations, such as Lake Eyre in central Australia or the Aral Sea in central Asia.
• Meromictic: A lake which has layers of water which do not intermix. The deepest layer of water in such a lake does not contain any dissolved oxygen. The layers of sediment at the bottom of a meromictic lake remain relatively undisturbed because there are no living organisms to stir them up.
• Fjord lake: A lake in a glacially eroded valley, which has been eroded below sea level.
• Oxbow: A lake which is formed when a wide meander from a stream or a river is cut off to form a lake. They are called oxbow lakes due to the distinctive curved shape that results from this process.
• Rift lakes: A lake which forms as a result of subsidence along a geological fault in the Earth's tectonic plates. Examples include the Rift Valley lakes of eastern Africa and Lake Baikal in Siberia.
• Underground: A lake which is formed under the surface of the Earth's crust. Such a lake may be associated with caves and aquifers and springs.

• Crater: A lake which forms in volcanic calderas or craters after the volcano has been inactive for some time. Water in these types of lakes may be fresh, or highly acidic, and may contain various dissolved minerals. Some also have geothermal activity, especially if the volcano is merely dormant rather than extinct.
• Lava: A pool of molten lava contained in a volcanic crater or other depression. Lava lakes that have partly or completely solidified are also referred to as lava lakes.
• Former: A lake which is no longer in existence. Such lakes include prehistoric lakes, and lakes which have permanently dried up through evaporation or human intervention. Owens Lake in California, USA is an example of a former lake. Former lakes are a common feature of the Basin and Range area of south-western North America.
• Shrunken: Closely related to former lakes, a shrunken lake is one which has drastically decreased in size over geological time. Lake Agassiz is a good example of a shrunken lake, which covered much of central North America. Some notable remnants of this lake are Lake Winnipeg and Lake Winnipegosis.

[edit] Characteristics

Lakes have numerous features in addition to lake type, such as (but not limited to) drainage basin (also known as catchment area), inflow, and outflow, nutrient content, dissolved oxygen, pollutants, pH, and sedimentation.

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 artificial sources from outside the catchment area. Output sources are evaporation from the lake; surface and groundwater flows; and any extraction of lake water by humans. As climate conditions and human water requirements vary, these will create fluctuations in the lake level.

Lakes can be also categorized on the basis of their richness of nutrients, which typically affects plant growth. Nutrient-poor lakes are said to be oligotrophic and are generally clear, having a low concentration of plant life. Mesotrophic 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. And hypertrophic lakes are bodies of water that have been excessively enriched with nutrients. These lakes typically have poor clarity and are subject to devastating 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 to humans, and have a poor ecosystem due to decreased dissolved oxygen.

Due to the unusual relationship between water's temperature and its density, lakes form layers called thermoclines which are layers of drastically varying temperature relative to depth. Fresh water is most dense at about 4 degrees Celsius (39.2 °F) at sea level. When the temperature of the water at the surface of a lake reaches the same temperature as deeper water (such as during the cooler months in temperate climates), the water in the lake can mix, bringing oxygen starved water up from the depths, and bringing oxygen down to decomposing sediments. Deep temperate lakes can maintain a reservoir of cold water year-round which allows some cities to tap that reservoir for deep lake water cooling.

Since the surface water of deep tropical lakes never reaches the temperature where water reaches its maximum density, there is no process that makes the water mix. The deeper layer becomes oxygen starved, and can become saturated with carbon dioxide, or other gases such as sulfur dioxide if there is even a trace of volcanic activity. Exceptional events, such as earthquakes or landslides, can cause mixing which rapidly brings up the deep layers, and can release a vast cloud of toxic gases which lay trapped at the bottom of the lake. An example of such a release is Lake Nyos in Cameroon. The amount of gas that can be dissolved in water is directly related to pressure. As the previously deep water surfaces, the pressure drops, and a vast amount of gas comes out of solution. Under these circumstances even carbon dioxide is toxic because it is heavier than air and displaces oxygen, so it may flow down the river valley to human or livestock settlements and cause mass asphyxiation.

The material at the bottom of a lake or lake bed may be composed of a wide variety of materials, including inorganics such as silt or sand sediments, and organic material such as decaying plant or animal matter. The composition of the lake bed has a significant impact on the flora and fauna found within the lake's environs by contributing to the amounts and the types of nutrients available.

[edit] Limnology

Main article: Limnology

Limnology is the study of inland bodies of water and related ecosystems, and divides lakes in three zones: littoral zone, which is a sloped area that is close to land; photic or open-water zone, where sunlight is abundant; and deep-water profundal or benthic 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, may be responsible for a yellow or brown color, while algae may 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 amount of algae (see aquatic trophic cascade). The light depth or transparency is measured by using a Secchi disk. This is a 20 cm (8 in) 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. For a detailed look at these processes, see lentic system ecology.

A lake moderates the surrounding region's temperature and climate because water has a very high specific heat capacity (4,186 J·kg⁻¹·K⁻¹). In the daytime, the lake can cool the land beside it with local winds, resulting in a sea breeze; in the night, it can warm it, forming a land breeze.

[edit] How lakes disappear

A lake may be infilled with deposited 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 water plants (typically reeds) accelerate this closing process significantly because they partially decompose to form peat soils that fill shallows of lakes. Conversely a peat soils in a marsh can naturally burn and reverse this process to recreate a shallow lake. 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 succession, more trees would grow in, eventually turning the wetland into a forest.

Some lakes can also disappear seasonally; they are called intermittent lakes and are typical of karstic terrain. A prime example of this is Lake Cerknica in Slovenia. On 3 June 2005 in Nizhny Novgorod Oblast, Russia, a lake called Lake Beloye vanished in a short period of time (minutes). News sources reported government officials theorized that this strange phenomena may have been caused by a shift on soil underneath the lake which drained water to channels leading to Oka River.^[1]

The presence of ground permafrost is also important to lake persistence. According to research published in the journal Science ("Disappearing Arctic Lakes," June 2005), thawing permafrost may explain the shrinking or disappearance of hundreds of large Arctic lakes across western Siberia. The idea here is that rising air and soil temperatures thaw permafrost, allowing the lakes to drain away into the ground.

Neusiedler See, located in Austria and Hungary, dried up several times for a number years during the past centuries. As of 2005, it is again rapidly losing water, giving rise to the fear that it will be completely dried up by 2010.

Some lakes disappear because of human development factors. The shrinking Aral Sea is described as being "murdered" by the intended diversion of rivers feeding the lake for irrigation.

See also: Prairie Lake

[edit] Extraterrestrial lakes

At present the surface of the planet Mars is too cold and has too little atmospheric pressure to permit pooling of liquid water on the surface. However geologic evidence appears to confirm that ancient lakes once formed on the surface. It is also possible that volcanic activity on Mars will occasionally melt the subsurface ice, forming large lakes. Under current conditions this water will quickly evaporate or freeze unless insulated in some manner, such as by a coating of volcanic ash.

Jupiter's small moon Io is volcanically active due to tidal stresses, and as a result sulfur deposits have accumulated on the surface. Some photographs taken during the Galileo mission appear to show lakes of liquid sulfur on the surface.

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.

On July 24, 2006 photos brought in by the Cassini-Huygens spacecraft give a strong evidence for the existence of methane or ethane lakes on Titan.

[edit] Notable lakes

• The largest lake in the world by surface area is the Caspian Sea. With a surface area of 394,299 km², it has a surface area greater than the next six largest lakes combined.
• The deepest lake is Lake Baikal in Siberia, with a bottom at 1,637 m (5,371 ft.) and is the world's largest freshwater lake by volume.
• The world's oldest lake is Lake Baikal, followed by Lake Tanganyika (Tanzania).
• The world's highest lake is an unnamed pool on Ojos del Salado at 6,390 m (20,965 ft), ^[2] the Lhagba Pool in Tibet at 6,368 m (20,892 ft) comes second.^[3]
• The world's highest commercially navigable lake is Lake Titicaca in Peru and Bolivia at 3,812 m (12,507 ft). It is also the largest freshwater (and second largest overall) lake in South America.
• The world's lowest lake is the Dead Sea bordering Israel, Jordan and the West Bank at 418 m (1,371 ft) below sea level. It is also one of the lakes with highest salt concentration.
• The largest freshwater lake by surface area, and third largest by volume, is Lake Superior with a surface area of 82,414 km². However, Lake Huron and Lake Michigan form a single hydrological system with surface area 117,350 km², sometimes designated Lake Michigan-Huron. All these are part of the Great Lakes of North America.
• The largest island in a freshwater lake is Manitoulin Island in Lake Huron, with a surface area of 2,766 km². Lake Manitou, located on Manitoulin Island, is the largest lake on an island in a freshwater lake.
• The largest lake located on an island is Nettilling Lake on Baffin Island.
• The largest lake in the world that drains naturally in two directions is Wollaston Lake.
• Lake Toba on the island of Sumatra is located in what is probably the largest resurgent caldera on Earth.
• The largest lake located completely within the boundaries of a single city is Lake Wanapitei in the city of Greater Sudbury, Ontario, Canada. Before the current city boundaries came into effect in 2001, this status was held by Lake Ramsey, also in Sudbury.
• Lake Enriquillo in Dominican Republic is the only saltwater lake in the world inhabited by crocodiles.

[edit] Largest by continent

The largest lakes (surface area) by continent are:

• Africa - Lake Victoria, also the second largest freshwater lake on Earth. It is one of the Great Lakes of Africa.
• Antarctica - Lake Vostok (subglacial)
• Asia - Caspian Sea, also the largest on Earth.
• Australia - Lake Eyre
• Europe - Lake Ladoga, followed by Lake Onega, both located in northwestern Russia.
• North America - Lake Superior
• South America - Lake Titicaca, which is also the highest navigable body of water on Earth at 3821 m above sea level.

Note: Lake Maracaibo can be considered as the largest lake in South America. It however lies at sea level with a relatively wide opening to sea, so it is better described as a bay.

[edit] Trivia

• Finland is known as The Land of the Thousand Lakes (actually there are 187,888 lakes in Finland, of which 60,000 are large) ^[4].
• The U.S. state of Minnesota is known as The Land of Ten Thousand Lakes.
• The license plate of the Canadian province of Manitoba used to claim "100,000 lakes" as one-upmanship on Minnesota.
• The Great Lakes of North America originated in the ice age.
• Only one lake in the English Lake District is actually a lake; other than Bassenthwaite Lake, the others are all "meres" or "waters".
• Over 60% of the world's lakes are in Canada; this is because of the deranged drainage system that dominates the country.
• Only six bodies of water in Scotland are known as lakes (the others are lochs): the Lake of Menteith, the Lake of the Hirsel, Pressmennan Lake, Cally Lake near Gatehouse of Fleet, the saltwater Manxman's Lake at Kirkcudbright Bay and The Lake at Fochabers. Of these only the Lake of Menteith and Cally Lake are natural bodies of fresh water.