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A lake is a body of water surrounded by land. The majority of lakes 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 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 (actually there are 187,888 lakes in Finland, of which 60,000 are large)

and Minnesota is known as The Land of Ten Thousand Lakes.

The Great Lakes of North America originated in the ice age. Over 60% of the world's lakes are in Canada;

this is because of the deranged drainage system that dominates the country.

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 latter 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 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 saltGreat Salt Lake, the Caspian SeaDead 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 is torn 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 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. The Crater Lake in Oregon is a lake located within the caldera of an extinct volcano.

Some lakes come into existence as a result of sinkhole activity.

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Characteristics:

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 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.

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.

Types of lakes:

A periglacial lake is one in which part of its margin is formed by an ice sheet, ice cap or glacier,

the ice having obstructed the natural drainage of the land.

A subglacial lake is one 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.

The 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.

Because of the unusual relationship between water's temperature and its density, the water in lakes in temperate climates mixes twice a year.

Fresh water is most dense at about 4 degrees Celsius.

When the temperature of the water at the surface of a lake reaches the temperature at which water is most dense all the water in the lake can mix,

bringing oxygen starved water up from the depths, and bringing oxygen down to decomposing sediments.

When the density of surface water differs from that of the deeper water there is a marked barrier layer, the thermocline, that prevents mixing.

Deep Temperate lakes can maintain a reservoir of cold water year-round.

The reservoir of deep, cold water allows cities to tap that reservoir for deep lake water cooling.

Since the 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, and possibly other gases, like sulfur dioxide,

if there is even a trace of volcanic activity. Exceptional events, like earthquakes or landslides, which do cause mixing, that brings up the deep layers,

can release a vast cloud of toxic gas. An example of a release as described is Lake Nyos in Cameroon.

The amount of gas that can be dissolved in water is pressure related.

As the water surfaces, and the pressure drops, a vast amount of gas cab comes out of solution.

Under these circumstances even carbon dioxide is toxic. Carbon dioxide is heavier than air, and the released carbon dioxide flows down the river valley.

Artificial lakes:

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.

Abiotic and biotic limnology:

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 forcesfishaquatic 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.

For a detailed look at these processes, see lentic system ecology. acting upon it.

The turbidity is a decisive factor in the transparency of the water. Bottom-dwelling detritivorous 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.

A lake moderates the surrounding region's temperature and climate because waterspecific 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.

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 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 peatfen. 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 June 3, 2005 in Bolotnikovo, Russia, a lake called White Lake 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.

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