A lake (from the Latin word lacus) is an inland body of water, not part of the ocean, that is larger and deeper than a pond and is localized at the bottom of a basin. There is, however, a lack of consensus on definitions used to distinguish between lakes and ponds. Some have defined a lake as a water body with a minimum surface area of 2 hectares (5 acres, 20,000 square meters), others have put the figure at 8 hectares (20 acres, 80,000 square meters). In ecology, the environment of a lake is described as lacustrine. The study of lakes, ponds, and other inland bodies of water and related ecosystems is called limnology.
A lake may receive water from one or more of the following sources: melting ice, streams, rivers, aquifers, and direct rainfall or snowfall. If the rate of replenishment is too low, the lake may lose water by evaporation or underground seepage or both.
A lake sustains a variety of living organisms and thus forms its own ecosystem. In addition, it moderates the area's temperature and climate by regulating wind temperature. If fed by a stream, it regulates the flow of the stream. For humans, a lake is usually valuable as a source of freshwater that may then be used for drinking and irrigating fields. It may also be used for recreational activities. Some saltwater lakes are useful for the minerals they contain. Some lakes have been artificially constructed or modified for hydroelectric power generation and industrial use.
The term "lake" is occasionally 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. In addition, a pool of molten lava may be called a "lava lake." Large lakes are occasionally referred to as "inland seas," and small seas are occasionally called lakes.
There is considerable uncertainty about definitions that distinguish between lakes and ponds. For example, limnologists have defined lakes as water bodies that are simply larger versions of ponds, or that have wave action on the shoreline, or where wind-induced turbulence plays a major role in mixing the water column. None of these definitions completely excludes ponds, and all are difficult to measure. Furthermore, in common usage, many lakes bear names ending with the word "pond," and a lesser number of bodies of water known as "lakes" could be better described (in quasi-technical terms) as ponds. In short, there is no current internationally accepted definition of either term across scientific disciplines or political boundaries. Within disciplines, authors are careful to define environmental geographic circumstances.
In light of these uncertainties, simple, size-based definitions are increasingly used to distinguish between ponds and lakes. In the United Kingdom, for example, the charity Pond Conservation has defined lakes as water bodies of 2 hectares (5 acres) or more in surface area. Elsewhere, other researchers have treated lakes as water bodies of 5 hectares (12 acres) and above, or 8 hectares (20 acres) and above. Charles Elton, one of the founders of ecology, regarded lakes as water bodies of 40 hectares (99 acres) or more, a value larger than what most modern researchers consider appropriate.
In naming lakes (at least in North America), the word "lake" is often placed after the name of a smaller lake, as in Green Lake (Seattle), but the word order is often inverted when naming larger lakes, as in Lake Ontario. In some places, the word "lake" does not correctly appear in the name at all (such as Windermere in Cumbria).
In the English Lake District, only one lake (Bassenthwaite Lake) is actually called a lake; the others are called "meres" or "waters." 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 freshwater.
The vast majority of lakes on Earth are freshwater, and most lie in the Northern Hemisphere at higher latitudes. More than 60 percent of the world's lakes are in Canada, as a result of the deranged drainage system that dominates the country. Many lakes occupy the basins and valleys created by glaciers in past epochs.
Finland, known as The Land of the Thousand Lakes, has 187,888 lakes, of which 60,000 are large. The U.S. state of Minnesota is referred to as The Land of Ten Thousand Lakes,, and issues automobile license plates boasting of its "10,000 lakes." The license plates of the Canadian province of Manitoba used to claim "100,000 lakes," as one-upmanship on Minnesota.
Globally, lakes are greatly outnumbered by ponds. Of an estimated 304 million standing water bodies worldwide, 91 percent are 1 hectare (2.5 acres) or less in area. Also, small lakes are much more numerous than big ones. However, large lakes contribute disproportionately to the area of standing water with 122 large lakes of 1,000 square kilometers (390 sq mi, 100,000 ha, 247,000 acres) or more representing about 29 percent of the total global area of standing inland water.
A lake may be formed by any of a number of natural processes. For example, a recent tectonic uplift of a mountain range can create bowl-shaped depressions that accumulate water and form lakes. Alternatively, the advance and retreat of glaciers can scrape depressions in the surface where water accumulates. Such lakes are common in Scandinavia, Patagonia, Siberia, and Canada. Among the most notable examples are the Great Lakes of North America.
Lakes can also be formed by landslides or glacial blockages. An example of the latter occurred during the last ice age in the U.S. 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. Some lakes, such as Lake Jackson in Florida, came into existence as a result of sinkhole activity.
Small, crescent-shaped lakes, called oxbow lakes, may form in river valleys as a 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.
Crater lakes are formed in volcanic calderas which fill up with precipitation more rapidly than they empty via evaporation. An example is Crater Lake in Oregon, located within the caldera of Mount Mazama. The caldera was created in a massive volcanic eruption that led to the subsidence of Mount Mazama around 4860 B.C.E.
Lake Vostok is a subglacial lake in Antarctica, possibly the largest in the world. Based on the pressure from the ice atop it and its internal chemical composition, one may predict that drilling the ice into the lake would produce a geyser-like spray.
Most lakes are geologically young and shrinking, since the natural results of erosion will tend to wear away the sides and fill the basin. Exceptions are lakes such as Lake Baikal and Lake Tanganyika that lie along continental rift zones and were created by the crust's subsidence as two plates were pulled apart. These lakes are the oldest and deepest in the world. Lake Baikal, which is 25-30 million years old, is deepening at a faster rate than it is being filled by erosion and may be destined over millions of years to become attached to the global ocean. The Red Sea, for example, is thought to have originated as a rift valley lake.
Lakes may be classified according to their manner of formation or current characteristics. Various types of lakes are noted below.
There is also evidence of extraterrestrial lakes, although they may not contain water. For instance, NASA has announced "definitive evidence of lakes filled with methane" on Saturn's moon Titan, as recorded by the Cassini Probe.
Lakes have a variety of characteristics in addition to those mentioned above. Their features include a drainage basin (or catchment area), inflow and outflow, nutrient content, dissolved oxygen, pollutants, pH, and sediment accumulation.
Changes in the level of a lake are controlled by the difference between the input and output, compared to the total volume of the lake. 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 in nutrients, which typically affects plant growth:
Due to the unusual relationship between the temperature and density of water, lakes form layers called thermoclines—layers of drastically varying temperature relative to depth. Freshwater 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, as it does 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.
Given that the surface water of deep tropical lakes never reaches the temperature of 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 may release a vast cloud of toxic gases that lay trapped in solution in the colder water at the bottom of the lake. This is called a limnic eruption. An example of such a release is the 1986 disaster at Lake Nyos in Cameroon. The amount of gas that can be dissolved in water is directly related to pressure. As the once-deep water rises, 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 it, so it may flow down a river valley to human settlements, causing mass asphyxiation.
The material at the bottom of a lake, or lake bed, may be composed of a wide variety of inorganics, such as silt or sand, 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.
Limnology is the study of inland bodies of water and related ecosystems. In this field of study, lakes are divided into three zones:
The depth to which light can penetrate a lake depends on turbidity of the water, which in turn is determined by the density and size of suspended particles. The 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 produce greenish water. In very shallow water bodies, iron oxides make the water reddish brown. Biological particles include algae and detritus. Bottom-dwelling detritivorous fish can be responsible for turbid waters, because they stir the mud in search of food. Piscivorous fish contribute to turbidity by eating plant-eating (planktonivorous) fish, thus increasing the amount of algae.
Light depth or transparency is measured by using a Secchi disk, a 20-centimeter (8-inch) disk with alternating white and black quadrants. The depth at which the disk is no longer visible is the Secchi depth, a measure of transparency. The Secchi disk is commonly used to test for eutrophication.
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, a lake can cool the land beside it with local winds, resulting in a sea breeze; in the night, it can warm it with a land breeze.
On geologic time scales, lakes—including those created by man-made concrete edifices—are temporary bodies, as ongoing geologic forces will eventually either break the earth and rock dams that hold them, or fill the basin with sediments forming a fresh geologic record.
A lake may be infilled with deposited sediment and gradually become a wetland such as a swamp or marsh. Large water plants, typically reeds, accelerate this closing process significantly because they partially decompose to form peat soils that fill the shallows. Conversely, peat soils in a marsh can naturally burn and reverse this process to recreate a shallow lake. Turbid lakes and lakes with many plant-eating fish tend to disappear more slowly.
A "disappearing" lake (barely noticeable on a human timescale) typically has extensive plant mats at the water's edge. These 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, producing a fen. In lowland river valleys, where a river can meander, the presence of peat is explained by the infilling of historical oxbow lakes. In the very last stages of succession, trees may grow in, eventually turning the wetland into a forest.
Some lakes disappear seasonally. They are called intermittent lakes and are typically found in karstic terrain. A prime example of an intermittent lake is Lake Cerknica in Slovenia.
Sometimes a lake will disappear quickly. On 3 June, 2005, in Nizhny Novgorod Oblast, Russia, a lake called Lake Beloye vanished in a matter of minutes. News sources reported that government officials theorized that this strange phenomenon may have been caused by a shift in the soil underneath the lake that allowed its water to drain through channels leading to the Oka River.
The presence of ground permafrost is important to the persistence of some lakes. 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, has dried up many times over the millennia. As of 2005, it is again rapidly losing water, giving rise to the fear that it will be completely dry by 2010.
Some lakes disappear because of human development factors. The shrinking Aral Sea is described as being "murdered" by the diversion for irrigation of the rivers feeding it.
At present, the surface of the planet Mars is too cold and has too little atmospheric pressure to permit the pooling of liquid water on its surface. Geologic evidence appears to confirm, however, that ancient lakes once formed on the surface. It is also possible that volcanic activity on Mars will occasionally melt subsurface ice creating large lakes. Under current conditions, this water would quickly freeze and evaporate 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.
Dark basaltic plains on the Moon, similar to but smaller than lunar maria, are called lacus (Latin for "lake") because early astronomers thought they were lakes of water.
The largest lakes (in terms of surface area) are listed below, with their continental locations.
All links retrieved July 22, 2014.
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