Lake
For other uses, see Lake (disambiguation).
An area of lakes in Germany
A lake is an area of variable size filled with water, localized in a basin, that is surrounded by land, apart from any river or other outlet that serves to feed or drain the lake.[1] Lakes lie on land and are not part of the ocean(except for sea lochs in Scotland and Ireland), and therefore are distinct from lagoons, and are also larger and deeper than ponds, though there are no official or scientific definitions.[2][3] Lakes can be contrasted withrivers or streams, which are usually flowing. However most lakes are fed and drained by rivers and streams.
Natural lakes are generally found in mountainous areas, rift zones, and areas with ongoing glaciation. Other lakes are found inendorheic basins or along the courses of mature rivers. In some parts of the world there are many lakes because of chaotic drainage patterns left over from the last Ice Age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.
Many lakes are artificial and are constructed for industrial or agricultural use, for hydro-electric power generation or domestic water supply, or for aesthetic or recreational purposes.
Etymology, meaning, and usage of "lake"
Oeschinen Lake in the Swiss Alps
The Caspian Sea is either the world's largest lake or a full-fledged sea.[4]
The word lake comes from Middle Englishlake ("lake, pond, waterway"), from Old Englishlacu ("pond, pool, stream"), from Proto-Germanic *lakō ("pond, ditch, slow moving stream"), from the Proto-Indo-European root *leǵ- ("to leak, drain"). Cognates include Dutchlaak ("lake, pond, ditch"), Middle Low Germanlāke ("water pooled in a riverbed, puddle"),German Lache ("pool, puddle"), and Icelandiclækur ("slow flowing stream"). Also related are the English words leak and leach.
There is considerable uncertainty about defining the difference between lakes andponds, and no current internationally accepted definition of either term across scientific disciplines or political boundaries exists.[5] For example, limnologists have defined lakes as water bodies which are simply a larger version of a pond, which can 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. For this reason, simple size-based definitions are increasingly used to separate ponds and lakes. One definition of lake is a body of water of 2 hectares (5 acres) or more in area;[6]:331[7]however, others[who?] have defined lakes as waterbodies of 5 hectares (12 acres) and above,[citation needed] or 8 hectares (20 acres) and above [8] (see also the definition of "pond"). Charles Elton, one of the founders ofecology, regarded lakes as waterbodies of 40 hectares (99 acres) or more.[9] The term lakeis also used to describe a feature such asLake Eyre, which is a dry basin most of the time but may become filled under seasonal conditions of heavy rainfall. In common usage, many lakes bear names ending with the word pond, and a lesser number of names ending with lake are in quasi-technical fact, ponds. One textbook illustrates this point with the following: "In Newfoundland, for example, almost every lake is called a pond, whereas in Wisconsin, almost every pond is called a lake."[10]
One hydrology book proposes to define the term "lake" as a body of water with the following five chacteristics:[5]
- it partially or totally fills one or several basins connected by straits[5]
- has essentially the same water level in all parts (except for relatively short-lived variations caused by wind, varying ice cover, large inflows, etc.)[5]
- it does not have regular intrusion ofseawater[5]
- a considerable portion of the sediment suspended in the water is captured by the basins (for this to happen they need to have a sufficiently small inflow-to-volume ratio)[5]
- the area measured at the mean water level exceeds an arbitrarily chosen threshold (for instance, one hectare)[5]
Distribution of lakes
The majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes. Canada, with a deranged drainage system has an estimated 31,752 lakes larger than 3 square kilometres (1.2 sq mi)[13] and an unknown total number of lakes, but is estimated to be at least 2 million.[14] Finland has 187,888 lakes 500 square metres (5,400 sq ft) or larger, of which 56,000 are large (10,000 square metres (110,000 sq ft) or larger).[15]
Most lakes have at least one natural outflow in the form of a river or stream, which maintain a lake's average level by allowing the drainage of excess water.[16] Some lakes do not have a natural outflow and lose water solely by evaporation or underground seepage or both. They are termed endorheiclakes.
Many lakes are artificial and are constructed for hydro-electric power generation, aesthetic purposes, recreational purposes, industrialuse, agricultural use or domestic water supply.
Evidence of extraterrestrial lakes exists; "definitive evidence of lakes filled withmethane" was announced by NASA[citation needed] as returned by the Cassini Probe observing the moon Titan, which orbits the planet Saturn.
Globally, lakes are greatly outnumbered by ponds: of an estimated 304 million standing water bodies worldwide, 91% are 1 hectare (2.5 acres) or less in area (see definition ofponds).[17] Small lakes are also much more numerous than large lakes: in terms of area, one-third of the world's standing water is represented by lakes and ponds of 10 hectares (25 acres) or less.[citation needed]However, large lakes account for much of the area of standing water with 122 large lakes of 1,000 square kilometres (390 sq mi, 100,000ha, 247,000 acres) or more representing about 29% of the total global area of standing inland water.[citation needed]
Origin of natural lakes
A portion of the Great Salt Lake inUtah, United States
A lake in the Andes Mountains
Since they progressively become filled by sediment, lakes are considered ephemeral over geological time scales, and long-living lakes imply that active processes keep forming the basins in which they form.[18]There are a number of natural processes that can form lakes.
Tectonic lakes
The longest-living lakes on Earth are related to tectonic processes. A tectonic uplift of a mountain range can create depressions that accumulate water and form lakes.[18]
Landslide and ice-dam lakes
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 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
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 Aral Sea and theDead Sea.
Oxbow lakes
Small, crescent-shaped lakes called oxbow lakes can 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
Crater lakes are formed in volcanic cratersand calderas which fill up with precipitation more rapidly than they empty via evaporation. Sometimes the latter are called caldera lakes, although often no distinction is made. An example is Crater Lake in Oregon, in the caldera of Mount Mazama. The caldera was created in a massive volcanic eruption that led to the subsidence of Mount Mazama around 4860 BC.
Glacial lakes
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. The most notable examples are probably the Great Lakes of North America. As a particular case, gloe lakes are basins that have emerged from the sea as a consequence of post-glacial rebound, and are now filled with freshwater.
Other lakes
Some lakes, such as Lake Jackson in Florida, USA, come into existence as a result ofsinkhole activity.
Lake Vostok is a subglacial lake in Antarctica, possibly the largest in the world. The pressure from the ice atop it and its internal chemical composition mean that, if the lake were drilled into, a fissure could result that would spray somewhat like a geyser.
Most lakes are geologically young and shrinking since the natural results of erosionwill tend to wear away the sides and fill the basin.[18] Exceptions are those such as Lake Baikal and Lake Tanganyika that 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. 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.
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