Lakes occur where depressions on the surface allow water to accumulate. Landslides blocking rivers, subsidence or collapse of the surfaces, glacial erosion and deposition, and volcanic activity are some of the common geological processes that can create the necessary basins.

Water filling the lake basin is usually derived from rivers, overland flow and direct precipitation, while groundwater is an important component in many regions. Although most lakes have inlets, not all have outlets; some do not have inlets or outlets.

Location and climate determine the chemical nature of lake water. Essentially, lakes are found in all regions of the world, from polar to equatorial, and where precipitation varies from abundant to minimal, resulting in water that varies considerably in salinity as well as acidity.

Lakes have one characteristic that is common to them all. They are relatively shortlived. Although tectonic movements may extend their lifespan, lakes gradually fill with sediment carried in by streams from within their watersheds. In arid environments a considerable amount of sediment may be blown onto the surface, then sinking to the lake floor. Vegetation growing along the edges and lake bottoms adds to the accumulation and helps to diminish the lifespan of the lake.

Groundwater has several roles in the life a lake; the most important is the position of the water table, the upper surface of the zone of saturation. The surface of a lake is also the surface position of the local water table, which rises and falls with the amount of precipitation that, in turn, determines the elevation of the lake surface. Groundwater in limestone areas dissolves the rock and produces caves, which sometimes contain underground lakes. In addition, as caves enlarge, the roofs often collapse forming depressions on the surface. Where the water table is high enough, lakes called sinkholes form in the depression.

After volcanic activity has ceased, even temporarily, rainwater fills some craters at the top of volcanic edifices. Where large calderas form, large lakes, sometimes many miles wide, may develop. A good example is Crater Lake in Oregon, where a major eruption occurring 6,600 years ago destroyed most of the volcano, scattering the rocks over great distances and forming a basin that is now occupied by a lake.

In arid regions, lake basins may receive water during a short rainy season and leave dry lakebeds as evaporation continually reduces the volume of water. The Great Salt Lake of Utah is a mere remnant of a vast lake that covered most of Utah some 8,000 years ago, when precipitation was greater. As the climate became drier, the decrease in rainfall caused the lake to shrink to its present size.

In cold climates soil is frozen most of the year, except for a short summer season when

it thaws, leaving, in places, numerous lakes. Lake sediments are important tools that geologists use to determine past climate. By dating the sediments collected on the lakebed and identifying the spores and pollen, and thus the nature of the ancient vegetation contained within it, geologists can determine the local climate.

Lakes can also be created by people, as their activities modify the surface of the earth by blasting out rock quarries, digging pits for gravel, creating lakes for farms, and the damming of stream and rivers for water supply, flood control, and recreation.

—Sidney Horenstein See also: Freshwater; Rivers and Streams Bibliography

Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Wetzel, Robert G. 2001. Limnology: Lake and River Ecosystems. San Diego: Academic Press.

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