LAKE

any relatively large body of slow-moving or standing water that occupies an inland basin. Lakes occur throughout much of the world, but they are most abundant in high northern latitudes and in mountain regions, particularly those that have been glaciated in recent geologic times. They commonly occur along rivers with low gradients and wide flats and are associated with variations in the river channel. Many lakes are found in lowlands near the sea, especially in wet climates. A lake may contain either fresh water or salt water. Freshwater lakes constitute about 0.009 percent of the Earth's free surface water, while saline lakes account for a somewhat smaller percentage. In general, salt lakes occur in arid regions where the lakes occupy landlocked basins and the surrounding lands are rich in saline and alkaline compounds; the salts brought into the lakes in solution remain and are concentrated as the lake waters evaporate. Extremely salty lakes, such as the Great Salt Lake in northern Utah, occur in areas where the climate has changed from humid to arid. Lakes vary significantly in size. Most have a surface area of about 260 square km (100 square miles) or less, but a few exceed 3,900 square km (1,506 square miles). Lake Superior, located in North America and covering roughly 82,100 square km (31,700 square miles), is the largest freshwater lake in terms of surface area. The freshwater lake with the largest volume, however, is Lake Baikal in Siberia. Its total volume is approximately 23,000 cubic km (5,500 cubic miles). Lakes may form in any large undrained depression or in any sizable depression that has an outlet somewhat above the lowest parts of the depression where there is an adequate supply of water to keep the depression filled. The primary sources of lake water are melting ice and snow, springs, rivers, immediate runoff from the land surface, and direct precipitation. Unquestionably, more of the world's existing lakes have been produced by glacial action than by any other single agent. The extensive ice sheets that covered Canada, the northern United States, Finland, and portions of Sweden during the Pleistocene Epoch (1,600,000 to 10,000 years ago) gouged out undrained depressions in the bedrock of these areas, thereby producing tens of thousands of rock-shored lakes. Such continental glaciers also created lakes by depositing rock debris across preexisting drainage patterns. The sedimentary material dammed up streams and creeks, causing lakes to form. In some cases, glacial debris spread indiscriminately over the landscape left scattered shallow swales, or depressions, and these formations now hold lakes. When glacial ice melts, it sometimes leaves deep pits called kettle holes, which fill with water and become lakes. Most lakes in mountain valleys are glacial in origin, having been formed by mountain, or valley, glaciers by the same process of erosion and deposition. A few mountain lakes were formed as a result of landslides or mudflows blocking streams. Ordinarily, such a lake is relatively short lived because the outlet at the lower end is quickly cut down through the fairly unresistant material in the natural dam. Lakes so formed by lava flows, however, are likely to be permanently established. Other barriers to preexisting drainage that result in lake formation are sand dunes deposited across a small stream, beach deposits across the mouth of a river, or the alluvium laid down by a large river across the course of a smaller tributary. Deltaic deposits that are laid down in a sluggish stream by a silt-laden, more swiftly flowing stream may dam the former and produce a lake upstream. The floodplains of rivers contain bodies of water called oxbow lakes that were formed in what were once river channels. The craters of extinct or dormant volcanoes commonly contain lakes. Crater Lake in Oregon is one of the best known examples. At various places, the Earth's crust has been warped or broken into depressed blocks filled with water. Several large African lakes (e.g., Nyasa and Tanganyika) lie in the Great Rift Valley, an enormous down-dropped block of the terrestrial crust. Regions underlain by highly soluble limestone develop depressions known as sinkholes, which may fill with water under certain conditions to become lakes. The lake region of north-central Florida is of this type. Viewed on the geological time scale, lakes are short-lived. Almost as soon as they are formed, three processes begin their eventual destruction. All inflowing streams carry sediment into a lake, thereby starting the process of filling the basin. If the basin is filled enough for the lake to overflow, the outflowing stream tends to erode a notch through the lip of the basin and thereby drains the depression. Lastly, the accumulation of organic deposits from vegetation may cause shallow lakes to become bogs or swamps and, ultimately, dry land. In the upper part of lakes there is a good supply of light, heat, oxygen, and nutrients, well distributed by currents and turbulence. As a result, a large number of diverse aquatic organisms can be found there. The most abundant forms are plankton (chiefly diatoms), algae, and flagellates. In the lower levels and in the sediments, the main forms of life are bacteria. Although organic wastes are efficiently digested through the symbiotic relationship between algae and bacteria, the presence of toxic matter in some industrial and domestic wastes may seriously disrupt a lake's normal biological activity. Anaerobic conditions (the absence of free oxygen) may prevail, and the diversity of life may also be harmed by agricultural pesticides and herbicides carried by streamflow, and by air pollutants carried by rain. In modern industrial societies, requirements for watera large percentage of which is derived from lakesinclude its use for dilution and removal of municipal and industrial wastes, for cooling purposes, for irrigation, for transportation, for power generation, and for recreation. With each of these uses is associated a variety of abuses of the very characteristics of lakes that make them desirable for any of these purposes. Municipalities and industries have polluted lakes chemically and thermally, the shipping that plies large inland water bodies leaves oil and other refuse in its wake, water used for irrigation often contains chemical residues from fertilizers and biocides when it is returned to lakes, and the populace that demands clean bodies of water for its recreation often ignores basic sanitary and antipollution practices, to the ultimate detriment of the waters enjoyed. These abuses of lake waters, however, are being ameliorated by better resource management. any relatively large body of slowly moving or standing water that occupies an inland basin of appreciable size. Definitions that precisely distinguish lakes, ponds, swamps, and even rivers and other bodies of nonoceanic water are not well established. It may be said, however, that rivers and streams are relatively fast moving; marshes and swamps contain relatively large quantities of grasses, trees, or shrubs; and ponds are relatively small in comparison to lakes. Geologically defined, lakes are temporary bodies of water. This article treats lake basins and sedimentation; the physical and chemical properties of lake waters; lake currents, waves, and tides; and the hydrologic balance of lakes. For information on related systems, see the article river. The place of lakes within the hydrologic cycle is further dealt with in hydrosphere, as are certain aspects of lake sedimentation and water chemistry. See inland water ecosystem for information on lacustrine life-forms. any of a class of pigments composed of organic dyes that have been rendered insoluble by interaction with a compound of a metal. The interaction may involve the precipitation of a salt in which the proportions of dye to metal are fixed, or it may be a less well defined attraction between the dye and the surfaces of particles of the inorganic compound. Some lakes are prepared by a combination of both processes. Lakes considerably extend the range of colours available in the production of paints, cosmetics, and inks for printing and lithography. Dyes of several chemical classes are made into lakes by techniques that vary according to the nature of the salt-forming groups in the dye molecule. Mordant dyes and acid dyes form insoluble salts with metal ions, such as those of calcium and aluminum. Basic dyes contain amino groups and form insoluble salts with inorganic metal-containing acids such as phosphotungstic or phosphomolybdic acids. Additional reading Mary J. Burgis and Pat Morris, The Natural History of Lakes (1987); and Laurence Pringle et al.,, Rivers and Lakes (1985), are two treatments written for the general reader. G. Evelyn Hutchinson, A Treatise on Limnology, vol. 1, Geography, Physics, and Chemistry (1957, reissued 1975), still a classic work, contains a comprehensive treatment of the physical and chemical aspects of lakes and includes an excellent bibliography of previous work. Less fundamental but more recent summaries of scientific lake studies include Gerald A. Cole, Textbook of Limnology, 4th ed. (1994); Charles R. Goldman and Alexander J. Horne, Limnology (1983); Robert G. Wetzel, Limnology, 2nd ed. (1983); Abraham Lerman (ed.), LakesChemistry, Geology, Physics (1978); and Werner Stumm (ed.), Chemical Processes in Lakes (1985). Up-to-date books on hydrology and hydrogeology are R. Allan Freeze and John A. Cherry, Groundwater (1979); Patrick A. Domenico and Franklin W. Schwartz, Physical and Chemical Hydrogeology (1990); and Rafael L. Bras, Hydrology (1990). The sedimentology of lakes is the subject of J. McManus and R.W. Duck (eds.), Geomorphology and Sedimentology of Lakes and Reservoirs (1993). Prediction of the behaviour of lakes is dealt with in B. Henderson-Sellers, Engineering Limnology (1984); and A. James (ed.), An Introduction to Water Quality Modelling, 2nd ed. (1993). The dynamics of lakes are covered by K. Hutter (ed.), Hydrodynamics of Lakes (1984); James Lighthill, Waves in Fluids (1978); and Jerome Williams and Samuel A. Elder, Fluid Physics for Oceanographers and Physicists (1989). An up-to-date book on general freshwater biology is Brian Moss, Ecology of Fresh Waters, 2nd ed. (1988). Also of interest on this subject are the studies by C.S. Reynolds, The Ecology of Freshwater Phytoplankton (1984); and by Max M. Tilzer and Colette Serruya (eds.), Large Lakes: Ecological Structure and Function (1990). The consequences of building large-scale dams are analyzed in Edward Goldsmith and Nicholas Hildyard, The Social and Environmental Effects of Large Dams (1986). Kent W. Thornton, Bruce L. Kimmel, and Forrest E. Payne (eds.), Reservoir Limnology (1990), analyzes the ecology of reservoirs in their similarities to and differences from lakes. The effects of pollution on lakes are discussed in B. Henderson-Sellers and H.R. Markland, Decaying Lakes: The Origins and Control of Cultural Eutrophication (1987); E.B. Welch and T. Lindell, Ecological Effects of Wastewater: Applied Limnology and Pollution Effects (1992); Theodora E. Colborn et al., Great Lakes, Great Legacy? (1990); and William Ashworth, The Late, Great Lakes: An Environmental History (1986). Continuing research can be found in Limnology and Oceanography (bimonthly). Robert K. Lane The Editors of the Encyclopdia Britannica