natural bodies of standing water masses of various compositions which occupy depressions in the continental surfaces of the earth. Lake basins originate as a result of various relief-forming processes and are divided by origin into tectonic, glacial (erosion and aggradation), river, littoral, collapse (karst and thermokarst), aeolian, volcanic (crater lakes and lakes filling lava depressions), and dammed up lakes. Intensive use of water resources has caused a steady increase in the number of regulated storage lakes, including Baikal, Onega, and Victoria. Often several factors are involved in the formation of the basin, for example, tectonics and glaciers. The shape and size of the lake basins change considerably in the course of time through the accumulation of bottom deposits and the re-formation of banks. The lake bed—the part of the basin that is filled with water—is divided into a littoral, or shallow shore area subject to wave action, and a profundal, or open, deeper part where the waves do not affect the bottom.

The dimensions of a lake include its surface area, length, width, the extent and indentation of its shoreline, the volume of water, the average and maximum depth, and the ratio between area and volume at various depths. The volume of water and the changes in it over time depend on the lake’s water balance, that is, its intake and loss of water. The main components of the intake are the surface and underground inflow from the basin and the atmospheric precipitation on the lake’s surface, and water loss consists chiefly of surface and underground runoff from the lake and evaporation from its surface. In terms of their water balance, lakes are divided into those with outlets, un-drained lakes, and lakes with intermittent drainage. Geographic zonality, altitude, and size and shape play an important role in a lake’s water balance and regime. In wet regions, both the intake and loss of water essentially result from drainage. Lakes in arid regions lose their water through evaporation, and most of them have no outlets. By retaining the water that flows in from their catchment basins and slowly giving it up to outflowing rivers, lakes regulate river runoff.

The world’s lakes cover about 2.7 million sq km, or about 1.8 percent of the land area, and have a volume of about 230,000 cu km. In the USSR there are more than 2.8 million lakes with a total area of some 490,000 sq km. Of these, about 37,000 have a surface area of 1 to 10 sq km; 2,400 cover from 10 to 100 sq km; and 185 have an area exceeding 100 sq km. The distribution of lakes throughout the world is uneven and depends primarily on the water balance, which is determined by the climate.

The water level of lakes undergoes seasonal as well as long-term fluctuations. Whereas seasonal fluctuations, which are related to the water balance in large lakes, rarely exceed 1 m, long-term fluctuations may reach 3–7 m. In arid regions, lakes frequently dry up. Wind produces waves that are smaller than ocean waves (up to 3–5 m high), but they are steeper and more closely spaced. Lake currents are caused chiefly by winds, and seiches are created by wind or changes in air pressure. Lake water is warmed primarily by direct and scattered solar radiation. Heat is lost mainly through evaporation, the release of heat into the air, and radiation. Heat is carried to the depths and distributed in the water mass by mixing and currents.

During the summer the temperature of lakes in the temperate zone gradually decreases with distance from the surface (direct temperature stratification). Between the warm, less dense surface layer (epilimnion) and the cold, dense, deep layer (hypolimnion) there is usually a stratum of rapid temperature change (thermocline), in which the temperature drops as much as 10°C per m. During the winter these lakes have an inverse temperature stratification—the temperature rises as one moves from the surface to the bottom (from 0° to 4°C). In the spring and autumn there is homoeothermy, that is, the same temperature and density prevail throughout the entire body of water, and this promotes mixing. In tropical lakes there is direct stratification throughout most of the year; lakes in the frigid zone have inverse stratification.

The ice of lakes is layered, usually uneven, and hummocky. Freezing and ice breakup depend on the loss and influx of heat. Because of their large reserve of heat and wave action, large lakes freeze and break up later than rivers; most of the ice thaws in the lakes and only a small portion is carried into the rivers. Salt lakes sometimes do not freeze during the winter despite temperatures below 0°C, and during the summer temperatures beneath the surface freshwater layer may reach 60°C or higher.

Lake waters are complex polydispersion systems containing, in addition to H₂O, ions, dissociated molecules, gases, mineral and organic particles (ranging from colloidal to large particles), and organisms and their remains. The salt content of lakes varies from several mg to 300 g or more per liter in mineral lakes. Natural regions tend to have characteristic hydrochemical facies in their lake water: Si and HCO₃⁻ predominate in the tundra zone; the ions Ca²⁺ and HCO₃⁻, in the forest zone; the ions Na⁺ and SO₄^2- or Na⁺ and Cl⁻, in the steppe zone; and the ions Na⁺ and Cl⁻, in the desert and semidesert zone. In addition to the main ions of mineralization—HCO₃⁻, CO₃², SO₄^2-, Cl⁻, CA²⁺, Mg²⁺, Na⁺ and K⁺—the biogenic elements N (in its bonded form), P, Si, Fe, Mn, Cu, and Zn are important for the development of life and are often scarce.

Gases penetrate the surface waters of the lakes and diffusing through the water are transported by the water masses; surplus gases are released into the atmosphere. The gases affect the lake’s hydrochemical conditions and the existence of organisms. In most instances, the water’s acidity or alkalinity depends on the ratio between nondissociated and dissociated carbon dioxide, its ratio of bicarbonate and carbonate salts. The oxygen content and the amount of hydrogen sulfide, methane, and hydrogen determine the characteristics of the oxidation and reduction zones of the water layers and the bottom. A shortage of oxygen causes the asphyxiation of fish in summer and winter and the destruction of invertebrates and plants. Only bacterial forms of life can survive without oxygen. In the process of photosynthesis aquatic plants release oxygen and create organic matter. Using gases and biogenic elements, photosynthetic and chemosynthetic organisms create autochthonous organic matter. Matter brought to a lake is called allochthonous.

Deposits are formed on the bottoms of lakes from mineral and organic particles brought by runoff and wind from lake basins or formed in the lakes through the destruction of shores and the dying of plants and animals. The same processes cause the silting up of lakes. The color and transparency of the water depend on the quantity of mineral and organic suspended matter. A light blue color and great transparency (up to 40 m in Baikal) are characteristic of lakes with pure water, usually large ones. As the turbidity increases, the color of the water becomes green or brown, and the transparency decreases to 1 m or less. The thickness of the photosynthesis layer depends on transparency. A compensation level is established in lakes, above which the photosynthetic production of organic matter and release of oxygen exceeds the total expended through decomposition.

According to their location in lakes and their processes of adaptation, aquatic organisms are divided into bottom organisms (benthos), surface organisms (pleuston), floating organisms (plankton), and freely swimming organisms (nekton); hygrophils live along the shores.

In terms of biological productivity, lakes are divided into highly productive lakes, which are rich in biogenic elements (eutrophic), unproductive lakes, which are poor in biogenic elements (oligotrophic), and lakes enriched with humic matter (dystrophic). In addition to seasonal cycles in the regime and in the development of plant and animal life, lakes have long-term cycles and undergo successive states resulting in their disappearance. In the course of their evolution, lakes are filled with debris, become overgrown, and are transformed into bogs in wet climates and into salt marshes in dry climates.

Lakes contain a large part of the world’s scarce fresh water (123,000 cu km) and thus ensure the normal vital activity of man and valuable plants and animals. The water resources of lakes and the products obtained from them are widely used in the national economy for such purposes as water supply, water transport, water power, fisheries, irrigation, and obtaining raw materials for industry. Peat and such bottom deposits as sapropels and salts are extracted. Therapeutic muds, called peloids, are used extensively in medicine. In the USSR and the other socialist countries, great importance is attached to the integrated use of lakes. Lakes are important for organizing recreation and for resort medical treatment using muds and brines. With careless management, the removal of waste water and the runoff from agricultural land and forests where fertilizers and toxic chemicals are used can change the regime of lakes and undermine their resources. In the industrially developed and densely populated nations the quality of lake water is deteriorating because of pollution. The Great Lakes in North America are a vivid example of this trend. The water of the lakes is used by more than 250 cities which each day remove more than 15 billion liters, about equal to the waste water dumped into the lakes daily. In the USSR and in many foreign countries, laws have been passed to protect natural waters, and much attention is being given to problems of water toxicology and processes of self-purification. (See Table 1 for data on the world’s largest lakes.)