Lake

From the point of view of planetology, a lake is an object with dimensions between a sea and a pond, filled with liquid matter, and stable in time and space.

In geography, a lake is a hollow (or depression) on the land surface where water flows and accumulates. Lakes are not considered part of the world's oceans.

Although the chemical composition of lakes is relatively stable over long periods of time, unlike rivers, their water source and chemical composition are often more consistent, and these factors do not typically dominate the flow regime.

Lakes regulate the flow of rivers by trapping water in their basins and releasing it again at other times. Various chemical reactions occur in the waters of lakes. Some elements sink to the bottom, while others do not. In a number of lakes, mainly those with no outflow, evaporation increases and the concentration of salts increases. As a result, the mineralization and salinity of the lake change significantly.

Due to the significant thermal inertia of the water mass of large lakes, the climate and temperature in nearby areas are softened, and annual and seasonal fluctuations in weather conditions are reduced.

The shape, size, and topography of a lake basin change significantly as sediment accumulates at the bottom. Vegetation growth creates new landforms, including plains and hills. Lakes, especially reservoirs, often act as a reservoir for groundwater, leading to the swamping of nearby dry areas. Due to the continuous accumulation of organic and mineral particles, thick sediments accumulate at the bottom of the lake. These change as the lakes develop and become marshy or dry. Under certain conditions, organic matter is converted into igneous rocks.

There are many dead lakes in the desert and arid steppes. They are called dead lakes because there is no water flowing out of them. The water that enters the lake evaporates, and the salt sinks to the bottom of the lake. The water in some lakes is so salty that plants and animals cannot live there. That is why one such lake in the Middle East was called the Dead Sea. Lake Victoria is located on the equator, almost in the center of Africa. There is no other lake in the world that is as large yet as shallow. Its average depth is 40 m. The northern part of Lake Victoria is very shallow, only a few meters deep. The water is warm all year round to the bottom. In the warm water, countless algae grow. Many fish feed on these algae.

High mountain lakes are formed when loose rocks and cliffs collapse and block a narrow valley through which mountain water flows. The water in Lake is clean and cold. It flows into the lake from a high mountain glacier. Trout thrive in such cold water. They feed more on flies and insects that land on the surface of the lake than on small creatures that live in the water.

There are also lakes underground. Underground water fills caves and forms lakes. Man-made reservoirs also resemble lakes. They are small lakes formed when water is dammed, and very large reservoirs formed when large rivers are dammed by high dams for hydroelectric power plants.

Classification by origin:

• tectonic: formed by water filling cracks in the earth's crust;
• glacial: formed by melting glaciers; for example, Lake Arbersee at the foot of the Great Arber Mountains;
• moraine lakes;
• river lakes;
• near the sea or maritime (lagoons and coastal lakes); most famously the Venetian lagoon;
• sedimentary or pitted (karst, thermokarst); their characteristic feature is that they disappear and reappear periodically, depending on the dynamics of groundwater; for example, Lake Ertso in South Ossetia.
• floodplains - lakes formed by the spreading of water, floods;
• Landslide-dammed lakes: formed when part of a mountain collapses and blocks a valley;
• mountainous: located in mountain valleys and hollows;
• volcanic: occurs in the craters and vents of extinct volcanoes; such lakes exist in the Eifel region of Europe (Germany);

By status or location:

• surface waters, which actively participate in the natural water cycle,
• underground (e.g., subglacial lakes in Antarctica).

According to water balance:

• Exorheic (or Flow-through): having an outflow, often in the form of a river;
• Endorheic (or Terminal): having no outflow, with water loss primarily due to evaporation.

By type of mineralization:

• fresh;
• ultra-fresh
• mineral (salty);
  • Brackish (slightly salty);
  • Saline (salty).

Mineral lakes are divided into the following groups based on the chemical composition of their water:

• carbonate (soda),
• sulfate,
• chloride (salty).

Three types of nutrients (trophicity) are distinguished in the lake:

• Oligotrophic (low in nutrients) - such lakes are characterized mainly by large or medium depths, water masses at the bottom of temperature fluctuations, transparency, water color from blue to green, and a gradual decrease in oxygen towards the bottom.
• Eutrophic (rich in nutrients) - well-warmed lakes (the bottom layer is not very sensitive to temperature fluctuations), medium transparency, green to brown color, organic sediments spread on the bottom. The water is rich in nutrient salts, and the oxygen content towards the bottom is sharply reduced, often completely absent.
• Dystrophic (poor in nutrients) - marshy lakes with low transparency and yellow or brownish water color. The water has low mineralization, and the oxygen content is reduced due to the oxidation of organic matter.

In modern hydrology and hydroecology, intermediate levels of trophic classification are distinguished: mesotrophic (between oligotrophic and eutrophic) and hypertrophic.

The total area of lakes on Earth may account for an estimated 2.7% of the land area (estimated at 4.1 million km²). The size of a lake is often estimated by its water surface area.

• The largest lake in the world by area is the Caspian Sea-Lake. It is so big (371,000 km²) that it is classified as a sea-lake and washes five countries.
• The largest freshwater lake by area is Verkhnee Lake (North America, part of the Great Lakes).
• The deepest lake on Earth is Baikal (Russia). Its depth reaches 1642 meters.
• The most ancient lake in the world is Baikal. Its age is estimated at 25-35 million years, which makes it a unique geological object.
• In Baikal, about 20% of all the fresh water of the planet is concentrated.
• The Dead Sea (Israel/Jordan) is the lowest point of land on Earth (about 430 meters below sea level) and one of the saltiest lakes in which it is impossible to drown.
• Lake Nicaragua is the only freshwater lake in the world where bull sharks live, adapted to this environment.
• Lake Tanganyika (Africa) is the second deepest and longest freshwater lake in the world (length 676 km).
• Lake Titicaca (Peru/Bolivia) is the highest navigable lake in the world, located at an altitude of more than 3800 meters.
• Finland is often called the «Land of a Thousand Lakes» because there are about 188,000 of them on its territory.
• About 60% of all lakes in the world are located on the territory of Canada.
• Lake Vostok (Antarctica) is the largest subglacial lake, isolated from the Earth's atmosphere for a million years.
• Jellyfish Lake (Palau) contains millions of non-stinging jellyfish that do not have access to the ocean.
• Lake Peach Lake (Trinidad) is the world's largest natural reservoir of asphalt.
• Most of the lakes in the world (about 68%) have a glacial origin, that is, they were formed as a result of the retreat of glaciers.