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Lakes are commonly defined as accumulation of water in a delimited depression or depressed surface on a generally impermeable floor. We often talk about inland lakes, in order to distinguish them from the word "sea."
The speed of outflow of water from a lake is so low, that a person may not even notice it, which leads to the name of standing water. Smaller lakes are called, depending on the type, pool, pond, swamp, or shallow lake. Large lakes are sometimes called "sea," for example, Dead Sea.
Inland lakes, with a total of 2.5 million square kilometres, occupy 1.8 percent of the land surface. The largest European inland lake is Lake Ladoga in Russia. The world's largest lake without outflow is the Caspian Sea.
Globally, the distribution of lakes is very irregular, but there are regions where there is a larger concentration of lakes. These are mainly regions that have lake plates, which were formed by glaciers (particularly during the Pleistocene), found for example in Scandinavia and North America. The lakes receive their water fromriver mouths, atmospheric precipitation, or even from springs on the bottom of the lake.
There are freshwater and saltwater lakes. Saltwater lakes form in places, where water has no outlet, since in such a case the water evaporates and the salt concentration increases. The saltiest lake on earth is the Dead Sea, which is about nine times saltier than the oceans.
Freshwater lakes are found in places where water can flow in and flow out, for example by means of sluices, over which the water sometimes flows. Freshwater lakes are very important for our lives. They contain 95 percent of the global freshwater supply.
Only hundred years ago, the prevalent opinion was, that the lakes are remnants of ancient seas. This theory, however, applies in the case of just a few lakes. For example, the Caspian Sea was probably connected with the Black Sea some 50 million years ago. It is possible, that Lake Ladoga, situated in Russia, formed a part of Baltic Sea. These bodies of water, separated from seas, are called residual or regressive lakes. The water of the original sea, however, is long gone, because water of every lake is totally renewed every few decades.
Many lakes formed in depressions or by sinking ground. Crater lakes are the result ofvolcanic processes. When the summit of a volcano collapses or is blown off, it creates a circular depression, which fills with rainwater.
Tectonic lakes are those that formed as a result of the movement of the earth's crust, for example, trench lakes. When the earth layers reform and break apart, this creates basins, or deep trenches and kettles. Tectonic lakes are often very deep. One of this type of lakes is Baikal in southeast Siberia (it is 1620 metres deep). Blind river lakes form, when a loop of a river meander separates, forming a semi-circular lake. This type is called oxbow lake.
Mountain lakes (moraine lakes) formed thanks to theglaciers. Glaciers grind depressions in the soil, and when they melt, they deposit the transported sediments. Water collects behind these sediments. End or lateral moraines often form natural dykes.
Artificial lakes are built as regulating reservoirs in the case of flooding. However, they also supply utility water, serve asfisheries, and are used for irrigation.
Inland lakes are very different as far as their origin is concerned. The configuration of their basins contains many common characteristics. In general, we find a gently sloping coastal area, the so-called littoral, consisting of dry, sandy belt, followed by an area which is flooded when the water level rises. Underneath, there are stable coastal sandbanks. The sandbanks are connected to steeply descending lake slope, ending on the bottom of the lake. This zone is also called profundal.
The lake watertemperature depends on the intensity of the sunlight, the air, and the stirring of the water surface by the wind. In tropical regions, the water temperature remains the same. On the surface, it ranges between 28 and 40 degrees Celsius, and it is several degrees lower in deeper waters.
In temperate geographical latitudes, the water temperature fluctuates according to theseason. Only the upper regions of the water are warm, which we can easily test. If we bathe our feet in a lake, it seems warm. When we lower our feet, we feel immediately a big temperature difference. This difference may be of several degrees within a one-metre depth.
Large lakes also have impact on theclimate of their surroundings. They warm up rather slowly, but do not cool down quickly. The warmth collected during the summer is released in wintertime only gradually, which results in temperate climate in lake regions.
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