home Roach Why are lakes salty? Why do some lakes become salty?

Why are lakes salty? Why do some lakes become salty?

On the geographical maps the lakes are painted either blue or lilac. Blue color means that the lake is fresh, and lilac - that it is salty.

The salinity of the water in the lakes is different. Some lakes are so saturated with salts that it is impossible to drown in them, and they are called mineral lakes. In others, the water is only slightly salty in taste. The concentration of dissolved substances depends on what kind of water the rivers bring them. If the climate is humid and the rivers are full of water, the lakes are fresh. In deserts, there is little rainfall, rivers often dry up or they don’t exist at all, which is why the lakes are salty.

Among the large lakes of the world, most of all are fresh. This is due to the fact that the water in them is flowing and does not stagnate, which means that the salts brought by the rivers are carried away by them into the seas and oceans.

The freshest lakes on the planet- this is Baikal in Asia, Onega and Ladoga in Eastern Europe, Upper in North America. But the freshest of them should still be considered Lake Benern - the largest of the lakes in Western Europe. Its water is the closest to distilled, there are slightly more soluble minerals in Baikal and Lake Onega.

Freshwater lake of the largest water surface area - Lake Superior - one of the Great Lakes North America. Its area is 83,350 square kilometers.

Mountains are especially poor in salts. glacial lakes, whose waters feed glaciers and snowfields.

If the reservoir is not flowing, then the water in it becomes first slightly brackish, and then salty.

The most saline lakes on our planet can be considered lakes in which the salt content per liter of water is more than 25 grams. Such lakes, in addition to Lake Tuz in Turkey, include Lake Air in Australia, the Dead Sea on the Arabian Peninsula, Molla-Kara in Turkmenistan, Lake Dus-Khol in Tuva and others.

In the center of Turkey, south of Ankara, at an altitude of 900 meters above sea level, there is a lake on which you can walk on foot in summer. This drainless lake Tuz has a length of 80 kilometers, a width of about forty-five kilometers and an average depth of two meters. It is not only small, but also very salty - up to three hundred and twenty-two kilograms of salt per ton of water. In spring, due to winter and spring precipitation, the lake overflows and increases almost seven times, occupying a huge area of 25,000 square kilometers. In the summer, when the water evaporates, the lake becomes quite small, and a dense crust of salt forms on its surface from several centimeters to two meters thick.

The Dead Sea is the deepest and saltiest of the salt lakes. Its greatest depth is over 400 meters, and it is located 395 meters below the level of the oceans. One liter of water Dead Sea it contains 437 grams of salt.

Some of the lakes are brackish-fresh. The most amazing of them is Lake Balkhash. Its western part is fresh, and the eastern part is brackish. The reason for this peculiarity lies in the fact that the Ili River flows into the western part of the lake, and the eastern part lies surrounded by deserts, where water evaporates very strongly. Therefore, on maps West Side Balkhash is shown blue, and the eastern one is lilac.

The huge Lake Chad, located on the outskirts of the Sahara, is fresh on top and brackish at the bottom. Fresh river and rain water, falling into the lake, does not mix with brackish water, but rather floats on it. Freshwater fish live in the upper layer, and marine fish that got into the lake in ancient times stay at the bottom.

The lake is very shallow (from 2 to 4 meters deep). Its shores are flat and swampy, and from the north the desert comes close to them. The hot sun dried up all the northern and eastern tributaries of Chad, turning them into waterless channels - wadis. And only the Shari and Lagoni rivers flowing into it from the south feed the "Sahara Sea" with their waters. Long time Lake Chad, or Ngi Bul as it is called locals, was considered drainless, which was its main riddle. Usually in large, shallow and endorheic lakes on Earth, the water is completely salty, and the upper layer of Lake Chad is fresh. The riddle turned out to be simple.

Approximately 900 kilometers northeast of Chad is the vast Bodele Basin, lying approximately 80 meters below lake level. A water stream hidden under the ground stretched to it from the lake. So, through underground runoff, Lake Chad slowly but constantly renews its waters, preventing them from becoming salty.

Even more surprising is Lake Mogilnoye. It is located on Kildin Island, not far from the northern coast of the Kola Peninsula, and has a depth of 17 meters. The lake consists, as it were, of several layers - "floors". The first "floor" at the bottom of the lake, almost lifeless, consists of liquid silt and is saturated with hydrogen sulfide. The second "floor" stands out in cherry color - this color is given to it by purple bacteria. They are, as it were, a filter that traps hydrogen sulfide rising from the bottom. The "third" floor is a "piece of the sea", hidden in the depths of the lake. This is the usual sea water, and its salinity here is the same as in the sea. This layer is filled with life, jellyfish, crustaceans, stars, sea anemones, sea bass, cod live here. Only they look much smaller than their counterparts at sea. The fourth "floor" is intermediate: the water in it is no longer sea, but not fresh either, but slightly brackish. The fifth "floor" is a six-meter layer of pure spring water suitable for drinking. Animal world here common for freshwater lakes.

The unusual structure is explained by the history of the lake. It is very ancient and was formed on the site of the sea bay. Mogilnoye Lake is separated from the sea only by a small bridge. At high tide, sea water seeps through it in the place where the "marine" layer is located. And the distribution of water in the lake by layers is due to the fact that salt water, as heavier, is at the bottom, and lighter fresh water is at the top. That's why they don't mix. Oxygen does not enter the depths of the lake, and the bottom layers become contaminated with hydrogen sulfide.

An unusual lake called Drutso is located in Tibet. The locals consider it magical. Every 12 years, the water in the lake changes: it becomes either fresh or salty.

Terrestrial reservoirs arose for various reasons. Their creators are water, wind, glaciers, tectonic forces. The water washed out the hollow on the surface of the earth, the wind blew out a depression, plowed and polished the glacier depression, the mountain landslide dammed the river valley - and the bed of the future reservoir is ready. The depressions will be filled with water - a lake will appear.

lakes the globe are divided into two large groups - fresh and salt water. If less than one gram of salts is dissolved in one liter of water, the water is considered fresh, if there are more salts, then salty.

The lakes have the most varied salinity - from fractions of a gram to several tens and hundreds of grams per liter of water. There are, for example, reservoirs in which the water is so saturated with salts that it surpasses the ocean in this respect (35 grams of salts per liter of water); such lakes are called mineral. It all depends on what kind of tribute the rivers bring to them. If the climate is humid and the rivers are full of water, then the rocks in the catchment area are well washed, and therefore the river and lake waters are poorly mineralized.

In a drier climate, where precipitation is scarce and rivers are shallow, their waters contain significantly more salts. Therefore, in the deserts, salt (mineral) lakes are the most widespread. A vivid example of this is Central Kazakhstan, where there are few freshwater lakes, and salty ones are found almost at every step. And yet, among the greatest lakes in the world, freshwater reservoirs predominate.

They are flowing, water does not stagnate in them, salts brought by rivers are discharged into the ocean or sea. And it is worth making such a reservoir drainless - and after some time it will become salty. Take, for example, the Caspian Sea. This huge body of water became largely saline because it had no outlet to the ocean. There were many similar cases on Earth.

In the center of Turkey, south of Ankara, at an altitude of 900 meters above sea level, there is a lake on which you can walk on foot in summer. This drainless Lake Tuz has a length of 80 kilometers, a width of about forty-five kilometers and an average depth of two meters. It is not only shallow, but also very salty - up to three hundred and twenty-two kilograms of salt per ton of water. In spring, due to winter and spring precipitation, the lake overflows and increases almost seven times, occupying a huge area of 25,000 square kilometers. In the summer, when the water evaporates, the lake becomes quite small, and a dense crust of salt forms on its surface from several centimeters to two meters thick.

The Dead Sea is the deepest and saltiest of the salt lakes. Its greatest depth is over 400 meters, and it is located 395 meters below the level of the oceans. One liter of Dead Sea water contains 437 grams of salt.

Some of the lakes are brackish-fresh. The most amazing of them is Lake Balkhash. Its western part is fresh, and the eastern part is brackish. The reason for this peculiarity lies in the fact that the Ili River flows into the western part of the lake, and the eastern part lies surrounded by deserts, where water evaporates very strongly. Therefore, on geographical maps, the western part of Balkhash is shown blue, and the eastern part is lilac.

The lake is very shallow (from 2 to 4 meters deep). Its shores are flat and swampy, and from the north the desert comes close to them. The hot sun dried up all the northern and eastern tributaries of Chad, turning them into waterless channels - wadis. And only the Shari and Lagoni rivers flowing into it from the south feed the "Sahara Sea" with their waters. For a long time, Lake Chad, or Ngi-Bul, as the locals call it, was considered drainless, which was its main mystery. Usually in large, shallow and endorheic lakes on Earth, the water is completely salty, and the upper layer of Lake Chad is fresh. The riddle turned out to be simple.

Even more surprising is Lake Mogilnoye. It is located on Kildin Island, not far from the northern coast of the Kola Peninsula, and has a depth of 17 meters. The lake consists, as it were, of several layers - "floors". The first "floor" at the bottom of the lake, almost lifeless, consists of liquid silt and is saturated with hydrogen sulfide. The second "floor" stands out in cherry color - this color is given to it by purple bacteria. They are, as it were, a filter that traps hydrogen sulfide rising from the bottom. The "third" floor is a "piece of the sea", hidden in the depths of the lake. This is ordinary sea water, and its salinity here is the same as in the sea. This layer is filled with life, jellyfish, crustaceans, stars, sea anemones, sea bass, cod live here. Only they look much smaller than their counterparts at sea. The fourth "floor" is intermediate: the water in it is no longer sea, but not fresh either, but slightly brackish. The fifth "floor" is a six-meter layer of pure spring water suitable for drinking. The fauna here is typical for freshwater lakes.

Everyone who was on the beach could make sure that the water in the sea tasted salty. But where does salt come from if fresh water enters the ocean through rains, rivers and? Why the sea is salty and has it always been so - time to figure it out!

How is the salinity of water determined?

Salinity refers to the content of salts in water. Salinity is most commonly measured in ppm » (‰). A ppm is one thousandth of a number. Let's give an example: the salinity of water is 27 ‰, which means that one liter of water (that's about 1000 grams) contains 27 grams of salt.

Fresh water is considered to be water with an average salinity of 0.146 ‰.

Medium the salinity of the oceans is 35 ‰. Sodium chloride, also known as table salt, makes the water directly salty. Among other salts, its share in sea water is the highest.

The most salty sea is the Red Sea. Its salinity is 41‰.

Where does the salt in the seas and oceans come from

Scientists still disagree about whether sea water was originally salty or acquired such properties over time. Depending on the versions, different sources of the appearance of salts in the World Ocean are also considered.

Rains and rivers

Fresh water always has a large number of salts, and rainwater is no exception. It always contains traces of substances dissolved in it, which were captured during the passage through the atmosphere. Getting into the soil, rainwater washes away a small amount of salts and eventually brings them to lakes and seas. From the surface of the latter, water evaporates intensively, falls again in the form of rain and brings new minerals from the land. The sea is salty because all the salts remain in it.

The same principle applies to rivers. Each of them is not completely fresh, but contains a small amount of salts captured on land.

Confirmation of the theory - salt lakes

The proof that salt comes through rivers are the most saline lakes: Bolshoye Salt Lake and the Dead Sea. Both are about 10 times saltier than sea water. Why are these lakes salty?, while most of the world's lakes are not?

Usually lakes are temporary storages for water. Rivers and streams bring water to lakes, and other rivers carry it away from these lakes. That is, water enters from one end, and leaves from the other.

The Great Salt Lake, the Dead Sea and other salt lakes have no outlets. All the water that flows into these lakes leaves only by evaporation. When the water evaporates, the dissolved salts remain in the water bodies. Thus, some lakes are salty because:

rivers carried salt to them;
the water in the lakes evaporated;
salt remained.

Over the years, the salt in the lake water has accumulated to its current level.

Interesting fact: The density of salt water in the Dead Sea is so high that it practically pushes a person out, preventing him from sinking.

The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and replenish rivers, but the salts remain in the ocean.

hydrothermal processes

Rivers and rains are not the only source of dissolved salts. Not so long ago, on the ocean floor were discovered hydrothermal vents. They represent places where sea water has seeped into the rocks of the earth's crust, has become hotter and is now flowing back into the ocean. Along with it comes a large amount of dissolved minerals.

underwater volcanism

Another source of salts in the oceans is underwater volcanism - volcanic eruptions underwater. It is similar to the previous process in that sea water reacts with hot volcanic products and dissolves some mineral components.

Hydrological types of lakes

The water mass of lakes is created by atmospheric precipitation and groundwater. Sometimes fresh water replaces the sea that filled the basin in the geological past. These are relict lakes, among them are the Caspian Sea, Ladoga and Onega lakes.

Lakes are waste (from which rivers flow), flowing and endorheic (having no flow, mainly in semi-deserts and deserts). The endorheic Lake Chany is very interesting, which, depending on fluctuations in seasonal or annual precipitation, is subject to sharp changes in shape. The wandering lakes include: Lop Nor, Airy Chad.

Due to the large volume of water, the hydrological and thermal regimes of lakes are not as pronounced as those of rivers (see Rivers.). There are no such impressive water rises on the lakes during floods and floods, freezing and ice drift (most lakes in Russia freeze in winter) occur more slowly than on rivers. But there are strong waves, including seiches.

Fresh and salt lakes

Flowing lakes in the vast majority of their fresh water and often have a unique quality drinking water(the most striking example is Baikal). endorheic lakes to some extent mineralized, they accumulate salts (from 1 to 24.7% - brackish lakes, and from 24.7 to 47% - salty), contained even in the fresh water of their tributaries. There are also mineral lakes(which contain more than 47% salts), including flowing ones, formed due to the inflow of mineralized waters from the depths of the Earth. Salts may precipitate out of them. For example, self-sustaining lakes Elton and Baskunchak.

A lake is a closed depression of land filled with water. It has a slow water exchange, unlike rivers, and does not flow into the waters of the oceans, unlike the seas. These reservoirs on our planet are distributed unevenly. total area lakes of the Earth is about 2.7 million km 2, or about 1.8% of the land surface.

The lakes have a number of differences among themselves both in external parameters and in the composition of the water structure, origin, etc.

Classification of lakes by origin

Glacial reservoirs were formed due to the melting of glaciers. This happened during periods of severe cooling, which fettered the continents repeatedly over the past 2 million years. The result of the ice ages were modern lakes located on the territory of North America and Europe, namely in Canada, Baffin Island, Scandinavia, Karelia, the Baltic States, the Urals and in other areas.

Huge blocks of ice, under the weight of their weight, and also because of their movements, formed considerable pits in the thickness earth's surface sometimes even pushed apart tectonic plates. In these pits and faults, after the melting of ice, reservoirs were formed. One of the representatives of glacial lakes can be called Lake. Arbersee.

The cause of the occurrence was the movement of lithospheric plates, as a result of which faults were formed in the earth's crust. They began to fill with water from melting glaciers, which led to the emergence of this type of reservoir. The clearest example is Lake Baikal.

River lakes appear when some sections of flowing rivers dry up. In this case, the formation of chain reservoirs arising from one river takes place. The second variant of river formations are floodplain lakes, which appear due to water barriers that interrupt the water channel.

Seaside lakes are called estuaries. They appear when lowland rivers are flooded by the waters of the seas or as a result of the lowering of sea coasts. In the latter case, a strip of land or shallow water appears between the newly formed bay and the sea. In estuaries, which appeared from the confluence of the river and the sea, the water has a slightly salty taste.

Karst lakes are earthen pits that are filled with water. underground rivers. Pit pits are failures of the lithosphere, consisting of limestone rocks. As a result of the failure, the bottom of the reservoir is lined, which affects the transparency of its filled waters: they are crystal clear.

Karst lakes have one distinctive feature - they are periodic in their appearance. That is, they can disappear and form again. This phenomenon depends on the level of underground rivers.

They are located in mountain valleys. They are formed in several ways. Due to mountain landslides that block the river flow and thereby form lakes. The second way of formation is the slow descent of huge blocks of ice, which leave behind deep land failures - basins that are filled with water from melted ice.

lakes volcanic type appear in the craters of dormant volcanoes. Such craters are of significant depth and high edges, which prevents runoff and inflows of river waters. This makes the volcanic lake practically isolated. Craters fill with rainwater. The specific location of such objects is often reflected in the composition of their waters. The increased content of carbon dioxide makes them dead, unsuitable for life.

These are reservoirs and ponds. They are created intentionally for industrial purposes. settlements. Also, artificial lakes can be the result of earthworks, when the remaining earthen pits are filled with rainwater.

Above was a classification of lakes depending on the origin.

Types of lakes by position

It is possible to make a classification of lakes depending on their position in relation to the earth as follows:

Terrestrial lakes are located directly on the surface of the land. These are involved in the constant water cycle.
Underground lakes are located in underground mountain caves.

Mineralization classification

You can classify lakes by the amount of salts as follows:

Fresh lakes are formed from rainwater, melting glaciers, groundwater. The waters of such natural objects do not contain salts. In addition, fresh lakes are a consequence of the overlapping of river beds. The biggest fresh lake- Baikal.
Salt water bodies are subdivided into brackish and saline.

Brackish lakes are common in arid areas: steppes and deserts.

Salt lakes in terms of the content of salts in the thickness of their waters resemble oceans. Sometimes the salt concentration of lakes is somewhat higher than in the seas and oceans.

Classification by chemical composition

The chemical composition of the lakes of the Earth is different, it depends on the amount of impurities in the water. Lakes are named based on this:

In carbonate lakes, there is an increased concentration of Na and Ca. Soda is mined from the depths of such reservoirs.
Sulfate lakes are considered curative due to the content of Na and Mg in them. In addition, sulfate lakes are a place for the extraction of Glauber's salt.
Chloride lakes are salt lakes, which are the place where common table salt is mined.

Water balance classification

Waste lakes are endowed with the help of which a certain amount of water is discharged. As a rule, such reservoirs have several rivers flowing into their basin, but there is always one flowing one. An excellent example is large lakes- Baikal and Teletskoye. The water of sewage lakes is fresh.
Endorheic lakes are saline lakes, since the flow of water in them is more active than its inflow. They are located in the desert and steppe zones. Sometimes they produce salt and soda on an industrial scale.

Classification according to the amount of nutrients

Oligotrophic lakes contain a relatively small amount of nutrients. The peculiarities are the transparency and purity of the waters, the color from blue to green, the depth of the lakes is significant - from medium to deep, the decrease in oxygen concentration closer to the bottom of the lake.
Eutrophic plants are saturated with a high concentration of nutrients. The peculiarities of such lakes are the following phenomena: the amount of oxygen sharply decreases towards the bottom, there is an excess of mineral salts, the color of the water is from dark green to brown, which is why the transparency of the water is low.
Dystrophic lakes are extremely poor in minerals. There is little oxygen, the transparency is low, the color of the waters can be yellow or dark red.

Conclusion

The water basin of the Earth is made up of: rivers, seas, oceans, glaciers of the oceans, lakes. There are several types of lake classifications. They have been reviewed in this article.

Lakes, like other bodies of water, are the most important Natural resources which are actively used by man in various fields.