home Fishing methods Why is the sea salty, and some lakes are even saltier. Lakes with unique salinity Deep fresh or salt lake

Why is the sea salty, and some lakes are even saltier. Lakes with unique salinity Deep fresh or salt lake

why lakes are salty other fresh and got the best answer

Answer from Zhenok Rain[guru]
On 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 are 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.
The freshwater lake of the largest area of the water surface - Lake Superior - one of the Great Lakes of North America. Its area is 83,350 square kilometers.
Mountain glacial lakes are especially poor in salts, the waters of which 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 very small, and a dense crust of salt forms on its surface from a few 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 the water evaporates very strongly. Therefore, on geographical maps, the western part of Balkhash is shown blue, and the eastern part 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.
Source:

Answer from Department for interaction with POWC[newbie]
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Answer from Albina Sapitsyna[active]
PATAMU))))))

Answer from Katya Shubina[newbie]
Not quite right with stock. and the matter is in feeding the lakes, if the lake is fed from a salty spring (mineral spring), then it is salty. Nearby it is in 10-15 meters.

Answer from Victoria Volgina[newbie]
Salt water becomes in drainless lakes in a hot arid climate. Due to the high evaporation, the salts contained in the water accumulate in the lake, making it salty.

In natural waters there are always impurities in the form of suspended or dissolved substances, and they can be considered as a very complex solution, representing a true solution in relation to some substances, and colloidal in relation to others.

With the help of special methods of filtration from natural waters, it is possible to isolate the colloidal part, separating it from water and electrolytes. In this way, for example, it was established that in the entire water mass of Lake Baikal there are about 55 thousand tons of solid matter in a finely divided phase. In general, in the hydrosphere (on the whole Earth), the order of concentration of colloids is expressed by the value x · 10 -5%, where x does not exceed the order of hundreds.

However, water is of particular interest as a true solution, because the concentration of molecularly dissolved substances is immeasurably more significant than the concentration of colloids.

Any water contains certain salts in solution, but if the salinity of the water is less than 0.3‰, then such water is called fresh water. Consequently, less than 0.3 g of dissolved salts are contained in 1 thousand g of fresh water. When the salinity is from 0.3 to 24.695‰, the water is called brackish, and when the salinity is above 24.695‰, it is called salty. The value of 24.695‰ was chosen as a boundary between brackish and saline waters because only at this salinity value, the freezing point of water and its highest density are equal (-1.332°). If the salinity is less than 24.695‰, then with continuous cooling, the water will first reach its highest density, and then it will freeze; if the salinity is more than 24.695‰, then water under similar conditions will freeze before it reaches its highest density.

For obvious reasons, it is difficult to expect a high concentration of salts in flowing waters. But in stagnant reservoirs, especially those deprived of runoff and subjected to increased evaporation, a lot of salts accumulate. In accordance with this, lakes are divided into fresh and salt, or mineral.

In lakes of marine origin, i.e., in reservoirs detached from the sea, the presence of salts in the water is, as it were, a "hereditary" phenomenon. In the process of further independent existence of such a relict lake, its hereditary features either increase (it becomes saltier than the parent water basin) or weaken (desalination). As for continental salt lakes, salts enter them due to chemical weathering of crystalline rocks, leaching of various sedimentary rocks, dissolution of ancient salt deposits by groundwater, etc.

The main factors in the distribution of mineral lakes are the climate and the presence of drainage basins, as well as the composition of the rocks that make up the area, and the groundwater regime. Steppes and deserts are the birthplace of salt lakes, since there is little precipitation, evaporation is high, and the relief is predominantly flat, and therefore the runoff is weak. The large lakes of Tibet - Namtso (Tengri-Nur), Kukunor, and others - are salt lakes.

But salt lakes can also be in a humid climate if there are salt deposits nearby; in this case, the origin of the salt lake is influenced not by the modern climate, but by the climate of the geological past, in which salt deposits could form. Thus, the small salt lakes of the Lena-Vilyui Plain are fed by salt springs that come out of the salt-bearing strata of Paleozoic rocks.

Mineral lakes are quite diverse in terms of the composition of dissolved salts. Soda lakes are widely represented in Western Siberia (Lake Tanatar, Petukhovskie lakes, etc.), in Transbaikalia (Lake Doroninskoe), and Yakutia. Bitter-salty, or sulfate lakes, which precipitate mainly Glauber's salt, are found in the Kulunda steppe, in the Crimea, in the Caucasus (Batalpashinsky lakes), in the deserts of Central Asia, etc. Salt (chloride) lakes are among the most common - their many in the Crimea, in the Kulunda steppe in the Volga region and other places. The Kulunda steppe, in terms of the number of mineral lakes (there are several thousand of them here) and their diversity (soda, salt, Glauber lakes), is undoubtedly an area for the development of a large chemical industry in the future.

The concentration of salts in mineral lakes varies over a very wide range. It differs not only in different lakes, reaching up to 37% in some cases, but often changes noticeably in the same lake depending on the water level in the latter, i.e., depending on the volume of the water mass. Thus, the salinity of the Great Salt Lake in Utah, in accordance with fluctuations in the water level in the lake, varies from 13 to 22%.

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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 small amount of salt, 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

Evidence that salt comes through rivers are the most saline lakes: the Great 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.

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. The total area of the Earth's lakes 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 of the 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 earth pits that are filled with the waters of 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.

Volcanic-type lakes appear in the craters of dormant volcanoes. Such craters have a significant depth and high edges, which impede the runoff and inflow 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 the industrial purposes of 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 largest fresh lake is Baikal.
Salt water bodies are divided 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

Sewage 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 the 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 world ocean, 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 that are actively used by man in various fields.

Geography lesson in grade 6

Teacher: Neborak T.I.

Lesson topic: "Blue Eyes of the Planet" ( lakes).

Target: get acquainted with the variety of lakes of our planet and their origin.

Tasks:

educational: to form students' ideas about the lake, types of lake basins, waste and drainless, fresh and salt lakes;

developing: development of individual cognitive interests of students;

educational : to instill in students a sense of love for their homeland, pride in their land.

Lesson type: explanation of new material.

At the end of the lesson the student should:

Know: what is a lake; types of lake basins; waste and non-drainage, salt and fresh lakes;

Be able to: show lakes on the map.

Technology: lesson using ICT and problem-based learning.

Equipment:

TCO tool: computer, projector, screen.

Didactic and visual material: Route sheets, textbooks, a map of the hemispheres and a physical map of Russia, atlases, slides are shown in parallel.

Lesson stages: 1. Organizational

2. The study of new material with a phased consolidation.

3. The results of the lesson.

4. Homework

During the classes.

1. Organizational.

Hello guys! The lesson of geography today will be taught by me - Tatyana Ilyinichna Neborak. I hope that our cooperation will be not only pleasant, but also fruitful.

2. Learning new material.

(against the background of music).

Teacher. Listen and guess the riddle that will tell us the topic of today's lesson. Slide number 1.

There is a mirror in the middle of the field.

Blue glass, green frame.

Young mountain ash look at him,

Colored their own, directing headscarves

Young birch trees look at him,

Straightening her hair in front of him.

And the month, and the stars - everything is reflected in it ...

What is this mirror called? (Lake).

Teacher. Correctly. This lake. The topic of our lesson is “The Blue Eyes of the Planet”. (Lakes). Slide number 2.

Teacher. During the lesson, we will consider the following questions:

The variety of lakes of our planet and their origin;

Which lakes are called sewage, and which are drainless;

Let's get acquainted with the concepts of salt and fresh lakes. Slide number 3.

Teacher. We will keep notes in the lesson today not in notebooks, but in Route sheets, ( Application No. 1) that are on your desk. Slide number 4.

Teacher. Our study road is long and therefore we set off without delay.

Teacher. Guys, which of you saw the lake? Hands up.

Teacher. I invite you to close your eyes for a few seconds and imagine a lake. (Music). Now let's open our eyes and tell what we saw. Before my eyes, the lake was round and shallow. And what word do you associate it with? (small and large, deep and shallow, etc.). (Children's answers)

Exercise. Try to formulate the definition of the concept of "lake".

(Children answer).

Teacher. Let's compare with the definition that the authors of the textbook offer us. p. 95. Who was the first to find the definition?

A lake is a closed body of water formed on the land surface in a natural depression.

We write down the definition in the Route sheet. Slide number 5.

Teacher. This depression is called the lake basin.

Teacher. Guys, the lake is not part of the ocean, like the sea. (explanation)

Teacher. We have already found out that a lake is a natural depression or a lake basin formed on the earth's surface. What natural processes and phenomena can lead to the formation of lake basins? Yes, the question is complicated, but we will try to answer it now.

For further work, we will need atlases on page 16 "Physical map of Russia".

Teacher. So, pay attention to the screen.

Teacher's story.

Tectonic in troughs (residual). They were formed due to the slow sinking of vast areas of the earth's crust, which were filled with water. (Aral, and the largest lake in the world - Caspian). Slide number 6,7.

2. Tectonic in faults. During the movement of sections of the earth's crust, faults were formed, which were filled with water. As a rule, such lakes are very deep. (Tanganyika, Nyasa on the African mainland. What do you think, but in Russia there are such lakes). - The deepest lake is Baikal. It is unique. It knows no equal in fame and glory. The greatest depth is 1620m. Baikal contains one tenth of the fresh water of the entire Earth. Baikal ranks first in the world in terms of water transparency. 336 rivers and streams flow into the lake, and one Angara flows out. Slide number 8,9.

3. Lakes of glacial origin were formed on the site of depressions deepened by a glacier. (Onega. Lake Ladoga has a glorious history: during the Great Patriotic War, the Road of Life passed through its ice - the only connection between the country and besieged Leningrad).

Slide number 10,11.

4. Dams were formed as a result of a collapse or shedding of rocks into the river valley, an example is Lake Sarez in the Pamirs.

(Student's story). On a February night in 1911, the inhabitants of Bartang woke up from an incredible rumble coming from the bowels of the earth. The rumble was accompanied by a deafening stone rumble. It seemed that somewhere above an invisible genie was raging, breaking the Pamir giant. Giant fragments of rocks flew from the peaks. Frightened people felt as if they were on unsteady waves; the ground was moving underfoot. There was a strong earthquake. And over the village of Usoy, clouds of large dust swirled for several days. And when the dust cleared, in the riverbed people saw a miraculous stone wall half a kilometer high. A wall formed from fragments of rocks blocked a stormy river. This is how the Sarez Lake was born in the Pamirs. Slide number 12,13.

5. Volcanic. Their lake basins are located in the crater of extinct volcanoes. (Kronotskoye, Kurilskoye.) Slide number 14,15.

6. Lakes-oxbow lakes are often found in floodplains, they are the remains of former river channels. These lakes are small, so they are not marked on the map. They have an arched shape. Slide number 16.

7. Karst. Easily soluble rocks (limestone, salt, etc.) are found in the earth's crust and on its surface. When they dissolve, voids, caves are formed, and on the earth's surface hollows are formed, which are filled with water. (Many in the Urals, Caucasus). Slide number 17.

Dynamic pause. (Close screen)

Teacher. There is a table "Types of lake basins" in the Route sheets. Working with the textbook pp. 96-97 and based on the material that I told you, you fill in the table. We will work in pairs. I gave you cards in advance, on which only one type of origin of lake basins is written. You will describe it.

Guys, pay attention to the example of filling. Slide number 18.

Basin type	Reason for education
Tectonic in troughs	Subsidence of vast areas of the earth's crust	Aral, Caspian
Tectonic in faults	Lowering sections of the earth's crust along the fault	Baikal, Nyasa, Tanganyika.
Glacial	The basin is deepened by ancient glaciers.	Onega, Ladoga.
Zaprudnye	The riverbed is blocked by a collapse or flow of hardened lava.	Sarez.
Volcanic	Craters of extinct volcanoes	Kronotskoye, Kurilskoye.
Karst	It is formed after the failure of the upper layer covering the underground cavity, and fills it with water.
	Sections of the former river bed that has changed its direction.	Small ones are not marked on the maps.

Teacher. Did you complete the task? Now let's make a general table.

(Those who wish to report on the work done in groups).

Teacher. Guys, do you know the name of the lakes of the Kargat region? (Eastern part of Lake Ubinskoye, Kargan, Small and Big Toroky, Atkul, Kankul, Kayly, Bizura). The lakes of our region are the remains of an ancient lake system. Slide number 19.

Teacher. A lake is a depression filled with water. Where do you think the water in these recesses comes from? What do lakes feed on? (precipitation by atmospheric, groundwater, waters of inflowing rivers). Slide number 20,21.

Teacher. All lakes can be divided into 2 groups according to the inflow and outflow of water.

In the diagram you see lakes and rivers. Please note that rivers can flow into and out of lakes .

Children's answers.

Teacher. We have two new concepts: waste and drainless lakes. Let's try to formulate definitions. Slide number 22.

A wastewater lake is a lake into which rivers flow in and out (or only flow out)

Endorheic lakes - lakes into which rivers only flow.

Teacher. What type of lakes do you think Lake Baikal belongs to? (children's answers)

Why? (children's answers)

Examples of sewage lakes are also Onega and Ladoga lakes.

What type of lakes does the Aral Sea-Lake belong to? (children's answers)

Why? (children's answers)

Examples of endorheic lakes are also the Caspian and Balkhash.

Write examples of lakes in the table. Slide number 23.

	Baikal, Onega, Ladoga
Drainless	Caspian, Balkhash, Aral

Teacher. Lakes differ not only in the flow of water, but also in the presence of salts. Let's remember, what is salinity? (the amount of minerals in 1 liter of water). And what does it express? (in ppm-thousandths of a number). Slide number 24,25.

Teacher. All lakes can be divided into two groups according to salinity: fresh - up to 1% o; salty - from 1%o to 35%o. Fresh on the map are indicated in blue, salty in pink.

Teacher: Find Lake Balkhash in the atlases. Pay attention to its color. Part of the lake is colored blue and the other pink. Why do you think? (salty lakes are most often endorheic, since mineral substances brought by rivers gradually accumulate in lakes)

Teacher. Is Lake Baikal fresh or salty? ? (Children's answers)

Exercise: write examples of lakes in the table.

	Baikal, Ladoga, etc.
	Balkhash, Caspian, Dead (270 ppm)

Student reports about Dead Lake. Slide number 26. Listen to what a person who has visited its shores tells about one of the most saline lakes, Dead Lake: “We stood on a deserted shore, the dull look of which evoked sadness: a dead land - no grass, no birds. On the other side of the lake reddish mountains rose steeply from the green water. We decided to swim, but we were dissuaded. We only washed ourselves with water thick as a steep brine. A few minutes later, the face and hands were covered with a white coating of salt, and an unbearably bitter taste remained on the lips. Sometimes fish swim from the Jordan River into the Dead Lake. She dies in a minute. We found one such fish thrown ashore. She was hard as a stick, in a strong salty shell.

Teacher. How does a person use lakes? Slide number 28.(fishing, for navigation, waterfowl are bred, salt is mined, the coast of lakes is a favorable zone not only for life, but also for recreation, recovery of health).

Teacher. Lakes have been used by humans since ancient times. But today, like other water bodies, many lakes are polluted with oil products, industrial and domestic waste, and pesticides from the fields. The “blue eyes” of the planet, including the lakes of the Kargat region, ask for help from people, shedding clean tears.

Slide number 29. Even the tiniest lake filled with “living water” should be treated as a most valuable gift in order to bring this cup of priceless life-giving moisture to future generations.

3. The results of the lesson.

Teacher. Our lesson is coming to an end. I would like to know how successful it was. Answer the questions: What did you learn? what did you learn? Start your answers with... Slide number 30.

I found out…

I can…

4. Homework. Slide number 31. Now pay attention to homework. Read 31 paragraphs, describe the lake Baikal according to a standard plan (on route sheets). Prepare messages about unusual lakes of the world (optional). (they do not write down in the diary because the homework is written on the route sheets).

And yet, I would like to know the attitude to the lesson of each of you. Choose the emoticon that most clearly reflects your mood. And I, in turn, will give you my emoticons as a keepsake of this lesson.

Thank you for the lesson. Slide number 32.

Route sheet

Lesson topic: “ Blue eyes of the planet”(lakes).

Lake - _____________________________________________________________________

_____________________________________________________________________________

Types of lake basins

Basin type	Reason for education	Examples
Tectonic in troughs (residual)
Tectonic in faults
Glacial
Zaprudnye
Volcanic
Karst

According to the consumption and income of water


Drainless

Salinity

Fresh up to 1%o
Salty from 1%o and above

Homework: read the text of the textbook item 31, using the plan to describe Lake Baikal, prepare messages about unusual lakes (optional).

Fill in the table:

Description of Lake Baikal according to a standard plan

Lake description plan	Characteristics of the lake
1. Title
2. What continent is it on and in what part of it
3. Between which meridians and parallels is
4. Origin of the basin
5. Waste or drainless. Inflowing and outflowing rivers
6. Salty or fresh

Unusual lakes of the world: ink lake, pan lake, burning lake, ghost lake, asphalt lake, sweet lake.

The lake is one of the components of the hydrosphere, the water shell of the Earth. Lakes are natural water bodies. They are a kind of bowl (lake bed), filled to the brim with water. There are more than five million lakes on Earth, the total area of which is more than 2.7 million square meters. kilometers.

The science of planetology defines a lake as an object that is stable

existing in time and space and filled with liquid matter. The size of the lake, according to the same science, is the average between the sea and the pond. If we consider the lakes from the point of view of geography, then this is a depression in the earth's surface into which water flows and accumulates. It is important to know that such reservoirs are not part of the oceans.

The chemical composition of lake water is considered to be quite stable. Water in the lakes practically does not circulate, so the fill fluid is updated quite rarely. Lakes perform an important function - they retain water in their basins and release them at different times.

Such reservoirs have significant thermal inertia, therefore, contribute to climate mitigation in adjacent areas. In lakes, processes of accumulation of sediments (minerals and) are constantly taking place, which in turn form bottom sediments. During the subsequent development of the reservoir, bottom sediments can be transformed into land, swamps, or mountain sediments.

Large lakes are able to have a mitigating effect on the climatic conditions of adjacent territories. The lakes that exist on the planet are classified according to several criteria. They can be ground and underground, mountain, river, crater, failure. They can be anthropogenic, that is, artificial and natural. According to the water balance, they are divided into waste and non-drainage.

endorheic lakes

On Earth, there are many land areas with river systems that are not connected to the oceans. River basins located in such areas are called endorheic basins. And the bottom of such pools is, as a rule, an endorheic lake. Science gives the following definition: a drainless lake is a body of water that does not have a drain underground and surface runoff. Simply put, one or more rivers can flow into such reservoirs, but none of them flow out.

Endorheic lakes are formed in areas with an arid climate, where moisture is much lower than evaporation. Endorheic lakes are scattered all over the planet, they are on all continents, even in Antarctica. There, such lakes are located on the territory of the land of Victoria and the dry McMurdo Valley.

The most famous are Frixell, Vostok, Ellsworth, Don Juan. The area of Lake Fryxell is 7 sq. km, and its entire surface is permanently covered with ice about 5 meters thick. Vostok is the largest subglacial freshwater lake in Antarctica. Its uniqueness lies in the fact that for several million years it was isolated from the surface of the Earth. Don Juan is a very small lake, interesting, first of all, because it currently claims the status of the most salty lake in the world. The salt content in Lake Friksell is more than 40%, the salinity of the famous Dead Sea is slightly less than 35%. Due to the high salt content, the reservoir does not freeze even at a temperature of -53 degrees.

Another amazing fact about Fixell: its waters contain a lot of nitrous oxide, which appears as a result of the vital activity of microorganisms. Meanwhile, scientists failed to detect a single microorganism in the waters of the lake.

Lake Wanda- Another one of the mysteries of Antarctica. The fact is that, despite the low ambient temperatures, the waters of the lake always maintain a temperature of about +26 degrees. The reason was unknown until recently, and only recently scientists found out that the water is simply heated by the Sun. This happens because the ice above the lake has the shape of a lens, which means it focuses the heat of the sun.

In Australia, such reservoirs include Air, Corangamite, George, Torrens. In North America - Pyramid Lake, Sevier, Mona, Atitlan. The inner basin is a large part of Central and Western Asia. Sasykkol, Balkhash, Zhalanoshkol, Issyk-Kul are located on this territory. The Caspian and Aral Seas are not seas by origin, but residual lakes, relics formed after the disappearance of the ancient Tethys Ocean.

Baikal- the largest freshwater lake in Russia, is the deepest in the world. The water in it is so clean and transparent that objects can be found at a depth of 40 meters. This lake is one of the oldest on Earth, it was formed 20-25 million years ago. 336 rivers flow into it, but only one flows out - the Angara. Thus, Baikal is a sewage lake.

Endorheic lakes are almost always salty. This is explained by the fact that no rivers flow out of them that could carry this salt.

Waste lakes

Waste lakes are lakes that have a runoff (as a rule, these are rivers). Most of the lakes of this type are located in areas with a temperate and humid climate. Interestingly, several rivers can flow into such reservoirs, and only one can flow out. Dissolved substances (such as salt) are removed from the waters with the help of effluents. However, in some lakes, water exchange can be slow, which leads to salt accumulation and other biochemical processes. The way the water changes in a reservoir determines the amount of water in the lake, its chemical composition, and its ability to self-purify.

One of the subspecies of sewage lakes are flowing. They differ in that the flowing river carries away about the same amount of water as the flowing river brings. The flowing ones include Chudskoye, Kubenskoye, Zaisan. Water consumption in such lakes occurs mainly due to runoff and evaporation. Flow in these lakes is one of the most important characteristics, as it affects the filling, composition and water exchange. The largest lake on our planet is the Caspian Sea. Despite the presence of the word "sea" in the name, technically the Caspian is not one. The fact is that the sea is part of the oceans. If completely separated from the ocean by land, then this is a lake. The area of the Caspian Lake is 371,000 square kilometers.

Fresh lakes

Lakes are divided into several categories according to different criteria. By mineralization, they are divided into fresh, ultra-fresh, brackish and salty. Fresh lakes are those lakes, the salt content in the waters of which is minimal, that is, less than 1%. Freshwater lakes can be both sewage and flowing. Drainless - always salty.
There are tens of thousands of freshwater lakes on the planet, some of them have truly amazing characteristics, have a unique location and an interesting history. In the country of Nicaragua, for example, there is a lake with the same name. It has a tectonic origin, an area of about 8 thousand square kilometers. The uniqueness of Nicaragua is that it is the only freshwater lake on our planet in which sharks live. The distance from the lake to the Pacific Ocean is very small, so scientists admit that earlier the territory on which the reservoir is located was a bay of the sea.

Another amazing creation of nature is Lake Titicaca. It is located at an altitude of 4000 meters above sea level, and was also once part of the oceans. More than three hundred rivers flow into it, most of which flow from glaciers. Titicaca was studied by scientists who came to the conclusion that thousands of years ago the lake was much lower - at an altitude of about 250 meters. Then the reservoir was a sea bay, and its waters are still inhabited mainly by marine species of crustaceans and fish.
Fresh lakes located in a hot thermal zone are distinguished by the fact that the water near their surface is warm. As the depth increases, the water temperature decreases. The name of this phenomenon is direct thermal stratification. Interestingly, the lakes that are located in the cold zone have the water with the lowest temperature (about 0 degrees Celsius), but the greater the depth, the higher the temperature. If the temperature in a body of water is distributed in this way, it is called reverse thermal stratification.

Interesting Facts:

If you pay attention to the geographical map of the world, you will notice that some lakes are marked in blue, while others are purple. This is explained simply - fresh lakes are highlighted in blue, salty lakes are purple.
There are more fresh lakes on our planet than salty ones.
The world's largest freshwater lake is located in North America. This is Lake Superior, it is part of the Great Lakes group.
There are also peculiar "record holders" for salinity. These are considered water bodies, the salt content in which is more than 25 grams per liter. Examples are Ace (Turkey), Air (Australia), Dus-Khol (Tuva).
The most unsalted lakes are those that are in mountain glaciers.
One of the saltiest is Lake Tuz. It is 80 km long and about 45 km wide. When the lake overflows, it becomes huge - up to 25 thousand square kilometers. The salt content in its waters reaches 322 grams per liter of water.
The most salty and deepest is the Dead Sea. Its depth in some places reaches 400 meters. The salt content in it is 437 grams per liter of water.
There are some amazing lakes on the planet. For example, Balkhash, one part of which is salty and the other is fresh. And Lake Chad, located in Africa, is fresh from above, and closer to the bottom is salty. This is explained by the fact that fresh water (rain) does not mix with salt water when it enters the lake. Thus, Lake Chad is two-layered.
The largest basins, in which lakes subsequently formed, are of tectonic origin.

Lakes are depressions in the land filled with water, which arose naturally and occupy about 2% of the entire land. On the territory of Russia are located the deepest lake - Baikal and the largest lake in the world - the Caspian.

People use lakes for water supply. The lakes serve as communication routes, they are full of fish. At the bottom of some lakes, valuable minerals have been found: iron ores, salts, sapropel. The shores of the lakes are equipped for people's recreation, sanatoriums and rest houses have been built there.

Lake types

Lakes according to the nature of the runoff are divided into:

a) drainless;

b) waste.

Many rivers flow into flowing lakes and flow out of them, for example, they include Onega and Ladoga lakes. A waste lake is a lake that is replenished with water from a large number of rivers, and only one river originates from it. Lakes Baikal and Teletskoye belong to this type. Endorheic lakes are located mainly in arid regions and in the tundra, not a single river flows out of them. Representatives of such lakes are the Caspian, Aral, Balkhash.

Lake depressions arose as a result of various natural processes. The hollows that have arisen under the influence of the internal forces of the Earth are called endogenous. These include tectonic and volcanic. This is the origin of most large lakes in the world. Lakes that have arisen as a result of the activity of external forces are called exogenous, they are mainly shallow lakes. Tectonic basins were formed in places of sunken sections of the earth's crust. They could be formed as a result of faults along the cracks in the earth's crust or the deflection of its layers. In tectonic basins, the following lakes were formed: Aral - due to the deflection of the earth's layers, and Baikal, Upper, Huron, Michigan, Tanganyika- due to resets.

Volcanic basins are volcanic craters, depressions on the surface of lava flows, or flat areas covered by lava flows. In volcanic basins, Kronotskoye Lake in Kamchatka, lakes of New Zealand, the Kuril Islands and the island of Java were formed.

Lake basins of exogenous origin are also diverse. Oxbow lakes, found in river valleys and having an oblong shape, arose in the places of former river beds. Lakes of glacial origin are formed as a result of the long-standing advance of glaciers on land, that is, during the ice age. They were formed due to the movement of glaciers, which plowed huge furrows on the earth's surface, filled with water. Such glacial lakes are narrow and elongated in shape, located in Canada, Finland, in northwestern Russia. In places where the glacier retreated, leaving its fragments, wide, shallow oval lakes arose. Many such lakes are found in northern Europe and North America, for example, Ladoga, Big Bear.

In areas where water-soluble rocks are found - limestone, dolomite and gypsum - basins of karst origin are often formed. Water fills the voids of the earth's crust, forming karst lakes, many of which are very deep, for example, Svityaz. Thermokarst depressions resulting from the uneven thawing of permafrost are often found in the tundra and taiga.

Dammed lakes arose in the mountains when, as a result of strong earthquakes, rivers were blocked by blockages or lava flows. This is how Lake Tana was formed in Africa. And in the Pamirs in 1911, Sarez Lake was formed right before the eyes of people, when, during an earthquake, a fragment of a mountain range collapsed into the river valley and blocked it with a dam more than 500 m high.

A lot of basins - artificial reservoirs - are created by man. So, on many large rivers of our country (Volga, Angara, Yenisei) large reservoirs have been created due to the construction of dams on them, therefore, the flow of these rivers is regulated.

Many lake basins are of mixed origin. For example, Ladoga and Onega lakes are of tectonic origin, but their basins have undergone changes under the influence of glaciers and rivers. The remains of ancient seas, which, due to vertical shifts in the earth's crust, were cut off by land from the ocean, are called relict lakes. Conventionally, they are called seas, they include the Caspian Lake - the remnant of a large marine basin, the largest lake in the world (an area of about 371 thousand km2), and the Aral Sea.

The sources of lake waters are underground springs, precipitation and rivers flowing into them. Part of the water evaporates from the surface of the lake, goes to the underground runoff, is carried out of the lake into the rivers. Due to the inflow and outflow, the water level fluctuates, and therefore the area of the lakes also changes. Thus, Lake Chad in Africa in the rainy season has an area of up to 26 thousand km2, and in the dry season it decreases to 12 thousand km2.

The water level in the lake changes due to climatic conditions, namely, when water evaporates from its surface or a decrease in the amount of precipitation in the lake basin. The water level in the lake can also change due to tectonic shifts.

Lake waters contain many dissolved substances and, depending on their amount in water, lakes are divided into: fresh, brackish and salty. Fresh lakes contain less than 1%o of dissolved salts, brackish lakes - more than 1%o, and saline lakes - over 24.7%o.

Freshwater lakes include flowing and wastewater lakes, since the influx of fresh water in them exceeds the flow. Endorheic lakes are mostly saline or brackish. The salinity in these lakes increases due to the smaller inflow of water relative to its flow. Salt lakes are located in the steppe and desert zones (Baskunchak, Elton, Dead, Big Salt and a number of others). Some lakes contain a high content of soda, for example, in the soda lakes of Southwestern Siberia.

Lake life

Lakes develop depending on changes in environmental conditions. A lot of inorganic and organic substances enter the lakes, brought by river water and temporary water flows that accumulate at the bottom. The remains of vegetation are also deposited on the bottom, gradually filling the hollows. As a result of such accumulations, lakes become shallow and can turn into swamps. The lakes are zoned. In Russia, the largest number of lakes are located in areas of ancient glaciation: in Karelia, on the Kola Peninsula. Here the lakes are flowing with fresh water and quickly overgrowing. There are very few lakes in the steppe and forest-steppe zones of the southern regions. In the desert zone there are drainless salt lakes, which eventually dry up, forming salt marshes. In all belts, there are tectonic lakes that have more depth, so changes in them are difficult to distinguish.

We remember: What sources feed the lakes? What is evaporation? Keywords:feeding of lakes, waste and endorheic lakes, fresh and salt lakes.

1. Waste and endorheic lakes. The lakes are fed by river, underground runoff and atmospheric precipitation. Depending on the flow of water, the lakes are sewage and endorheic. Lakes with river flow, that is, from which rivers flow, are s t o c h n e lakes, and lakes that do not have a runoff - b i s s o c t i o n e. Waste lakes are located mainly in areas of excessive moisture, drainless - in areas of insufficient moisture.

The level of lakes in connection with the inflow and outflow of water does not remain constant, it changes. Especially large fluctuations in the level of lakes are observed in arid and dry regions. Changes in the area of lakes are associated with this.

** The Australian Lake Eyre North in the rainy season of wet years is a large body of water with an area of up to 9300 km 2, and in the dry seasons of dry years, water is retained only in a few bays in the southern part of the lake.

Fresh and salt lakes. According to the amount of dissolved substances, lakes are divided into fresh(salt content less than 1 g per liter of water), salty(from 1 to 24 g of salts per liter) and salty, or min eral(salt content more than 24 g per liter of water). In lakes with high salinity, salts precipitate. Usually sewage lakes are fresh, as the water in them is constantly updated. Endorheic lakes are most often brackish or salty. This is because evaporation dominates the water flow of such lakes. All minerals brought by rivers and groundwater remain and accumulate in the reservoir.

**One of the largest salt lakes on Earth - the Great Salt Lake in North America (salinity from 137 to 300 0 / 00) (Fig. 131). The most salty lake in the world is the Dead Sea - the maximum salinity is 310 ppm.

As a result of sedimentation and overgrowth of vegetation, lakes gradually become shallow, and then turn into swamps. They, like rivers, are the most important natural wealth. Lakes are used for navigation, water supply, fishing, irrigation, recreation, treatment, and obtaining various substances.

1. What are the lakes in terms of water consumption and salinity? 2. Why is the water in endorheic lakes most often brackish or salty? 3. Name the largest lake in your area. How is it used by the local population?

Practical work.

Divide these lakes into two groups (drainage and drainless): Baikal, Caspian Sea, Ladoga, Onega, Victoria, Tanganyika, Aral Sea, Chad, Air North.

Draw a sewage and drainless lake?

3. Describe on the map one of the world's lakes according to the plan (see Appendix 2).

& 45. Glaciers

We remember: What land waters have we studied? Remember what glaciers are. Name the properties of ice .

Keywords:snow, glaciers, continental and mountain glaciers, moraine

1. Glaciers and their formation. Accumulations of ice on the earth's surface are glaciers. They do not have the ice that covers our rivers and lakes in winter.

* On Earth, glaciers occupy an area of about 16.1 million km 2, which is approximately 11% of the land. Glaciers are found in all latitudes, but the largest area of glaciation occurs in the polar regions.

Glaciers are formed as a result of the accumulation and transformation of solid atmospheric precipitation, mainly snow. If more snow falls than it can melt, it accumulates, compacts and turns into transparent bluish ice.

Rice. 132. Scheme of the structure of the glacier

* The height at which as much snow falls in a year as it melts is called the snow boundary (line). In tropical latitudes, the snow limit is located at an altitude of 5000 - 6000 m and drops to the ocean level in polar latitudes. Below this limit, during the year less snow falls than can melt, and therefore its accumulation is impossible. Higher, due to low temperatures, snowfall exceeds its melting, and snow accumulates and transforms into ice. Here is the feeding area of the glacier. From here, ice, being a plastic substance, flows down in the form of a glacial tongue (Fig. 132).

Glaciers are moving slowly. The speed of movement of glaciers in most mountainous countries is from 20 to 80 cm per day, or 100 to 300 m per year. In the ice sheets of Greenland and Antarctica, the ice moves even more slowly - from 3 to 30 cm per day (10 - 130 m per year).

2. Cover and mountain glaciers. Glaciers are divided into cover and mountain.

C o r o v n e, or ma t e r i k o v e, glaciers occupy the surface of the land, regardless of its relief, which does not affect the shape of the glacier (Fig. 133). They have a plano-convex surface in the form of domes or shields. Ice accumulates in the middle part and slowly spreads to the sides. Glacier tongues often descend to the coastal part of the ocean, as, for example, in Antarctica. In this case, blocks of ice break off from it, turning into floating ice mountains - icebergs (Fig. 134).

Rice. 134. Formation of icebergs

The height of icebergs above the water surface is on average 70 - 100 m, most of them are under water.

** One of the icebergs off the coast of Antarctica was 45 km wide and 170 km long with an ice thickness of more than 200 m.

Icebergs move under the influence of currents and winds to warmer latitudes, where they melt. They are dangerous for navigation. Modern ships are equipped with means of their detection.

Continental ice sheets are developed in Antarctica and Greenland, on the islands of the Arctic Ocean. Ice sheets once extended across most of Europe, northern Asia, and North America.

Rice. 133. Ice sheet of Antarctica

* Continental glaciers occupy 98.5% of the area of modern glaciation. Antarctica is almost completely covered with ice (the area not covered with ice is 5% of the total). The average thickness of the ice cover of Antarctica is 2200 m, the maximum is 4776 m. A powerful ice sheet is carried by the island of Greenland .

Mountain glaciers, unlike coverslips, are smaller and differ in a variety of shapes. The shape of mountain glaciers is determined by relief. Some, like caps, cover the peaks, others are located in bowl-shaped depressions on the slopes, and others fill the mountain valleys (Fig. 135).

Rice. 135. Mountain glaciers

* The most common are valley mountain glaciers, which move down from the feeding areas along mountain valleys. They can receive tributaries and have icefalls. The thickness of mountain glaciers is usually 200 - 400 m. The world's largest mountain glaciers are the Malaspina glacier in Alaska in North America (100 km long) and the Fedchenko glacier in the Pamirs in Asia (71 km).

3. Significance of glaciers. Glaciers have large reserves of fresh water. They contain many times more water than rivers and lakes combined. Mountain glaciers often feed streams and rivers.

Glaciers, like flowing waters, change the relief of land. During their movement, they develop glacial valleys, expand and deepen them, erase irregularities that impede their movement, demolish loose rocks, transfer and deposit various materials in other places. At the same time, the work of glaciers takes place where there are no rivers - in high-mountainous and polar countries.

Solid material transported and deposited by glaciers is called sea. Moraine consists of sands, sandy loams, loams, clays, gravel, boulders and is deposited during the melting of glaciers. It composes moraine plains, ridges, hills, uplands (Fig. 136).

1. What natural formations are called glaciers? 2. What is a snow border? 3. How do continental (cover) glaciers differ from mountain ones? 4. What is the importance of glaciers? 5*. Show on a pie chart the ratio of continental and mountain glaciers.

On 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 are 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.

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

Mountain glacial lakes are especially poor in salts, the waters of which 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 very small, and a dense crust of salt forms on its surface from a few 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 the water evaporates very strongly. Therefore, on geographical maps, the western part of Balkhash is shown blue, and the eastern part 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. 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.

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 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.

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.