The mouth of the delta estuary near the river. Estuary - what is it? Short course of the school curriculum

An estuary is defined as a place where a partially enclosed body of water with one or more rivers (streams) flowing into it meets the open sea. An estuary is a transition zone between river and river water where a unique mixture of fresh and salt water is formed. Estuaries have brackish water, but less salty than sea water, making it suitable for many species of flora and fauna.

It should also be noted that the salinity and water levels in estuaries vary throughout the day because the water is constantly circulating and is subject to both river and sea influences. The influx of waters of varying salinity provides estuaries with high levels of nutrients and makes them one of the most favorable types of aquatic bodies.

Most existing estuaries were formed at a time (about 11,000 years ago) when sea levels began to rise and the valleys eroded were flooded.

There are many estuaries around the world, and some of them are very large. Some of the largest are located in North America, and they have different names such as bay, lagoon, or estuary, although some of these bodies of water do not strictly meet the above definition of estuaries and may contain entirely salt water.

Types and classification

Along with different sizes, estuaries also vary in type, and they are classified based on their geology and water circulation.

Classification of estuaries based on geology includes:

• Coastal Plains: such estuaries were formed thousands of years ago at the end of the last ice age. At that time, sea levels were lower than they are now, so more coastal land existed. As large glaciers on land melted around 10,000 to 18,000 years ago, sea levels began to rise and fill low-lying river valleys to create coastal lowland estuaries. These estuaries typically widen and deepen towards the sea. The water depth rarely exceeds 30 m.
• Barrier: These estuaries are semi-isolated from sea water by barrier beaches (barrier islands and barrier spits). Barrier beaches form in shallow waters and are typically parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is usually less than 5 m and rarely exceeds 10 m.
• Tectonic: these estuaries are formed by subsidence or land collapse associated with faults, volcanoes and landslides. Tectonic estuaries form over time in areas with fault lines. During an earthquake, depressions can occur as the ground sinks along fault lines. If the land drops below sea level and it is near the ocean, seawater fills the area. Over time, other faults allow rivers to do the same, and eventually fresh water and sea water meet to form an estuary.
• Fjords: are the final type of geological estuaries, and they are created by glaciers. As these glaciers move toward the ocean, they carve long, deep valleys into coastlines. After the glaciers later retreat, seawater fills the valleys to meet freshwater coming from the land and form estuaries. In the upper fjords the depth can exceed 300 m.

Classifications of estuaries based on water circulation include:

• Wedge-shaped: In this type of estuary, the circulation of river water is much stronger than sea water, while tidal influences are insignificant. Fresh water is located above salt water, and as it approaches the sea, its layer decreases. Denser seawater flows to the bottom of the estuary, forming a wedge-shaped layer. As the velocity difference between the two layers develops, mixing of salt and fresh water occurs.
• Partially mixed: As the tidal influence increases, the river's power decreases under the influence of sea load. Here the entire water column mixes, so the salinity changes laterally.
• Well Mixed: In this estuary, intense turbulent mixing and eddy effects occur, as a result of which river water mixes with sea water.
• Back: This type of estuary occurs in dry climates where evaporation greatly exceeds the influx of fresh water. A maximum salinity zone is formed, and both river and sea water flow near the surface towards this zone. This water sinks down and spreads along the bottom towards the sea, as well as towards the river.
• Intermittent: this type of estuary varies greatly depending on the amount of fresh water introduced, and is capable of transitioning from being entirely a marine bay to any of the other types of estuaries.

Meaning

Major cities around the world, including New York and Buenos Aires, are located near estuaries. This means that estuaries are extremely important economically. For example, US estuaries support more than 75% of the country's fishing industry and add billions of dollars to its economy. The city of New Orleans, Louisiana, depends on the profits of the fishing industry in the Mississippi and its estuaries.

These areas are also tourism destinations. Boating, fishing and bird watching activities boost the local economy.

In addition to providing economic benefits, estuaries are also extremely important to the world because they provide essential support for species that require brackish water to survive. Salt marshes are two types of ecosystems that exist thanks to estuaries. These areas are home to oysters, shrimp and crabs, as well as nesting bird species such as pelicans and herons.

Due to changes in salinity and water levels in estuaries, many of the species that live in them have also developed various unique adaptations to survive. For example, estuarine crocodiles have adapted to live in brackish water, but they can also survive in seawater or freshwater, feeding on a variety of species and going to sea during dry periods.

Examples

The Chesapeake Bay and San Francisco Bay in the United States, and the Gulf of St. Lawrence in Canada are very large and important examples of estuaries. Along their shores there are large cities with well-developed economies. They are also extremely important for the environment.

Chesapeake Bay

The Chesapeake Bay is a coastal lowland estuary and is the largest in the United States. The estuary has a drainage area of ​​165,759 km², and major cities, including Baltimore, Maryland, lie along its coastline.

San Francisco Bay

San Francisco Bay is a tectonic estuary, and it is the largest estuary in western North America. The catchment area covers 155,399 km². It is surrounded by cities such as San Francisco, San Jose and Oakland, and is home to many species of plants and animals, including Pacific herring, and a large number of endangered waterfowl. The estuary is an important economic resource, where industry is concentrated and whose fresh water irrigates agricultural land.

Gulf of St. Lawrence

The Gulf of St. Lawrence is also an incredibly important estuary as it provides access via the St. Lawrence River to the Atlantic Ocean.

This estuary has an area of ​​226,000 km². The Gulf of St. Lawrence is a wedge-shaped estuary that is very important to Canada's fishing industry, which provides many jobs for Quebecers.

These examples are not the only ones in the world; estuaries can be found on other continents, including South America (estuary in the Amazon River, La Plata, etc.), Europe (estuary of the Dniester, Ob, etc.) and Asia (Onemen, Amur and so on.).

Pollution and the future of estuaries

Despite the importance of estuaries such as the Gulf of St. Lawrence and San Francisco Bay, many estuaries around the world are currently experiencing severe damage that is harmful to their sensitive ecosystems. For example, many toxic substances, such as pesticides, oils and greases, pollute estuaries through stormwater runoff. As a result, many cities and conservation organizations, such as the Chesapeake Bay Program, have launched campaigns to educate the public about the importance of estuaries and ways to reduce pollution so that these important areas can thrive for years to come.

With one arm, widening closer to the sea. When sediment - earth and sand carried by wind or water - is removed either by sea currents or tides, and the part of the sea adjacent to a given location is deeper, an estuary is formed. The Yenisei, Don and many other rivers have their mouths in the form of an estuary. The opposite concept to an estuary in geography is a delta. rivers divided into streams. The Nile, Amazon and Volga have such a part of the water flow, but the latter, in turn, forms an estuary when it flows into the Caspian Sea.

How does an estuary appear?

Typically, a river estuary is the result of the submersion of one of the sections of the coast of a watercourse. This process is accompanied by flooding of its lower part. Tides have a strong impact on the estuary, resulting in salt (ocean and sea) as well as fresh (river) water entering. Tides often occur with such force that the flow of the stream is reversed, and salt and fresh water penetrate many kilometers deep into the earth. If such a tide hits a fairly narrow V-shaped estuary with very steep and high banks, the water level can rise so much that a huge wave called a bore is formed. In this case, it will penetrate deep into the earth until it completely wastes all its energy.

The largest estuaries

The estuary is a place convenient for navigation, as it is protected on all sides. Many areas even contain fairly large cities. For example, Lisbon is located on an estuary

The world's largest site of this type is called La Plata. It is located between the countries of Uruguay and Argentina. There, rivers such as the Paraguay and Parana flow into the sea. It is on the shores of the La Plata estuary that the cities of Montevideo and Buenos Aires are located.

Climatic conditions

An estuary is a place where the climate is very stable and rarely “pleases” with new and unexpected precipitation. For example, the monsoon pattern may most often prevail. It represents constant tropical winds. As a rule, they come from the land in summer, and from the sea in winter. Summer in such conditions is somewhat cool - about 15 degrees. And also the described climatic conditions make it clear that the estuary is an area that can constantly be recharged by rainwater. An example of such a territory can be called It is constantly visited by tourists and can always please with its landscapes.

Mouth - the place where a river flows into a reservoir, lake, sea or other river. The part of the river adjacent to the mouth can form a delta or estuary (lip, estuary).

Delta is a lowland composed of river sediments in the lower reaches of a river, cut through by an extensive network of branches and channels. Deltas, as a rule, represent a special mini-ecosystem both on the planet as a whole and in the basin of a particular river in particular.

Estuary (from Latin aestuarium - flooded mouth of a river) is a single-arm, funnel-shaped mouth of a river, expanding towards the sea.

The formation of an estuary occurs if sediments brought by a river are removed by sea currents or tides, and the part of the sea adjacent to the mouth has significant depths; in these cases, sediment deposition does not occur even with a large removal of sediment at the mouth area.

One of the largest estuaries in Europe, the Gironde is 72 km long.

Rivers such as the Amazon (wide, located after the delta), Yenisei (Yenisei Bay), Ob (Gulf of Ob), Thames, Amur (also desalinate the Amur Estuary) have mouths in the form of an estuary.

The opposite of an estuary is a delta - an estuary divided into several channels. Classic deltas are found in rivers such as the Nile, Volga, and Amazon.

34.Interaction between the channel and the stream. Hydromorphological types of channel processes.

The type of channel processes is a quasi-cyclic pattern of deformations of river channels (in a specific section of the river).

There are different types of channel processes. Among them are the main ones: meandering, channel multi-branch, floodplain multi-branch (branched channel), etc. There are also various intermediate and extreme manifestations of channel processes.

For many types of channel processes, regular patterns of development of river channels have been identified. For example, during meandering - displacement of bends, with multi-branch channel - downstream displacement of channel islands, with floodplain multi-branch - development, development and death of floodplain channels.

Assigning a specific river section to the corresponding type of channel processes helps to predict channel deformations.

There are various typifications and classifications of channel processes.

Meandering (from the ancient Greek Μαίανδρος Meandros - the ancient name of the winding river Great Menderes) is a type of channel processes, a pattern of deformations in the form of successive stages of river bed tortuosity.

There are developed and undeveloped meandering, free and limited meandering.

A large number of rivers that have tortuous outlines are characterized by the fact that planned reformations occur in them, due to the influence of the flow on the channel. By meandering we mean not only the external form of the planned outlines of the channel (see River meander), but a certain process that boils down to a change in the planned outlines riverbeds according to a certain pattern, namely in the form of development of smoothly curved meanders. In this case, the river can move its channel for a long time, maintaining sinusoidal sinuosity, or it can form well-defined loops of a wide variety of shapes, completing their development with a breakthrough of the isthmus.

Ocean currents, like rivers, can also meander, forming eddies in the ocean.

Channel multi-branching is a type of channel processes that includes the formation, displacement and disappearance of channel islands.

Channel multi-branching is characterized by a flattened channel along which, during periods of high waters, channel mesoforms haphazardly move, drying out to varying degrees during low water and creating a multi-branched appearance of the channel.

Channel multibranch is a case when a river (or other watercourse) is so overloaded with sediment that the maximum slope is insufficient for its transport. To ensure the movement of sediment, the river is forced to expand its channel, that is, to increase the front of sediment movement.

The division of the flow into branches occurs as a result of the drying of the unflooded tops of the ribbon ridges, moving in the flattened bed not in a chain, but scattered across the width of the river.

The main reason for the formation of channel branches is the appearance of stubs in the channel, which are subsequently covered with vegetation and sometimes turn into floodplain islands. Their formation is determined by the division of the flow into several dynamic axes that arise when the channel is significantly flattened, the wandering of the dynamic axis of the flow, accompanied by the rejection of the side one from the banks, the development of large ridges that dry out during low water - macroforms of the channel relief in the middle of the channel

The formation of sediments also occurs due to a sharp decrease in the slopes of the free surface along the flow, an increased supply of bottom sediments, an increase in their size, etc.

The condition for the transformation of sedges into islands is drying out during low water and the appearance on their surface of shrub vegetation of sufficient density, which, with subsequent flooding during floods or floods, contributes to the accumulation of suspended sediment - silt, which, in turn, favors the further development of vegetation.

Sometimes the cause of the formation of a lake is flooded trees, a stranded boat or another object that creates a local slow flow of water.

Floodplain multi-branch is a general name for different types of branched channels with different types of channel processes in them.

It is often impossible to identify the main channel among numerous channels. Deformations of the channel come down to the development of straightening channels, their death and renewal, accompanied by a redistribution of water flow between branches.

River mouths are either very favorable for the accumulation of alluvium, or extremely unfavorable. In the first case, deltas are formed in them, in the second - estuaries.

Deltas are called low-lying formations composed of river sediments and arising at river mouths, in areas previously occupied by the sea or lake. D.V. Nalivkin (1956) also identifies terrestrial, or dry, deltas formed in deserts from sediments lost in the sands of rivers (the Murghab, Tedzhen, etc. rivers). “In our deserts,” he writes, “alluvial deposits sharply predominate even over aeolian sands.” Deltas vary in shape. Most often they have a fan-shaped (Fig. 48) or even triangular outline (their name comes from the capital Greek letter “delta”, from the similarity with which the fan-shaped delta of the Nile River was named in ancient times).

When rivers flow into shallow bays, deltas are formed. The growth of such deltas is limited by the bar that fences off the entrance to the bay. Alluvium is deposited at the bottom of the bay, and at its top (near the confluence of the river) a series of accumulative islands appear, which gradually grow together to form deltaic land. Deltas of another type are formed on open coasts, when the river carries out a large amount of alluvium and the waves do not have time to carry it beyond the estuary area into the sea. Such deltas are called “advanced”. Sometimes the deltas of several rivers, flowing close to one another, are connected into a continuous belt of deltaic deposits, stretching along the coast for many hundreds of kilometers. The sizes of the largest deltas are measured in tens and sometimes hundreds of thousands of square kilometers, for example, the river delta. Mississippi - 150, delta. Niger - 40, r. Nila - 20, b. Lena - 45 thousand km2.

The growth rate of deltas - the rate of extension of the coastline towards the sea - is on average determined by the first meters per year, but for some rivers they are much higher. For example, the growth of the river delta. Amu Darya for 1943-1947. on site b. Taldyk Bay was 2 km/year. Separate branches of the river delta. The Mississippi River increases by 75 m annually. The Don annually moves towards the sea by approximately 11 m. It turns out that most of the coastal marine sediments are of alluvial origin, and products of abrasion (destruction of the coast by the sea) and the remains of marine organisms constitute only a minor admixture to the alluvium. Deltas lengthen the river without lowering the erosion base, and, together with bends, smooth out its slope, helping to fill the valley

alluvium.

Estuaries are called open or funnel-shaped river mouths, confined to the shores of seas with strongly pronounced tides (from the Latin aestuarium - shore flooded by tides). A tidal wave comes up such rivers twice a day, damming and carrying the river water with it. Then, during low tide, a huge mass of sea and tide-damped river water rushes back at a speed of sometimes up to 20 km/h and carries away all the loose sediments from the estuarine areas, forming estuaries.

A tidal wave spreads along rivers for tens and hundreds of kilometers (along the Amazon River - 900 km above the mouth, along the Yangtze River - 700 km, etc.). It moves in the form of a shaft (“wall”)* at high speed, although it is held back by the oncoming flow of river water. The ebb wave rolls down much faster, unrestrained and reinforced by the waters of the river. It seems to constantly wash the river mouth area and not only prevents the accumulation of sediment, but deepens and widens the channel.

Sometimes estuaries are called the near-mouth areas of river valleys flooded by the sea (for example, the Gulf of Ob), not associated with tidal currents**. It is better to call such bays Rias (if they are located on rias-type coasts) or estuaries (from the Greek limen - bay, an expanded mouth of a river flooded by the sea).

*On the river In the Amazon, the height of the tidal shaft is up to 5 m; on the river Hangzhou (PRC) about 3 m.

**Flooding of river mouths is associated either with tectonic subsidence of coasts or with a eustatic rise in sea level.