The delta is a lowland composed of river drifts in the lower reaches of the river , cut by an extensive network of sleeves 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.
A delta located not at the mouth of a river, but, for example, in its middle or upper course, is called an internal delta .
The opposite of the delta is an estuary - one-arm, funnel-shaped estuary, expanding towards the sea. Classical estuaries have such rivers as the Amazon River (wide, located after the delta), the St. Lawrence River , the Thames , the Dniester , the Yenisei (the Yenisei Bay ), the Ob ( Gulf of Ob ), the Amur (also desalinates the Amur Liman ).
The name comes from the capital letter of the Greek alphabet . It appeared in antiquity and is associated with the Nile Delta , which has a triangular shape.
Geography and Geology
Delta is formed as a result of interaction of river runoff , sediment runoff, sea waves, tides and surge currents.
Deltas of rivers flowing into relatively calm waters reach gigantic proportions:
- Ganga - 105.6 thousand km²;
- Amazon - 100 thousand km ²;
- Lena - 45.5 thousand km²;
- Mekong - 40.6 thousand km²;
- Indus - 41.4 thousand km²;
- Mississippi - 28.6 thousand km²;
- Nile - 24 thousand km ²;
- Volga - 19 thousand km ².
The Amazon Delta, for example, starts about 350 km from the Atlantic Ocean . Despite its ancient age, it has not moved into the ocean beyond the boundaries of the original shores . The reason for this is obviously the activity of tides, the influence of currents, as well as the sinking of the coastline. Although the river brings into the ocean huge masses of detrital material, the growth process of the delta is hindered by all of these phenomena. A similar situation is characteristic of some northern Russian rivers - the Amur , Ob , Taz , Pur and others. For the same reason, the presence and size of the delta is almost independent of the length of the river: The Salween , which is 300 kilometers longer than the Irrawaddy , unlike the Irrawaddy, hardly forms the delta. Delta even forms the Neva , whose length is 71 km.
Russian river deltas
A large number of rivers in Russia have large deltas with well-developed alluvial (alluvial) activities. Typical examples are, in addition to the above-mentioned Volga and Lena , the deltas of the Terek , Kuban , Northern Dvina , and Neva rivers. Russian deltas, especially in the southern regions, are characterized by well-developed floodlands , are distinguished by the richness of the plant and animal world, and are especially valuable for their fisheries resources.
CIS river deltas
In the CIS countries , located mainly south of the main body of the Russian Federation , river deltas have a predominantly southern steppe ( Ukraine ) or desert character ( Kazakhstan , Uzbekistan , Azerbaijan ). Deltas in places like the Volga Delta are formed in arid climate and are entirely supported by the freshwater flow of the river itself. In such conditions, chains of tugai forests form, flowing with reeds and reeds .
Similar characteristics have the delta of the Dnieper in Ukraine, and typical examples of the tugai semi-desert and desert deltas are the rivers of Central Asia , especially Ili with their arms Zhideli and Topar , and also (in abbreviated form due to intensive irrigation) the deltas of the Syr Darya and Amudarya rivers near the Aral Sea. of the sea . The deltas of the Central Asian rivers are especially rich in fish ( pike perch , carp , bream , marinka , ottoman , etc.). In tugai forests along the banks there is a significant amount of game (especially pheasant ), tolai , wild boar , corsac , saiga , reed cat .
Somewhat apart from semi-desert deltas are well-moistened Danube deltas ( Ukraine ) and low marshy deltas in the mouths of the rivers of the Kolkhi lowland (Western Georgia ).
Soil fertility and moisture determine the high value of land in a large part of the territory of many valleys, which are areas of intensive farming (for example, the Nile Valley, the Yellow River , the Ganges ). The Danube Delta is listed in the UNESCO World Heritage Site  .
- Kuban Delta
- Okavango Delta
- Inner Delta of Niger
- Mangrove Forest