The Seversky Donets () or Siverskyi Donets (), usually simply called the Donets (), is a river on the south of the East European Plain. It originates in the Central Russian Upland, north of Belgorod, flows south-east through Ukraine (Kharkiv, Donetsk and Luhansk Oblasts) and then again through Russia (Rostov Oblast) to join the river Don, about from the Sea of Azov. The Donets is the fourth-longest river in Ukraine, and the largest in eastern Ukraine, where it is an important source of fresh water. It gives its name to the Donets Basin, known commonly as the Donbas, an important coal-mining and industrial region in Ukraine.
Etymology
The names Don and its diminutive Donets are derived from Iranic, Sarmatian "the river". Scytho-Sarmatians inhabited the areas to the north of the Black Sea from 1100 BC into the early medieval times.
In the 2nd century CE Ptolemy knew the river Don, into which the Donets flows, as Tanais, and Western Europeans recognized that the Don had a significant tributary which they called either the small Tanais or Donetz. The Slavic name of Seversky Donets derived from the fact that the river originates from the land of Severians<!-- ("Sever" is "North" in Russian) [this is true but etymologically irrelevant]-->. As the Italian-Polish chronicler Alexander Guagnini (1538–1614) wrote: "There is also another, small Tanais, which originates in the Seversky Principality (for this reason it is called Donets Seversky) and flows into the large Tanais above Azov".
Geography and hydrology
thumb|left|Seversky Donets and [[Don (river)|Don rivers]]
The Donets is the largest river in eastern Ukraine and the largest tributary of the Don. Its total length is and the basin area is . Most of the river's length stretches across Ukraine. The average annual flow is near the source and at the confluence to the Don.
The Donets originates on the Central Russian Upland, near Podolkhi village, Prokhorovka area, north of Belgorod, at an elevation of above sea level. Its basin contains over 3000 rivers, of which 425 are longer than and 11 are longer than ; 1011 of those rivers directly flow into the Donets. These rivers are mostly fed by melting snow, and thus the water supply is uneven during the year. The spring flood lasts about two months, from February to April
thumb|Near [[Shypylivka]]
The width of the river mostly ranges between , The river valley is wide: from in the upper part and up to downstream, and is asymmetrical. The right bank is usually high, sometimes with chalk cliffs, and is dissected by gullies. The left bank is more flat, contains numerous swamps, lakes and oxbow lakes, which supplies water to the city of Kharkiv. Below the Pechenizke Reservoir, the Donets is fed by the Udy and by its largest tributary, the Oskil. There the valley widens and its floodplain creates numerous oxbow lakes. Within Ukraine, the river flows between the Cisdesna plateau and the Donets lowland. Each consists of a long concrete dam and a single-chamber sluice, long, wide and deep. At the end of the 19th - beginning of the 20th centuries, a number of attempts were made to revive shipping, in particular, for the sake of the possibility of water transportation of minerals in the Donbass. One of the enthusiasts of the project was D. I. Mendeleev, who wrote about the need to “arrange the Donets […] for our Russian needs […] because the Donets, due to the importance of the interests involved in it, is most necessary”. The developer of the project was the Russian hydraulic engineer Nestor Platonovich Puzyrevsky, known for his contribution to the Volga-Don Canal project. In 1903-1904, he conducted a detailed study of the channel of the Seversky Donets and proposed a project to restore the navigation of the Seversky Donets to the city of Belgorod, which involves the construction of a large number of locks were interrupted by World War I, Russian Civil War and lack of funds. The design of the dams and their old age slowed the navigation of the river, which is currently rather limited.
{| Class = "wikitable" style="margin:1em auto;"
|+ Major features on the Donets (from source to mouth)
|-
!
| Kharkiv
| 837
| 216
| Chuhuiv
| City
| Annual consumption of 20.5 m<sup>3</sup>/s]]
Pre-industrial era
The river played a crucial role for its ancient settlers as a source of water and food, means of transportation and trade route. The first archaeological evidence of settlers relates to Cheulean and Acheulean periods of Lower Paleolithic through stone tools (axes) found on the river banks near Izium city of Kharkiv Oblast and in Luhansk Oblast. Over the ages, the river banks were populated by tribals of various cultures, including Mousterian, Yamna, Catacomb, Scythian, Alan, Khazar and later Slavic cultures. Many of the related tribals had nomadic lifestyle characteristic of Kipchak people, Golden Horde and later of Cossacks. The river flows through the historic lands of Sloboda Ukraine as well as the lands of Don River Host. The many Cossacks became later assimilated into the strengthening Russian Empire, which had rebuilt and reinforced the fortress of Belgorod and cities of Kyiv, Izium, Luhansk, Chuhuiv and others in order keep defensive lines against the raids of nomads from the south-east. Later, the protective role of the river basin gave way to economic needs. In the 18th–19th centuries, the river was extensively used for watermills, which numbered by hundreds by the end of the 18th century, and the mill dams interrupted navigation on the river.
Industrial era
Industrialization in the 20th century shifted interests to mineral exploitation in Donbas, with water-hungry plants concentrated mostly in Kyiv, Luhansk and Donetsk. Already by the 1930s, Kharkiv, Donetsk and Luhansk were lacking water forcing the authorities to gradually create a network of canals and reservoirs. In 1936, the Kochetok Reservoir was created and coupled to the water system of Kharkiv. As a result of the industrial activity, the wild nature of Donets Basin transformed into an industrial settlement. After the breakup of the USSR, most of the basin territory became part of Ukraine.
Russo-Ukrainian War
During the War in Donbas, Luhansk Oblast was roughly split along the river between the Luhansk People's Republic, which controlled most of the portion of the oblast south of the Donets, and the Ukrainian government, which controlled most of the territory north of the Donets.
In May 2022, Russian attempts to cross the Donets above Lysychansk were stopped in the operationally decisive Battle of the Siverskyi Donets.
thumb|Donets in [[Luhansk Oblast, near Shypylivka.]]
Environment
Being one of the largest rivers in Ukraine, the Donets is very intensively used in farming and industry. Ukraine alone uses more than of river waters per year, half of which is returned as polluted discharges; this consumption effectively reduces river runoff by . The industrial development of the 19th century reduced the groundwater levels of the basin. This resulted in shallowing of the river and slowed navigation. Until the mid-19th century, the river was rich in fish which population rapidly declined since then. In Kharkiv Oblast, water is contaminated by industrial and communal wastes of Belgorod, Izium and Shebekino cities, but the water is partially purified through the Pechenizke Reservoir. The density of plants and thus the contamination increase downstream in Donetsk and Luhansk areas, especially around Lysychansk and Sievierodonetsk, about 400 km from the mouth. Some tributaries of the Donets, such as Kozenyi Butt, Bakhmut and Lugan are so polluted that consuming fish caught there is dangerous.
thumb|Carp
Flora and fauna
Fish
The Donets hosts 44 species of fish, predominantly small fishes such as European perch, rutilus and common rudd. Medium and large species include bream, perch, catfish and pike and are becoming increasingly rare. Near Pechenizke Reservoir a large hatchery of carp has successfully operated since 1967.
Amphibians and reptiles
thumb|Grass snake
On the banks of the river, in the floodplain wetlands, there are abundant water frogs, toads (especially green toad), smooth and great crested newts and less common grass snake, dice snake and European pond terrapin. The most picturesque part is probably near the town of Izium, which hosts a National Park of Holy Mountains.