Donets

The Seversky Donets (Russian: Се́верский Доне́ц) or Siverskyi Donets (Ukrainian: Сіверський Донець), usually simply called the Donets (Russian: Донец, lit. 'Don + - ets, suffix'), 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 100 km (62 mi) 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.

The names Don and its diminutive Donets are derived from Iranic, Sarmatian Dānu "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. 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".

The Donets is the largest river in eastern Ukraine and the largest tributary of the Don. Its total length is 1,053 km (654 mi) and the basin area is 98,900 km² (38,200 sq mi). Most of the river's length 950 km (590 mi) stretches across Ukraine. The average annual flow is 25 m³/s (880 cu ft/s) near the source and 200 m³/s (7,100 cu ft/s) 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 200 m (660 ft) above sea level. Its basin contains over 3000 rivers, of which 425 are longer than 10 km (6.2 mi) and 11 are longer than 100 km (62 mi); 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 – during this period the water level rises by 3 to 8 m (9.8 to 26.2 ft). Excessive flooding is rare due to numerous artificial water reservoirs constructed along the river.

The width of the river mostly ranges between 30 and 70 m (98 and 230 ft), sometimes reaching 100–200 m (330–660 ft) and even 4 km (2.5 mi) in the reservoir area. The river bottom is sandy and uneven, with the depth varying between 0.3 and 10 m (1 ft 0 in and 32 ft 10 in) and the average value of 2.5 m (8 ft 2 in). The river freezes from around mid-December until late March and is covered by 20–50 cm (7.9–19.7 in) thick ice. It flows into the Don 218 km (135 mi) from the latter's mouth, at an elevation of 5.5 m (18 ft) above sea level; thus the fall of the river is 195 m (640 ft) with the average gradient of 0.18 m/km.

The flow is slow, between 0.15 m/s (0.49 ft/s) at Chuhuiv and 1.41 m/s (4.6 ft/s) near Lysychansk. The river valley is wide: from 8–10 km (5.0–6.2 mi) in the upper part and up to 20–26 km (12–16 mi) 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, the largest of which is lake Lyman. The river is meandering, especially above the Oskil tributary.

Upstream, above Belgorod, the river contains several dams and small reservoirs. Downstream, below the confluence with the Wolf River (Volchiya River), there is the Pechenizke Reservoir (volume 86 km³), 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. In its middle, the river is partly fed by the Dnieper waters, which are brought though the Dnieper–Donbas–Seversky Donets channels which provide water to the coal industry of the Donets Basin (usually called the Donbas). Near the Russian city of Donetsk (Rostov oblast), the river crosses the Donets Ridge and flows in a narrow valley with steep and rocky slopes. In the lower part of the Donets lowland, the flow is interrupted by sluices and is slow. At the delta, it splits into three distributaries.