Donets River basin. The Oskol is the northernmost tributary
Oskol (Russian: Оскол) or Oskil (Ukrainian: Оскiл) is a south-flowing river in Russia and Ukraine. It arises roughly between Kursk and Voronezh and flows south to join the Seversky Donets which flows southeast to join the Don. It is 436 kilometres (271 mi) long, with a drainage basin of 14,680 square kilometres (5,670 sq mi).
The river has its sources on the Central Russian Uplands, and flows through Kursk and Belgorod Oblasts in Russia, and through the eastern part of Kharkiv Oblast in Ukraine, where it joins the Seversky Donets river. An artificial lake, the Krasno-Oskol Reservoir (Chervonooskil Reservoir, Червоноскільське водосховище), was opened in year 1958 to help with flood protection and as a source of electricity.
There are several towns along the Oskol: Stary Oskol, Novy Oskol and Valuyki in Russia, and Kupiansk in Ukraine.