The Irtysh River is a river in Siberia and Kazakhstan and is the chief tributary of the Ob River.
Irtysh’s main affluents are the Tobol River and the Ishim River. The Ob-Irtysh system forms a major drainage basin in Asia, encompassing most of Western Siberia and the Altai Mountains.
From its origins as the Kara-Irtysh (Black Irtysh) in the Mongolian Altay mountains in Xinjiang, China, the Irtysh flows northwest through Lake Zaysan in Kazakhstan, meeting the Ishim and Tobol rivers before merging with the Ob near Khanty-Mansiysk in western Siberia, Russia after 4,248 kilometres (2,640 mi).
The name Black Irtysh (Kara-Irtysh in Kazakh, or Cherny Irtysh in Russian) is applied by some authors, especially in Russia and Kazakhstan, to the upper course of the river, from its source entering Lake Zaysan. The term White Irtysh, in opposition to the Black Irtysh, was occasionally used in the past to refer to the Irtysh below lake Zaysan; now this usage is largely obsolete.