Irtysh, river, c.2,650 mi (4,260 km) long, W Siberian Russia and Kazakhstan. It is the chief tributary of the Ob and one of the two major rivers of W Siberia. As the Ertix, it rises in Xinjiang, China, in the Mongolian Altai Mts., flows NW through Lake Zaysan in Kazakhstan (where it is known as the Ertis), and enters W Siberia. There it receives the Ishim and Tobol rivers, its chief tributaries. The Irtysh flows past Semey in Kazakhstan and Omsk and Tobolsk in Russia and joins the Ob near Khanty-Mansiysk. Major hydroelectric stations are at Öskemen and Zhana Buktyrma, Kazakstan. The river banks were occupied by Chinese, Kalmyks, and Mongols until the Russians arrived in the late 16th cent. The Russian conquest of the basin was completed by the early 19th cent.