Kazakh Ertis River rising in the Altai Mountains in Xinjiang autonomous region, China. It flows west across the Chinese border and then northwest through Kazakhstan into Russian Siberia, where it becomes the largest tributary of the Ob River. It is 2,640 mi (4,248 km) long and is navigable for most of its course. Its main ports include Tobolsk, Tara, Omsk, Pavlodar, Semey, and skemen