The Ob is formed by the confluence of the Biya and Katun rivers in Siberia. Their meeting occurs in Russia’s Altai Krai, about 25 kilometres southwest of the city of Biysk.
The Biya flows from Lake Teletskoye, while the Katun descends from glaciers in the Altai Mountains, including the region around Mount Belukha. Once the two rivers meet, the resulting Ob flows north and northwest across western Siberia before reaching the Gulf of Ob and the Kara Sea.
The Ob is one of the three great Siberian rivers that drain toward the Arctic Ocean; the other two are the Yenisei and Lena. Its major tributaries include the Irtysh, Tom, Chulym and Vasyugan. Novosibirsk, Siberia’s largest city, stands on its banks, and the Trans-Siberian Railway crosses the river there.
A frequent source of confusion is the Irtysh. It is a major tributary of the Ob and, measured from its farthest headwater in China, makes the combined Ob–Irtysh system one of the world’s longest river systems. It does not, however, form the Ob at its origin: that role belongs to the Biya and Katun.