The Ob River (), also Obi, is a major river in western Siberia, Russia and is the world's seventh longest river. It is the westernmost of the three great Siberian rivers that flow into the Arctic Ocean (the other two being the Lena River and the Yenisei River). The Gulf of Ob is the world's longest estuary. Names The Ob is known to the Khanty people as the As, Yag, Kolta and Yema; to the Nenets people as the Kolta or Kuay; and to the Siberian Tatars as the Umar or Omass. Geography The Ob forms southwest of Biysk in Altai Krai at the confluence of the Biya and Katun rivers. Both t...