The Ob River , also Obi, is a major river in western Siberia, Russia and is the world's seventh longest river. It forms at the confluence of the Biya and Katun rivers which their origins in the Altay Mountains. It is the westernmost of the three great Siberian rivers that flow into the Arctic Ocean (the other two being the Yenisei River and the Lena River). The Gulf of Ob is the world's longest estuary.