A major river in western Siberia, Russia and is the country's fourth longest river. The river is formed by the junction of the Biya and Katun rivers, in the foothills of the Siberian sector of the Altai, from which it has a course of 2,268 miles (3,650 km). If, however, the Irtysh River is regarded as part of the main course rather than as the Ob's major tributary, the maximum length, from the source of the Black (Chorny) Irtysh in China's sector of the Altai, is 3,362 miles (5,410 km), making the Ob the seventh longest river in the world. The catchment area is approximately 1,150,000 square miles (2,975,000 square km). Constituting about half of the drainage basin of the Kara Sea, the Ob's catchment area is the sixth largest in the world.