Ob

Ob, river, western Siberian Russia, rising in south-western Siberia on the northern slopes of the Altai Mountains, near the border with Mongolia. The Ob, about 3,700 km (2,300 mi) long, flows generally north-west, where it receives its chief tributary, the Irtysh River; turns to the north; and empties into the Gulf of Ob, an arm of the Arctic Ocean. The river drains an area of more than 2,590,000 sq km (1,000,000 sq mi). An important water route for the region, the Ob is used principally to transport timber and grain, although navigation is impeded by ice in winter. A few industrial cities and a hydroelectric plant have been built on the banks of the river near its source. The combined Ob-Irtysh system, the longest river system of Asia, is about 5,410 km (3,362 mi) long.