The Ob is a major river in western Siberia, Russia, and the seventh-longest river in the world, flowing for 2,268 miles (3,650 km). It is a vital resource for many Siberian communities, providing irrigation, drinking water, hydroelectric energy and fishing. The Gulf of Ob, which forms a bay on the Arctic Ocean, is the world’s longest estuary at 620 miles (1,000 km) long.