The Gulf of Ob is an immense bay of the Arctic Ocean, at the head of which is the mouth of the Ob River. The Gulf is about 1000 km (600 mi) long, and varies from about 50 km (30 mi) to 80 km (50 mi) in width, running generally north and south. The Tazovskij Gulf is an eastern side-branch formed by the Taz River. There are several islands near the mouth of the Ob, but no others in the Gulf.