[4] Formed by the confluence of its major tributaries, the South Fork Coos River and the Millicoma River, it drains an important timber-producing region of the Southern Oregon Coast Range.

[5] The river enters the bay about 15 miles (24 km)[5] from where the bay—curving east, north, and west of the cities of Coos Bay and North Bend and passing by the communities of Barview and Charleston—meets the ocean.

Curving south, the river receives Vogel Creek and then Lillian Creek, both from the left, before passing under Chandler Bridge, which carries Route 241 from the right bank to the left bank about 2 miles (3 km) from the mouth.

The river then turns west and north as it enters the bay and splits into two distributaries separated by a marsh.

The Cooston Channel, which is on the right, continues north around the west side of the marsh for about 1 mile (1.6 km) to the mouth.