Formed by the confluence of Elk Lake Creek and the East Fork Collawash River in the Cascade Range, it flows generally north-northwest from source to mouth through the Mount Hood National Forest.

[4][6] The largest tributary of the upper Clackamas, it provides about a third of bigger river's low-flow volume.

[8][9][10] Formed by the confluence of Elk Lake Creek and the East Fork Collawash River, the river loses about 840 feet (260 m) in elevation over the 12 miles (19 km) between source and mouth.

The river receives Paste Creek from the right before Hot Springs Fork enters from the left at RM 4.0 (RK 6.4).

Over the last third of its course, the Collawash River receives Slide, Sluice, and Cap creeks, all from the right, passes the Raab Campground, then receives Jack Davis Creek from the left.