The Collawash River is a 12-mile (19 km) tributary of the Clackamas River in the U.S. state of Oregon. 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.[5] About 35 percent of its watershed of 150 square miles (390 km2) is protected as wilderness.[5]
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. Flowing generally to the north-northwest, the river receives Dunno Creek and Jazz Creek from the right bank, both near river mile (RM) 11 or river kilometer (RK 18), then Russ Creek and Blitzen Creek, both from the right. Happy Creek enters from the right at about RM 8 (RK 13), then Dickey Creek from the left bank, and Buckeye Creek from the right.[4][6]
Peat Creek enters from the right at about RM 6 (RK 10) and Farm Creek from the left shortly thereafter. 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. Two Rivers Picnic Area is on the right near the confluence with the Clackamas River, 57 miles (92 km) from the larger river's confluence with the Willamette River.[4][6]