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. The largest tributary of the upper Clackamas, it provides about a third of bigger river's low-flow volume. About 35 percent of its watershed of 150 square miles (390 km2) is protected as wilderness.