The watershed is split into two geographic regions: the High Cascades and the Western Cascades, both volcanic in origin. The Western Cascades compose the western two thirds of the watershed. This region is highly eroded, being between 17 and 38 million years old. Its unstable slopes are primarily made of pyroclastic rock. Due to the rock's high ability to absorb moisture, earthflows are common. The High Cascades are much younger, around three to seven million years old. Mount McLoughlin is the most prominent High Cascade volcano in the watershed, last erupting between 20,000 and 15,000 years ago. Basalt and andesite are the most common rock types in this region.