Arising near Crater Lake, the Rogue River flows from the geologically young High Cascades through the somewhat older Western Cascades and then through the more ancient Klamath Mountains. The High Cascades are composed of volcanic rock produced at intervals from about 7.6 million years ago through geologically recent events such as the catastrophic eruption of Mount Mazama in about 5700 BCE. The volcano hurled 12 to 15 cubic miles (50 to 63 km3) of ash into the air, covering much of the western U.S. and Canada with airfall deposits. The volcano’s subsequent collapse formed the caldera of Crater Lake.

Older and more deeply eroded, the Western Cascades are a range of volcanoes lying west of and merging with the High Cascades. They consist of partly altered volcanic rock from vents in both volcanic provinces, including varied lavas and ash tuffs ranging in age from 0 to 40 million years. As the Cascades rose, the Rogue maintained its flow to the ocean by down-cutting, which created steep narrow gorges and rapids in many places. Bear Creek, a Rogue tributary that flows south to north, marks the boundary between the Western Cascades to the east and the Klamath Mountains to the west.

(temporary information from the wikipedia article regarding Rogue River.)