ABSTRACT The field trip examines coupled hydrologic and landscape response after the cataclysmic eruption of Mount Mazama to form Crater Lake in the Cascade volcanic arc at ~7627 ± 150 cal. yr B.P. The Williamson River basin, east of Crater Lake and in the rain shadow of the Cascade Range, was buried beneath thick pumice and pyroclastic-flow deposits. The distinctive physical properties of pumice and volcanic ash affect the movement and retention of water and the ongoing evolution of the landscape. Three themes will be explored: (1) post-eruption transition from perched streams to losing streams along the eastern flank of the Cascade Range; (2) filling and catastrophic draining of a lake trapped behind a dam of pyroclastic flow deposits in the Williamson River canyon; and (3) post-eruption faulting and the hydrology of Klamath Marsh.