ABSTRACT This fi eld trip guide explores the interactions among the geologic evolution, hydrology, and fluvial geomorphology of the central Oregon Cascade Range. Key topics include the geologic control of hydrologic regimes on both the wet and dry sides of the Cascade Range crest, groundwater dynamics and interaction between surface and groundwater in young volcanic arcs, and interactions between rivers and lava flows. As we trace the Willamette and McKenzie Rivers back to source springs high in the young volcanic rocks of the Cascade Range, there is abundant evidence for the large permeability of young lava flows, as manifested in streams that dewater into lava flows, lava-dammed lakes in closed basins, and rivers that emerge from single springs. These dynamics contrast sharply with the older, lower permeability Western Cascades terrane and associated runoff-dominated fluvial systems. On the east side of the Cascades we encounter similar hydrologic characteristics resulting in complex interactions between surface water and groundwater as we follow the Deschutes River downstream to its confluence with the Crooked River. Here, deep canyons have cut through most of the permeable part of the geologic section, have been invaded by multiple large intracanyon lava flows, and are the locus of substantial regional groundwater discharge. The groundwater and surface-water interaction in the Deschutes Basin is further complicated by surface-water diversions and an extensive network of leaking irrigation canals. Our west-to-east transect offers an unparalleled opportunity to examine the co-evolution of the geology and hydrology of an active volcanic arc.