ON A TEMPLATE SET BY BASALT FLOWS, HYDROLOGY AND EROSIONAL TOPOGRAPHY COEVOLVE IN THE OREGON CASCADE RANGE

Young basalt terrains offer an exceptional opportunity to understand landscape and hydrologic evolution over time, since the age of landscape construction can be determined by dating lava flows. I use a chronosequence of watersheds in the Oregon Cascade Range to examine how topography and hydrology change over time in basalt landscapes. Western slopes of the Oregon Cascade Range are formed from lava flows ranging from Holocene to Eocene in age, with watersheds of all ages have similar climate, vegetation and relief. Abundant precipitation (2.0 to 3.5 m/yr) falls on this landscape, and young basalts are highly permeable, so Holocene and late Pleistocene lavas host large groundwater systems. Groundwater flowpaths dictated by lava geometry transmit most recharge to large springs. Spring hydrographs have low peak flows and slow recessions during dry summers, and springs and groundwater-fed streams show little evidence of geomorphically effective incision. In the Cascades, drainage density increases linearly with time, accompanied by progressive hillslope steepening and valley incision. In watersheds >1 Ma, springs are absent and well-developed drainage networks fed by shallow subsurface flow produce flashy hydrographs with rapid summer recessions. A combination of mechanical, chemical, and biological processes acting within and on top of lava flows may reduce permeability over time, forcing flowpaths closer to the land surface. These shallow flowpaths produce flashy hydrographs with peakflows capable of sediment transport and landscape dissection. From these observations, I infer that the geomorphic evolution of basalt landscapes is dependent on their evolution from deep to shallow flowpaths.