Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA

Young basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, since the age of the landscape itself can be determined by dating lava fl ows. These constructional terrains are also highly permeable, allowing one to examine timescales and process of geomorphic evolution as they relate to the partitioning of hydrologic fl owpaths between surface and sub-surface fl ow. The western slopes of the Cascade Range in Oregon, USA are composed of a thick sequence of lava fl ows ranging from Holocene to Oligocene in age, and the landscape receives abundant precipitation of between 2000 and 3500 mm per year. On Holocene and late Pleistocene lava landscapes, groundwater systems transmit most of the recharge to large springs (≥0·85 m s) with very steady hydrographs. In watersheds >1 million years old, springs are absent, and well-developed drainage networks fed by shallow subsurface stormfl ow produce fl ashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fl uvial incision also occur on this timescale. Springs and groundwater-fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fl uvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormfl ow as the dominant streamfl ow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced suffi ciently for surface fl ow to replace groundwater fl ow. Copyright © 2010 John Wiley & Sons, Ltd.