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    Description

    Young basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, as the age of the landscape itself can be determined by dating lava flows. 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 flowpaths between surface and subsurface flow. The western slopes of the Cascade Range in Oregon, USA, are composed of a thick sequence of lava flows 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 (>0ยท85 m3 S-1) with very steady hydrographs. In watersheds >1 million years old, springs are absent, and well-developed drainage networks fed by shallow subsurface stormflow produce flashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fluvial incision also occur on this timescale. Springs and groundwater-fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fluvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormflow as the dominant streamflow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced sufficiently for surface flow to replace groundwater flow.

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    Citation

    Jefferson, A.; Grant, G.E.; Lewis, S.L.; Lancaster, S.T. 2010. Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA. Earth Surface Processes and Landforms. 35 (7): 803-816.

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    Keywords

    bedrock geology, weathering, topography, mapping, hydrologic processes

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