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Bedrock channel adjustment to tectonic forcing: Implications for predicting river incision rates

¹ Grant Institute of Earth Science, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland, UK
² Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA
³ Joint Research School for Geological and Geophysical Sciences, Birkbeck College, University of London, Gower Street, London WC1E 6BT, UK

We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from drainage area upstream of the fault. Unit stream powers are 4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins.

Key Words: rivers • tectonics • faulting • channel geometry • geomorphology

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