HILLSLOPE RESPONSE TO KNICKPOINT MIGRATION IN THE SOUTHERN APPALACHIANS: IMPLICATIONS FOR THE EVOLUTION OF POST-OROGENIC LANDSCAPES

The southern Appalachians represent an active landscape characterized by locally high topographic relief, steep slopes, and frequent mass wasting in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for such activity in a post-orogenic landscape remain enigmatic. We present evidence from the non-glaciated Cullasaja River basin of southwestern North Carolina that highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network and adjacent hillslopes. The Cullasja River basin was chosen for this study because of its relatively uniform lithology, frequent debris flows, and the availability of high-resolution airborne lidar DEMs. We utilize values of normalized hypsometry, hypsometric integrals, and mean slope vs. elevation relationships for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. We capture the transient affect of knickpoint-driven channel incision on basin hillslopes by measuring the relief, mean slope steepness, and mass wasting frequency of tributary basins and comparing these results to the distance from major knickpoints along the main stem of the Cullasaja River. Finally, we present a conceptual model of area-elevation and slope distributions that may be representative of post-orogenic landscape evolution in geologic settings analogous to the Cullasaja basin. Our model underscores how knickpoint migration and channel-hillslope coupling are important factors in tectonically-inactive orogens for the maintenance of significant relief, steep slopes, and weathering-limited hillslopes.