LONGITUDINAL PROFILE ANALYSIS IN THE BLUE RIDGE, CENTRAL VIRGINIA: EVIDENCE FOR LANDSCAPE DISEQUILIBRIUM

Despite undergoing a long period of post-orogenic decay, the southern Appalachians maintain rugged topography with significant relief. Recent work on the origins of this topography has produced seemingly contradictory conclusions: the landscape is in a state of 1) dynamic equilibrium, with high topography maintained by isostatic adjustment to uniform erosion, or 2) disequilibrium driven by uplift by mantle forcing, stream piracy, and/or climate change, producing nonuniform erosion that has increased relief. Here we present river longitudinal profile analyses from the Blue Ridge in west-central Virginia. We have extracted profiles from 10 m NED DEMs in ~150 tributaries along ~100 km of the Blue Ridge between the James River and Shenandoah National Park. Profile convexities were located using slope-area plots, from which we determined the elevation, change in elevation, and slope for each identified profile convexity. The lithology underlying each convexity and the proximity to lithological contacts were determined from georeferenced geologic maps. We eliminated from our data set convexities that were less than 15 m in total height and/or less than 0.03 in slope. Using this criteria, we identified ~110 distinct convexities on 90 tributaries. Of these, over 60 could not be directly related to lithology. In one portion of the study area with an apparent high-elevation, low-relief relict surface, knickpoints on channels draining this surface have knickpoints that cluster around an elevation of 700-800 m. We conclude that, at least in a portion of our study area, there is evidence for migratory knickpoints that are increasing valley relief and hillslope angles, producing a rugged, high relief topography.