ANALYZING SYMMETRY OF VALLEYS AT STREAM BENDS TO CHARACTERIZE POSSIBLE NEOGENE UPLIFT IN THE BLUE RIDGE MOUNTAINS OF NORTH CAROLINA

Recent work has suggested that parts of the Appalachian Mountains were uplifted during the Neogene, based on stream knickpoint analysis (Gallen et al., 2013; Miller et al., 2013). Analysis of the symmetry of valleys at stream bends has the potential to indicate tectonic activity by determining migration trends of the channels. Tectonic quiescence promotes lateral migration of stream bends. This creates asymmetrical river bend valleys with shallow point bars and steep cut banks. Uplift promotes downcutting over lateral migration, producing symmetrical stream valleys. Using LIDAR data and ArcMap software, I quantified the symmetry of stream-bend valleys in the Blue Ridge Mountains of western North Carolina. The symmetry of valleys at stream bends can be quantified by dividing the inner valley slope by the outer valley slope to form ratios ranging from 0 to 1, with 1 indicative of a symmetrical valley. Where the inner slope exceeded the outer slope, the ratio was inverted to avoid numbers greater than 1. Slope ratios were measured at 190 stream bends and used to create a raster map representative of the symmetry of valleys at stream bends. The ratios of inner to outer slopes had an average of 0.6, showing that inner slopes were on average 60% as steep as outer slopes. These values showed no correlations with elevation, downstream slope, or normalized steepness, but a pattern exists between the symmetry of valleys at stream bends and topographic roughness. Large areas with asymmetrical valleys are located in gentle topography between mountainous areas, such as the French Broad River valley and the foothills. Areas of high, rough topography commonly host symmetrical stream valleys, as in the case of the Mount Pisgah and Mount Mitchell areas of the Blue Ridge Mountains. The relationship between topography and the symmetry of valleys at stream bends is apparent when comparing symmetry measurements with a local relief map, which shows a correlation between high local relief and zones with symmetrical valleys. This connection, in addition to agreements with Gallen et al.’s projection of uplift in the Cullasaja basin, indicates that the symmetry of valleys at stream bends may act as an accurate metric for determining areas of uplift; such uplift has likely occurred recently in the areas northeast and southwest of the French Broad River valley.