LANDSCAPE KNICKZONES IN THE VIRGINIA PIEDMONT: EVALUATING LANDSCAPE DISEQUILIBRIUM THROUGH GIS-BASED MORPHOMETRIC ANALYSIS

Concordant topographic highs within the central Virginia Piedmont suggest recent dissection of a once more continuous, low-relief surface. This interpretation is supported by documentation of rapid river incision and recognition of migrating knickzones along trunk rivers in the Virginia Piedmont. Taken together, these observations suggest a landscape in disequilibrium, with some portions of the landscape that have not yet responded to accelerated rates of fluvial incision. As fluvial knickzones migrate upstream, incision lowers the boundaries for the adjacent hillslopes, translating the wave of erosion to the surrounding landscape. Through a series of morphometric analyses in the James River system, we attempt to identify signatures of the “landscape knickzone” that represent the location of this wave of erosion in the non-fluvial portion of the landscape. Proposed characteristics of increased erosion in the landscape include steeper slopes, increased relief, and increased drainage density, all tending to promote increased landscape “roughness”. We focus on the James River watershed as previous work suggests a recent (~1 Ma) acceleration of incision on this trunk stream in the Piedmont. Landscape analyses are conducted on both individual sub-watersheds and along topographic swaths through the entire James River watershed. Within subwatersheds we attempt to identify the position of both fluvial and landscape knickzones through determination of basin hypsometry, relief over multiple scales, hillslope steepness, drainage density, and channel longitudinal profiles on topographic maps and from digital elevation models. Along topographic swaths we attempt to identify concordant summits and variations in relief and landscape roughness assessed by statistical measures.