MIGRATORY KNICKZONES WITHIN THE SHENANDOAH RIVER DRAINAGE NETWORK: EVIDENCE FOR DISEQUILIBRIUM?

We present analyses of river longitudinal profiles within the Shenandoah River watershed, Virginia, that illustrate potential disequilibrium in this river system. River longitudinal profiles along the main stem and tributaries of the North and South Forks of the Shenandoah River were extracted from Digital Elevation Models (DEMs) with 10 m grid spacing. Steepness indices, profile concavities and drainage area were calculated along all river profiles, allowing us to identify knickzones. We compared steepness indices to underlying lithology both quantitatively and geographically to determine whether knickzones are stationary. We attempt to correlate the position of knickzones that are not clearly related to lithology in an effort to link knickzones with a common origin. We have located what appear to be both stationary and migratory knickzones in the Shenandoah River system. Stationary knickzones are consistently located where rivers flow across the Neoproterozoic Catoctin Metabasalt and Mesoproterozoic granitic formations. These knickzones tend to occur near tributary headwaters within the Blue Ridge. Migratory knickzones appear to be located on the Shenandoah River approximately 50 km upstream from where the North Fork and South Fork join. Several tributaries entering the North and South Fork of the Shenandoah, including those draining the Blue Ridge and Massanutten region, also possess knickzones not clearly related to lithology. A second, less well expressed set of genetically related knickzones may be present further upstream within the Shenandoah system. We conclude that the presence of knickzones that are not localized on resistant lithologies may represent upstream-migrating waves of river disequilibrium, generated by base level change. As these streams form the boundary conditions for adjacent hillslopes, enhanced river incision associated with passage of these knickzones should result in more rapid hillslope erosion, possibly explaining previous observations of variable rates of erosion and relief growth within the adjacent Blue Ridge.