TOPOGRAPHIC EVIDENCE OF WIDESPREAD TRANSIENT INCISION ON THE “QUIET” SIDE OF NORTH AMERICA'S EASTERN CONTINENTAL DIVIDE

Episodes of transient incision in southern Appalachian fluvial networks have seen increasing study in recent years, with particular focus on the dramatic response of former landward drainages recently energized by capture into the Atlantic basin. While these captures represent an active modifier of Appalachian Highland topography at large, region-scale analysis of DEM topography and fluvial profiles indicates transient incision propagating through landward Appalachian drainages is also driving accelerated dissection of the Highlands. Landward base level drop has energized these streams to incise into the otherwise slowly-evolving landscape, producing a widespread topographic signature of knickpoints separating relict, pre-incision topography from areas actively responding to base level drop. This characteristic topographic regime can be identified throughout stream systems draining the Highlands to the Gulf of Mexico regardless of the relief and lithologic adjustment of pre-incision topography, which ranges from gently rolling upland plateaus to the more rugged ranges of western North Carolina and east Tennesee. Knickpoint elevations within discrete basins typically describe a gentle downstream slope, suggesting comparable rates of vertical migration following formation. Partially dissected relict surfaces also show general elevation accordance across deepening gorges, and grade smoothly into contiguous domains of relict topography in headwaters areas not yet reached by knickpoints. Complete adjustment of landward drainages will ultimately increase symmetry of the Eastern Continental Divide, reducing the energetic driver for expansion of the Atlantic basin by stream capture. The origin of landward base level drop is unclear, but its widespread effects along with preservation of relict topography suggest it may result from glacially-forced Plio-Pleistocene drainage rearrangement hundreds of kilometers from the study area. Improved constraint on the origin and migration rate of these landward basin transients should enhance understanding of the range of processes which can accelerate topographic evolution on nominally inactive continental margins.