Non-tectonic base level forcings drive widespread transient incision and relief production in the waning Appalachian orogen

The development of knickpoints and steep bedrock gorges within otherwise low-relief landscapes of the Appalachian Highlands has long interested geomorphologists. While large tracts of muted Appalachian topography appear consistent with slow, steady exhumation of a thickened crustal root, active gorge development into lower-relief uplands suggests that Appalachian river systems have been re-energized to incise into bedrock and increase relief. Orogen-scale analysis of incision patterns, fluvial profiles, and surficial deposits indicate that two main sources of non-tectonic base level drop superimposed on the slowly-exhuming Appalachian landscape can explain the Cenozoic relief production observed. Along the Eastern Continental Divide on the southeast margin of the Highlands, repeated capture of headwaters of elevated, landward-draining streams of the Blue Ridge by Atlantic basin streams maintains the oversteepened slopes and bedrock gorges of the Blue Ridge Escarpment zone. Capture events induce a transient erosional response in which rapid knickpoint retreat carves deep gorges into the captured basin, ultimately adjusting topography to match the rest of the Atlantic slope. In the lower reaches of landward Appalachian rivers still following courses to the continental interior, hillslope steepening in the wake of migrating knickpoints is apparent against the backdrop of a comparatively low-relief upland. This landscape suggests that landward base level drop, possibly due to rapid, glacially-forced Plio-Pleistocene drainage rearrangement, has initiated transgressive waves of incision to adjust Highlands topography to the recently-established continental interior drainage pattern. Encroachment of both the post-rift seaward base level and the modern landward base level forces a rapid release of potential energy stored in the elevated Highlands, energizing streams to incise into bedrock without the introduction of new tectonic energy. The southern Appalachian landscape offers unique insight into mechanisms through which "relict" potential energy within ancient mountain belts may be tapped to accelerate fluvial incision despite decaying rates of post-orogenic exhumation.