EVIDENCE FOR REGIONAL LANDSCAPE DISEQUILIBRIUM DUE TO EROSIONAL DESTRUCTION OF A MAIN DRAINAGE DIVIDE IN THE SOUTHERN APPALACHIANS, USA

Drainage divides are first-order geomorphic features that dictate the routing of surface water and sediment and act as gene-flow boundaries for many riverine species. Drainage divides can be transient and subject to progressive migration or discrete jumps by river capture as a landscape strives toward steady-state. At steady-state main drainage divides are expected to be quasi-static and coincident with the topographic crest defined by the highest mountain peaks. The southern Appalachians are traditionally viewed as a slowly-decaying, steady-state orogen; however, the range’s topographic crest roughly parallels the western margin of the Blue Ridge physiographic province (BRP) that is ~50 – 100 km west the Eastern Continental Divide, suggesting landscape disequilibrium. Using published erosion rates and digital topographic analyses I show that a swath of elevated erosion rates and river channel gradients is concentrated along the western flank of the southern Appalachian topographic crest. Analysis of westward draining rivers reveal a series of 200 – 500 m relief knickpoints upstream of water gaps that have cut deep gorges and migrated headword into the BRP. River profile modeling indicates that these knickpoints originated as a series of major river captures along a paleo-drainage divide that integrated a pre-existing, high-elevation drainage system. This data shows that the southern Appalachians are in disequilibrium due to erosional destruction of a regional drainage divide. These observations are best explained by differential response to base level fall of the resistant BRP rock-types relative to the adjacent, weaker Valley and Ridge Province (VRP) rock units. Base level lowering was geologically recent and possibly triggered by climate induced changes in erosional efficiency, vertical motions of the earth’s surface, and/or downstream river capture. I suggest that it was the rapid response of the VRP rivers to base level fall relative to the BRP rivers that increased river gradients and erosion rates along the entire western flank of the paleo-divide and set the stage for drainage network integration. These findings are relevant for interpreting the spatial patterns of erosion and the evolution of riverine species in the southern Appalachians, as well as the history of sediment flux to offshore basins.