THE OLD AND THE RESTLESS: ACTIVE RIVER BASINS ON A PASSIVE MARGIN

River networks shape mountain topography, transport water and sediment from continental interiors to the oceans, and serve as conduits for freshwater species. Tectonic and climatic perturbations can drive changes in river network structure that affect all of these functions, but it is typically difficult to determine the nature and extent of river network reorganization across an entire region. We use a technique for mapping erosional disequilibrium of river profiles to infer the migration directions of drainage divides, which reveal how the boundaries between neighboring river basins are shifting through time. In an effort to determine how the cessation of active mountain building affects river network reorganization, we focus particular attention on the southern Appalachian Mountains. In contrast to the notion of static drainage patterns in a landscape with declining relief, we find abundant evidence of river network reorganization. The implied directions of drainage divide migration are consistent with cross-divide differences in erosion rates measured with cosmogenic nuclides. We consider which perturbations are driving this reorganization. We also discuss the possibility that river network reorganization is partly responsible for the pronounced diversity of freshwater species in the region.