129-41 Topographic and Zoogeographic Evidence for River Network Reorganization Within the Southern Appalachian Mountains

Abstract:

Drainage divides that separate fluvial systems in the Blue Ridge Mountains of western North Carolina are dynamic because of differences in potential energy across the Blue Ridge Escarpment. Atlantic-draining rivers are steeper and erode faster than neighboring rivers that drain towards the continental interior leading to drainage reorganization across a variety of mechanisms. These changes may influence the distribution of aquatic organisms; when geographic connectivity of fluvial ecosystems is lost due to drainage reorganization, previously connected species become isolated. Gene flow is inhibited, and populations may experience genetic divergence and eventual speciation. At the same time, drainage reorganization can connect previous isolated populations. This is apparent in the Blue Ridge where there are a variety of sites where freshwater fishes and crayfishes exhibit disjunct geographic distributions and genetic connectivity across drainage divides. Yet, our understanding of historic interactions between river networks, and therefore connection points for aquatic populations, across basins remains relatively unknown. Here, we evaluate an assortment of possible paleo fluvial connections across drainage divides and between major river basins along the margins of the Linville, Green and Toxaway rivers systems with the goal of better understanding previous drainage patterns and possible historic connections between drainages. Specifically, we interrogate topographic evidence for possible stream captures and drainage reversals in areas where aquatic organisms exhibit disjunct distributions and evidence of cross-divide gene flow. To do this, we use χ and long-profile analyses, and document drainage divide asymmetry, evidence of capture elbows, and qualitative topographic observations to evaluate the likelihood of historic connections that may explain the observed genetic connectivity across drainage divides. These findings improve our understanding of how drainage reorganization influences the evolution and distribution of aquatic organisms, and more broadly, how escarpments retreat over time.

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