USING BASIN-AVERAGED EROSION RATES TO ASSESS THE RATE OF DRAINAGE CAPTURE ALONG THE DIVIDE BETWEEN THE NEW AND ROANOKE RIVERS, SW VIRGINIA

We present basin-averaged erosion rates obtained from small catchments along the eastern continental divide between the Roanoke and New River drainage basins in southwestern Virginia. Here the Atlantic-draining Roanoke River has breached the northern end of the eastward-facing Blue Ridge escarpment, and appears to be actively capturing drainage from the New River system. As a consequence of this breach, we hypothesize that the rates of incision and divide migration in the headwaters of the Roanoke River watershed exceed those on the Blue Ridge escarpment proper. To determine the rates of drainage divide migration, we sampled river sediment from active channels in low order drainage basins (4.5 to 51 km²) along the divide in both the Roanoke River (n=6) and New River (n=4) watersheds. We isolated quartz from the 0.25 to 0.50 mm grain size fraction for extraction and measurement of in situ-produced ¹⁰Be to obtain basin averaged erosion rates. In addition, we used a 30 m resolution DEM to perform topographic and stream longitudinal profile analysis on complementary streams along the divide. Landscape character differs strongly across this divide; while the New River basins are relatively flat and are commonly used for agriculture, the Roanoke River basins are rugged and largely forested. Considering basins on comparable lithologies on either side of the divide, channel gradients in the New River basins are much lower than adjacent Roanoke River basins. The average slope within sampled New River basins is ~7°, compared to ~15° in sampled Roanoke River basins. Through the combination of topographic analysis and basin-averaged erosion rates, we will attempt to identify where erosion is focused on the divide. We will also evaluate our results alongside published erosion rates previously measured on the main Blue Ridge escarpment to compare rates of drainage migration.