EPHEMERAL CHANNELS AND HILLSLOPE EROSION IN THE SOUTHERN VALLEY AND RIDGE, SOUTHWEST VIRGINIA

The topographic fabric and characteristic relief of the Valley and Ridge province of the southern Appalachians are controlled by variations in the erodability of folded Paleozoic quartzite, carbonate, and shale. Although the regional, long-term erosion of these landforms is seemingly quasi-stable, the mechanisms that erode resistant ridges and less resistant side-slopes and valleys are not the same. Perennial, alluvial trunk streams drain valleys, while intermittent ephemeral tributaries and associated mass wasting erode hillslopes. These mechanisms work in concert to erode the Valley and Ridge, yet little is known about the behavior and evolution of ephemeral channel systems or their coupled interaction with perennial trunk streams. We have begun to examine the behavior of ephemeral channels in the Valley and Ridge with a field characterization of Allen Hollow along Brush Mountain in southwest Virginia. To understand how channels evolve, we have compared channel characteristics to numerous boundary conditions including drainage area, bedrock variations, and the presence of woody debris. Initial results define systematic relationships between channel gradient, channel cross section, channel morphometry, and clast size with drainage area. In addition, bedrock variations appear to exert an important control on channel longitudinal profile. Intervals of sandstone and siltstone within a broadly coarsening-up sequence of shale in the Devonian Braillier and Chemung Formations result in oversteepened channel gradient and bedrock-floored channel reaches. These buttress the catchment and control the erosion and deposition on intervening shale-based alluvial reaches. The presence of woody debris, in contrast, appears to have only a minor influence on channel morphometry and sediment storage. Based on real-time observations, sediment transport along alluvial reaches appears dominated by rare, high precipitation events. Characterization and dating of sediments in excavations of the alluvial fan at the base of the channel may constrain the frequency of such events and the rate of catchment erosion, both important for understanding its dynamic interaction with the trunk stream.