SUBSTRATE INFLUENCES ON THE OCCURRENCE OF STREAM POTHOLES

Adjustment to bedrock-channel form largely occurs via either abrasion or plucking, with abrasion being the dominant mechanism of erosion in massive substrate. Localized abrasion occurs as sculpted forms; at this field site, primarily potholes. We measured the dimensions and surveyed the locations of potholes in four reaches along the Ocoee River, Tennessee in the Blue Ridge Province of the Southern Appalachians. The goal was to assess the inter- and intra-reach spatial occurrence of potholes in a bedrock channel with no defined thalweg and heterogeneous substrate of varying erosional resistance. Inter-reach differences in lithology and channel morphology lead to differences in both the size and spatial patterns of potholes. In reaches with more resistant rock and heterogeneous bed topography, pothole dimensions (i.e., radius and depth) are larger and follow an aggregated spatial pattern. The likelihood of pothole formation tends to be greatest at intermediate relative bed elevations, suggesting that local hydraulics and tools versus cover relationships govern pothole formation and maintenance. The relative abrasion of experimental concrete grinders supports the interpretation that local hydraulic environment, as reflected by bed elevation, tends to be more important than pothole dimensions in controlling the erosional efficiency of potholes.