Getting Beyond the Bankfull Shield's Parameter: A Continuum of Threshold Channel Types Illustrated by the White Clay Creek, PA, a Bedrock-Alluvial River with Cohesive Banks

The Shields parameter based on median grain size (D50) and bankfull depth is often used to interpret river morphology. Shields parameters of gravel-bed rivers usually plot near the threshold of motion, while sand-bed rivers are an order of magnitude higher. From these analyses, 3 categories of threshold channels arise: boulder-bed channels without bed material transport, gravel-bed channels with stable banks and mobile beds, and sand-bed channels with bankfull shear stresses adjusted to the erosion threshold of cohesive bank sediments. While threshold boulder-bed channels reflect controls by local bedrock and colluvium, near-threshold gravel-bed channels and sand-bed channels are believed to adjust their morphology to transport the given sediment supply and discharge, manifesting the concept of graded fluvial equilibrium. However, real rivers may not fit these categories. At 12 sites of the White Clay Creek (WCC), PA, the ratio of bankfull Shields stress to threshold Shields stress averages 1.43 (range 0.41-2.63), suggesting that these channels are equilibrium threshold gravel-bed rivers, but field mapping indicates confinement by bedrock and colluvium and a channel slope dominated by bedrock incision and knickpoint migration. A numerical model of WCC bed material transport and grain size, calibrated to bedload tracer data, demonstrates that the bed material is composed of a population of immobile cobble and boulder-sized bed material (average 22%, range 8-73%) supplied through local colluvial processes and bedrock erosion, and a separate population of mobile pebble-sized alluvium. Computations also indicate that channel morphology is uncoupled from upstream sediment supply. Additional data suggests that width adjustment reflects a balance between cohesive bank erosion and floodplain deposition, though bank stresses are likely close to cohesive bank erosion thresholds. WCC is thus a bedrock-alluvial river whose morphology is controlled by an immobile cobble-boulder bed and cohesive bank erosion processes, with an alluvial throughput load that is only weakly coupled to channel morphology. Analysis of the bankfull Shields parameter alone is misleading: understanding WCC morphology requires field mapping, stratigraphic analysis, and detailed measurement of sediment transport processes.