SPATIALLY EXPLICIT ASSESSMENT OF BANK EROSION RATES ALONG THE SEDIMENT-IMPAIRED, BEDROCK-ALLUVIAL, WHITE CLAY CREEK, PENNSYLVANIA

We employ a field calibrated, spatially explicit, GIS approach to assess the contribution of bank erosion to the sediment budget of White Clay Creek in southeastern Pennsylvania. Analyses of historical aerial imagery and dendrochronology at 13 sites throughout the watershed define bank retreat rates ranging from 10 to 36 cm/yr; these data are supplemented by reach-scale geomorphic mapping and measurements of vegetation density, channel morphology, and sediment bulk density. Bank retreat rates are a non-linear function of local channel centerline curvature and near-bank canopy cover (r² = 0.58). We developed a workflow in GIS to apply this correlation to all channels in the watershed with Strahler stream orders 2 or greater. All streambanks were outlined manually from aerial imagery, and channel centerlines were inserted between them in ARC-GIS. Centerline curvature was computed at 2 m intervals after smoothing using a Savitsky-Golay filter in MATLAB. We eliminated steep banks confined by bedrock, colluvial valley walls, and engineering structures using a threshold relief ratio. Relief was defined with search radii varying from 5.5m to 15m based on channel length; a threshold ratio of relief/local bank height of 1.7 correctly classified 82% of 2,316 observations of confined and unconfined banks. Canopy cover was quantified using 30m resolution data from the U.S. Geological Survey (USGS). Linear bank retreat rates were converted to mass fluxes by multiplying by bank height (from hydraulic geometry equations) and an average bulk density of 1310 (std. dev. 131) kg/m³. When summed, the bank erosion in 2^nd order and higher channels accounts for 69% of the annual suspended sediment load of 22000 Mg/yr measured by the USGS at Strickersville, PA (USGS stream gage 01478245). The uncertainty associated with our estimates is significant, however, rendering them of questionable value for management decisions.