QUATERNARY-PRESENT SEDIMENT TRANSPORT AND GEOMORPHOLOGY OF THE WHITE CLAY CREEK: INSIGHTS FROM GEOMORPHIC MAPPING AND RADIONUCLIDES

The longitudinal profile of the White Clay Creek (WCC) was once graded to alluvial fan deposits of the Quaternary Old College Formation near Newark, DE. Then, the WCC incised 60 m into the underlying Wissahickon Formation, and a knickpoint migrated 30 km upstream to Landenburg, PA, where a steep gorge reflects continuing bedrock erosion. Analyses of meteoric ¹⁰Be and ²¹⁰Pb document erosion from hilltops and 0.5-2 m of sediment accumulation in gently sloping valley sites, possibly related to periglacial conditions during the Late Pleistocene. Exposures along the WCC’s valley walls reveal folded and deformed bouldery diamicton deposits possibly created by periglacial processes; these are current sediment sources. Within the WCC’s valley, subrounded gravel consisting of pebbles, cobbles and boulders underlie Holocene fine-grained sediments dated by ¹⁴C; these gravels are similar to gravels that comprise the current streambed. Buried Holocene bars, and gravels interbedded within Holocene fine-grained sediments primarily consist of pebbles similar in size and shape to gravels comprising current bars of the WCC. Studies with RFID tracers and hydraulic computations suggest that bar deposits represent active bedload, most of which behaves as throughput load across a framework of immobile cobbles and boulders comprising the streambed. These observations suggest that Holocene bedload transport processes were broadly similar to current bedload processes, and that the WCC’s slope is set by bedrock erosion. ²¹⁰Pb and ¹³⁷Cs analyses document ongoing overbank sedimentation at rates varying from 0-1 cm/yr. Analyses of ⁷Be, ²¹⁰Pb, and ¹³⁷Cs, mass balance computations, and geochemical analyses during individual storms indicate that large, rare events are able to mobilize soils on hillslopes and deliver fine-grained sediments to stream channels, but that more moderate storms tend to mobilize sediments already stored within the channel perimeter. Contemporary sediment processes of the WCC, therefore, reflect a suite of drivers whose timescales include days (individual storms), centuries (legacy effects of European settlement), and millennia (glaciation).