LONGITUDINAL VARIATION IN THICKNESS AND COMPOSITION OF LEGACY SEDIMENTS AND BURIED ORGANIC SOILS IN HEADWATERS OF THE CHRISTINA RIVER BASIN, USA

Floodplains in the mid-Atlantic Piedmont of the United States are characterized by large post-settlement alluvium deposits originating from colonial-era deforestation and ongoing hillslope land disturbance. These deposits, often referred to as legacy sediments, overlay a comparatively organic-rich, pre-colonial buried floodplain soils characteristic of extensive grass-dominated wetland valley bottoms. Debate has emerged regarding the ubiquity of both the interpreted pre-disturbance grass-dominated wetlands, and the thickness of legacy sediment mantle in modern floodplains. When coupled with management concerns regarding the potential for legacy sediment to serve as a source or nutrient-rich fine sediment, it is imperative that we improve our understanding of the nature and extent of these floodplain deposits. In this study, we use spatially extensive field sampling of exposed river banks along White Clay Creek and Doe Run, PA, major tributaries to the Christina and Brandywine Rivers respectively, to characterize the nature and spatial variation of legacy sediment and buried soil thicknesses along the floodplain continuum. Floodplain deposits were analyzed for thickness, organic content (AFDM), grain size, and elemental content. Deposit thicknesses were mapped using GIS and investigated for correlation with known historical mills and dams. Our results indicate that floodplain deposits can vary greatly within and between watersheds. Buried soils were consistently richer in organic content than post-settlement alluvium, but both layers had similar characteristic grain size distributions. Post-settlement alluvium deposits varied widely in thickness within and between watersheds (20-160 cm), as did buried organic soils (0-80cm). No clear correlation was found between post-settlement alluvium thickness and the known location of historic mills or dams. Overall, our results suggest patchy distributions of pre-colonial floodplain conditions (e.g. grass dominated wetland, bottomland forest) as well as a patchy post-settlement depositional environment.