SPATIAL VARIATION IN CHARACTERIZED BURIED SOILS AND LEGACY SEDIMENTS OF THE NORTHEAST USA

In the New England and mid-Atlantic Piedmont physiographic provinces of the United States, large post-settlement alluvium “legacy” deposits originating from colonial-era deforestation and ongoing hillslope disturbances characterize floodplains. These legacy sediments are frequently deposited behind mill dams and overlay comparatively organic-rich, pre-colonial buried floodplain soils. Debate has emerged regarding the ubiquity of both the interpreted pre-disturbance land surface, and the thickness of the legacy sediment layers in modern floodplains. The potential for legacy sediment to serve as a source for nutrient-rich sediment pollution and the rise of a $1 billion stream restoration industry necessitates an understanding of the nature and extent of these floodplain deposits. In this study, field sampling of exposed riverbanks was carried out along two major tributaries to the Christina and Brandywine Rivers in Pennsylvania and two tributaries to the Salmon and Connecticut Rivers to characterize the nature and spatial variation of floodplain sediments. Deposits were analyzed for thickness, organic material, grain size, and elemental content, and were mapped using GIS in combination with known historical dams. Results indicated that floodplain deposits vary greatly within and between watersheds as well as different geographic settings. Buried soils were consistently richer in organic content than legacy alluvium, but both layers had similar characteristic grain-size distributions. Legacy deposits varied widely in thickness within and between watersheds (20-160 cm in PA, 51-143 cm in CT), as did buried organic soils (0-80cm in PA, 20-48cm in CT). No clear connection was found between post-settlement alluvium thickness and known locations of historic mills or dams. The overall trends suggest patchy distributions of pre-colonial floodplain conditions (e.g. grass dominated wetland, bottomland forest) as well as a patchy post-settlement depositional environment.