Spatial Variation in Characterized Buried Soils and Legacy Sediments of the Northeast USA

The role of historical mill dams in transforming river systems, especially throughout New England and the mid-Atlantic Piedmont regions of the United States has recently emerged as a topic of debate amongst the scientific community. In both physiographic provinces, large postsettlement alluvium “legacy” deposits originating from colonial-era deforestation, agriculture, and ongoing hillslope land-use disturbances characterize the floodplains. Furthering land-use impacts, water-powered milling from the late 17th to early 20th century was historically intensive throughout Southeastern New England and Pennsylvania, aiding in the floodplain storage of legacy deposits behind mill dams. These geologically recent legacy sediments overlay comparatively organic-rich, pre-colonial buried floodplain soils. Along mid-Atlantic Piedmont streams, 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. When coupled with management concerns regarding the potential for legacy sediment to serve as a source for nutrient-rich sediment pollution and the rise of a billion dollar stream restoration industry, it is imperative that our understanding of the nature and extent of these floodplain deposits is pushed further. In this study, field sampling of exposed riverbanks was carried out along two major tributaries to the Christina and Brandywine Rivers in Pennsylvania and along two tributaries to the Connecticut River in order 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 material, grain size, and color. A longitudinal survey accompanied the deposit measurements relative to the stream gradient and bank elevation. Deposit thicknesses were mapped using GIS and investigated for correlations with known historical mills and dams. Sites were cross-compared to explore the role of glacial history in legacy sediment deposition and variability of the pre-disturbance floodplain surface. Results indicated that floodplain deposits vary greatly within and between watersheds as well as within different glacial settings. 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 in PA, 51-143 cm in CT), as did buried organic soils (0-80cm in PA, 20-48cm in CT). Mill dams served as a source of legacy sediment preservation, but were not collectively coupled with sediment deposits. Differences in regional and glacial histories influenced the magnitude to which sediments were stored in the floodplains, but it was slope, sinuosity, and depositional environment that appeared to most significantly impact the preservation of sediments in the landscape. The overall trends in the 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