Patterns of legacy sediment deposits in a small South Carolina Piedmont catchment, USA

Abstract

Anthropogeomorphic changes to channels and floodplains occurred in response to destructive agricultural practices following the arrival of European settlers into the Americas. The southeastern Piedmont physiographic region of the USA experienced severe erosion and sedimentation after settlement in the 1700s and farming up through the 1930s. In places, deep floodplain aggradation formed uninterrupted alluvial deposits that extend many kilometers. This research examines anthropogenic impacts of land-use change on valley bottom sedimentation in the Chicken Creek Watershed of South Carolina. Abrupt contacts between precolonial floodplain soils and a thick overburden of historical legacy sediment are common and well exposed throughout the two-km study reach and provide clear evidence of extensive postcolonial sedimentation. Soil stratigraphic and sedimentologic evidence are presented to identify and contrast pre- and postcolonial sediment characteristics and thicknesses. LiDAR-based spatial analysis is used to examine patterns of legacy sediment deposition and floodplain storage and—together with field observations of bank stratigraphy—examine the environments in which this occurred. Precolonial geomorphic stability is indicated by a buried soil epipedon (Ab horizon) and other features on precolonial floodplain surfaces.Legacy sediment 2 to 4 m thick rests on top of exposed precolonial soils throughout the stream corridor, with the pre−/postcolonial contact ~1.5 m above the channel bed on average. Linear regression analysis shows that legacy sediment thickness varies statistically with changes in valley width and proximity to tributary sediment sources. Thin precolonial alluvium over bedrock suggests modest erosion and sedimentation rates prior to the arrival of EuroAmerican settlers. Mean grain-sizes are similar between the pre- and postcolonial alluvium, but substantial contrasts in the degree of stratification and bulk density document differences between the respective depositional environments and post-depositional changes to the two units. Channel adjustments since the time of maximum aggradation include bed incision to at least as low as the precolonial longitudinal profile and widening that is ongoing.