CHANGES IN VALLEY SEDIMENTATION IN SOUTHERN PIEDMONT STREAMS

In the southern Piedmont of the eastern U.S. basin-wide erosion of upland soils and alteration of surface hydrology from historic land clearance and agriculture have combined with consistent summer drought and reduced annual precipitation to alter processes of valley sedimentation. Here we present results of stratigraphic and hydrologic analyses conducted in southern Piedmont watersheds to better understand changes in valley sedimentation related to historic landuse change and contemporary drought. Results suggest that there is more than one contemporary alluvial surface utilized by the studied Piedmont streams, and in some cases, the contemporary floodplain consists of in-channel alluvial benches and not the main valley flat. This is in contrast with the prevailing concept of a single floodplain in equilibrium with the contemporary climatologic and geologic regimen. In addition, a shift to in-channel sediment storage instead of sedimentation within the main valley flat suggests that these Piedmont streams have evolved to be more alluvial as opposed to the semi-confined, bedrock nature with which they are usually associated. This alluvial shift is likely the result of transport-limited conditions and channel widening created by historic landuse change combined with hydrologic variability associated with contemporary drier conditions. These results highlight the sensitivity of geomorphic systems with legacies of human impacts to shifts in extrinsic controls, even at short timescales (decades), and the need for a more nuanced understanding of the relationship between cross-sectional form and hydrologic conditions in systems with human impacts.