Hydrologic, geomorphic, and stratigraphic controls on suspended sediment transport dynamics, Big Harris Creek restoration site, North Carolina, USA

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Abstract Water quality monitoring records were combined with geomorphic and stratigraphic data to determine the controls on suspended sediment transport dynamics within the headwaters of Big Harris Creek, the site of one of the largest stream restoration projects in North Carolina, USA. Land-use change associated with European settlement resulted in spatially variable geomorphic responses that produced reaches possessing semi-homogeneous landforms and processes, referred to as process zones. Downstream, process zones were dominated by entrenched channels possessing banks characterized by sandy post-settlement deposits that overlie finer-grained pre-settlement deposits. Spatial variations in the resulting process zones strongly influenced modern suspended sediment concentrations (SSC) and loads in the catchment, which are among the highest reported for the southeastern United States. The source and transport dynamics of suspended sediment differed between low magnitude floods (characterized by minimal changes in water level elevations) and moderate to high magnitude floods. Low magnitude floods were characterized by SSCs that varied over several orders of magnitude, and that were unrelated to flow conditions. Precipitation, during these events, rapidly mobilization fine-grained pre-settlement deposits associated with bank failures and cattle crossings along entrenched alluvial channels. Moderate to high magnitude floods within larger tributary basins exhibited more systematic discharge-SSC relationships. Suspended sediment transport was dominated by sand-sized particles derived from post-settlement legacy sediments eroded from the channel banks and headwater gullies. The observed temporal and spatial differences in SSC between low and moderate to high magnitude floods complicates the quantification of water quality, and shows that comparisons of water quality before and following the implementation of restoration projects need to differentiate between distinct populations of suspended sediment transport.