Spatial Distribution of Fine Sediment and Juvenile Freshwater Mussel Settling in the Gravel Bedded Dan River, North Carolina

Abstract

Fine sediment plays an important role in the benthic habitat and water quality of gravel bed rivers. Fine particles, such as sand and silt, are often suspended in the channel but can settle into the bed during low flows. The deposition of these particles is dependent on the bathymetry of the river as well as the flow conditions. Shear velocity determines the particle size that can remain suspended in a given flow and therefore whether a particle can settle and accumulate on the riverbed. The purpose of this study is to investigate the spatial patterns of fine sediment deposition in a gravel bedded river and to determine the correlation between deposition and shear velocity. The study area was a section of the Dan River in Stokes County, North Carolina. A 2D hydrodynamic model was used to simulate shear velocity in this reach and compare to field measurements of sediment deposition in sediment traps. Our results showed a variation of shear velocity and fine sediment accumulation across the channel with significant correlations between them. Sand accumulation was positively correlated with shear velocity. A linear trend was fit between sand accumulation and the log of shear velocity, although the relationship may show a threshold response where sand shifts from bedload to suspended load. The silt accumulation when normalized by the water depth was negatively correlated with shear velocity. This indicates that most silt accumulated in the sediment traps was suspended in the water column and settled into the traps at a low local shear velocity. These theories for sediment deposition were related to the settling of juvenile freshwater mussels to predict areas in the reach where mussels may be able to settle based on the local shear velocity. Our model results suggest that there are areas in the margins of the channel that will allow juvenile mussels to settle when flows are less than 2 m3/s. Large boulders may increase the potential for juvenile freshwater mussel settling, although more work should be done to assess the extent of their effect. The results of this study are important for understanding fine sediment transport and deposition in rivers, for understanding where freshwater mussels can settle, and for restoring in-stream habitat.