Analyzing Near Channel Inputs to Suspended Sediment Concentration

High concentrations of suspended sediment (SSC) in a river can represent a critical water quality concern, reduce the storage capacity of reservoirs, and impact aquatic habitat. Within a river, SSC can be conceptualized as a function of local reach scale hydraulics and watershed processes. Reach-scale hydraulic factors control the available shear stress during a flood, while watershed processes and properties may dictate the types and quantities of sediment supplied to the river. Local hydraulic controls can be measured at the reach scale, however untangling SSC requires determining which factors impact sediment supply. To explore the role of sediment supply, we utilized SSC data from over a 1000 US Geological Survey gages. We find that SSC at a site can generally be well described by its average and standard deviation of the natural log transformed data with the entire collection of field sites spanning several orders of magnitude in average SSC. These data reveal an intriguing geospatial pattern with clustered high and low values throughout the country suggesting a dependence on regional watershed properties. Here we utilize geospatial watershed and reach scale attributes (topography, soils, lithology, land use, rainfall, aridity, etc.) to explore how these quantities may impact decadal scale SSC patterns throughout the country. Initially, we find that catchment-wide and local point scale geospatial variables provide few explanatory univariate trends for the observed mean SSC pattern. Principal components analysis on the catchment-wide data yields that mean SSC is positively correlated with fine grained soils, larger catchments, and the percentage of agriculture within the catchment, and negatively correlated with mean annual rainfall normal, coarser soils, and forest land use types. We further showcase how near channel catchment properties can be used to understand the average SSC at a gage by examining how geospatial properties change along the stream network upstream for nested field sites within selected watersheds across the USA.