How are River Discharge - Suspended Sediment Relations Influenced by Watershed and Channel-Floodplain Morphology?

Abstract:

Erosion, transport and deposition of fine sediment (clay, silt and fine sand) influence the form and function of river systems. Excess suspended sediment degrades stream ecosystems and is implicated as a leading cause of water quality and aquatic life impairment. Consequently, understanding the factors that control fine sediment transport regimes is an interesting topic for basic science and one that has important management and policy implications. Fine sediment is mostly transported in suspension as a non-capacity load; transport rates are dependent on sediment supply in addition to a river’s transport capacity. Many studies have investigated watershed-scale topographic, hydrologic, climatic, and land use influences on fine sediment erosion and transport regimes. Several recent studies in a wide range of landscapes have demonstrated that the majority of suspended sediment may be sourced from the near-channel environment; therefore, near-channel morphological characteristics may provide better predictive power compared to watershed averages.

This study analyzes recent total suspended solids (TSS) data from 45 gages on 35 separate rivers. The rivers span the state of Minnesota, draining basins ranging from 33 km² to 68100 km² with distinct settings in terms of topography, land cover, hydrology and geologic history. We generate rating curves of the form TSS = aQ^b, where Q is normalized discharge and a and b are parameters that describe the shape of the relations. Values of a range from 4 to 138 mg/L; b values range from -0.53 to 1.86. We use high resolution lidar topography data to characterize the near-channel environment upstream of gages. In addition to commonly studied metrics describing the topographic, climatic/hydrologic and land use setting of the basin, we extract near-channel morphometrics that we hypothesize to influence fine sediment generation and transport: the difference in height of banks/bluffs (a measure of the amount of material available to be entrained by fluvial erosion) and stream power (a measure of the erosive power of the stream) upstream of gages. We analyze the relationship between the basin and channel features and the rating parameters to identify morphological characteristics that provide the greatest explanatory power regarding controls on the transport regime.