Fine-grained channel margin deposits in a typical gravel bed river: Spatial and temporal controls on the distribution, quantity, and residence time and implications for centennial-scale sediment and mercury cycling

Fine-grained channel margin (FGCM) deposits are a key geomorphic feature that has been identified on South River. These deposits are the primary focus of this research. South River is a gravel-bed bedrock river that has a legacy of mercury contamination from a historical industrial release (from 1929 to 1950). Fine-grained channel margin (FGCM) deposits refer to deposits consisting of sand, mud, and organic matter that form in areas of reduced velocity behind accumulations of large, woody debris (LWD). These deposits store about 28% of the annual suspended sediment load. The deposits average 42 +/- 58 m long, 5 +/- 2 m wide, and 0.34 +/- 0.18 m thick. No FGCM deposits occur where the slope of the river exceeds 0.0025. A sediment rating curve was generated for South River and it is determined that the average and median annual masses of suspended sediment transport at Waynesboro are 5150 Mg/yr and 2900 Mg/yr, respectively. Isotopic dating analysis of sediments indicates that sediments in FGCM deposits have ages that range from less than one year to more than 70 years, an interpretation that is supported by the Hg concentrations in the deposits. An integrated conceptual model for FGCM deposits in South River incorporates a hierarchy of temporal and spatial scales governing the distribution of sediment and LWD. The age distribution of sediment, mass of sediment, and mercury concentrations are used to assess historical water quality in South River. This analysis indicates that mercury concentrations in the water column of the South River associated with suspended particles were about 1200 ppm during the release period.