The role of gravel channel beds on the particle size and organic matter selectivity of transported fine-grained sediment: implications for sediment fingerprinting and biogeochemical flux research

The preferential erosion, delivery, and transport of sediment within a drainage basin can result in downstream changes in both particle size and organic matter content. The physical and biological properties of transported and deposited sediment are important considerations in many sediment management and investigative tools, including sediment fingerprinting, and aid in the interpretation of sediment-associated nutrient and contaminant data. A recirculating flume (2 x 2 x 40 m) was used to assess the changes in particle size, organic matter content, and geochemical composition of fine-grained sediment (< 125 mu m) over a 31-h period (representing a travel distance of 24 km) under three contrasting channel bed conditions. The three channel gravel bed conditions investigated were 0-, 5-, and 40-cm gravel bed depths. Suspended sediment samples were collected throughout the duration of the experiment and gravel-stored sediment were collected along the length of the flume at the end of the experiment. Both suspended and gravel-stored sediment were analyzed for particle size and organic matter content. In addition, suspended sediment samples were analyzed for a broad suite of geochemical elements. The channel bed characteristics had a significant effect on both the particle size and organic matter selectivity of the transported suspended sediment. Furthermore, it was shown that a smooth planar channel bed as compared to a planar gravel bed (i.e., 0- vs. 5-cm gravel treatments) introduced small-scale roughness which resulted in the preferential deposition of larger particles into the channel bed. Increasing the gravel bed from 5 to 40 cm increased the amount of intra-gravel flow and reduced the potential for resuspension resulting in a further reduction in the particle size as well as resulting in a significant increase in the organic matter content of the suspended sediment. The relation between geochemical concentrations and particle size of the suspended sediment, in terms of linearity, magnitude, and direction, were not consistent between the different elements investigated. This research helps to understand the processes that control the particle size and organic matter selectivity of fluvial transported sediment. This information is an important part of many sediment management tools, including sediment fingerprinting, as it provides context when selecting sampling sites and interpreting the data they provide. The inconsistent relation between particle size and the concentration of different geochemical elements highlights the uncertainty associated with commonly used particle size correction factors.