Fine-Particle Deposition, Retention, and Resuspension Within a Sand-Bedded Stream Are Determined by Streambed Morphodynamics

Fine-particle dynamics occupy an integral role in nutrient, contaminant, and pathogen transport in rivers. Due to low settling velocities, fine particles are often treated as if they pass through the river without interacting significantly with bed sediments. However, fine particles are advected toward and into the bed by turbulence and hyporheic exchange, leading to deposition by filtration. Extensive remobilization of fine particles is commonly observed during high flow conditions, but these dynamics are not well understood. Here we analyze the remobilization of fine tracer particles within the context of sediment morphodynamics during an experimental flood. Field data collected within Clear Run, a sand-bedded stream, by Harvey et al. (2012, https://doi.org/10.1029/2012JG002043) represent subsurface and water column time series of solute and fine particulate tracers. Overhead time lapse photography of the motion and growth of bed forms during the flood are used to extract and quantify morphodynamics, which we relate to the resuspension of fine particles by transforming bed form wavelength statistics into scour depth through a set of geometric relations derived from laboratory experiments. Particles deposited near the bed surface such that scour from larger bed forms remobilized the majority of particles within the streambed (>72%) accounting for the increase in resuspended fine particles during the flood. This analysis demonstrates that the inclusion of bed form dynamics and scour is necessary for understanding the remobilization of fine particles in Clear Run and that interpreting fine-particle dynamics during and beyond the event scale requires coupling with bed morphodynamics.