Tracing river gravels: Insights into dispersion from a long-term field experiment

Sediment dispersion is a fundamental component of the sediment transfer process in gravel-bed rivers. Modeling this process requires an understanding of the collective movement of mixed-size clasts. This study explores the temporal evolution of gravel dispersion to underscore the importance of field observation in informing modeling efforts. Magnetically tagged gravels deployed in Carnation Creek have been monitored repeatedly over 17 years. Four metrics used to describe the extent of dispersion document that the overall shape in the spatial distribution of grain location changes over time. The general trends mask the complexity of the dispersion process, expressed by channel sections where tracers are concentrated regardless of grain size. The distribution of total grain displacement responsible for dispersion evolves as tracers become well mixed. Results demonstrate that observations from the field are crucial to the understanding and modeling of sediment dispersion because they provide key insights into the dispersion process that must be known a priori for mathematical modeling and similar observations cannot be collected using laboratory flumes.