Interactions between interfacial turbulence and hyporheic processes in rivers (Invited)

Rivers represent extremely complex environments because they are highly structured and dynamic over many scales. Turbulent flow is a defining feature of rivers, and this produces rapid internal mixing and strong interactions with channel boundaries. Recent studies have emphasized that rivers are also intimately connected to the underlying pore space, leading to a distinct region - the hyporheic zone - where physical, chemical, and biological processes are explicitly influenced by the interaction of rivers and groundwater. Most investigations of hyporheic exchange have focused on Darcy-scale potential flows induced by flow-boundary interactions and elevation gradients along river channels. However, it is abundantly clear that there must be a transition from free-stream turbulence to Darcy-flow conditions near the stream channel interface. Transport in this interfacial region appears to contribute substantially to observed patterns of solute transport, biogeochemical transformation, particle dynamics, and microbial growth in rivers. Yet little is known about the structure of the flow transition because it is spatially very limited in most rivers and common experimental methods do not adequately resolve the interfacial behavior. I will review the available evidence suggesting why interfacial turbulence is generally important in rivers, outline the challenges in characterizing turbulent hyporheic processes, and suggest strategies for making progress in resolving both the structure of these flows and important biophysical and biogeochemical outcomes.