Oxygen distribution and uptake in the hyporheic zone regulate various redox-sensitive reactions and influence habitat conditions. Despite the fact that fine-grain sediments in streams and rivers are commonly in motion, most studies on biogeochemistry have focused on stagnant sediments. In order to evaluate the effect of bed form celerity on oxygen dynamics and uptake in sandy beds, we conducted experiments in a recirculating indoor flume. Oxygen distribution in the bed was measured under various celerities using 2D planar optodes. Bed morphodynamics were measured by a surface elevation sensor and time-lapse photography. Oxygenated zones in stationary beds had a conchoidal shape due to influx through the stoss side of the bed form, and upwelling anoxic water at the lee side. Increasing bed celerity resulted in the gradual disappearance of the upwelling anoxic zone and flattening of the interface between the oxic (moving fraction of the bed) and the anoxic zone (stationary fraction of the bed), as well as in a reduction of the volumetric oxygen uptake rates due shortened residence times in the hyporheic zone. These results suggest that including processes related to bed form migration are important for understanding the biogeochemistry of hyporheic zones.

Fine particles (0.1–100 microns) are ubiquitous within the water column. Observations on the interactions between suspended fine particles and sediment beds remain limited, reducing our ability to understand the interactions and feedbacks between fine particles, morphodynamics, and hyporheic flow. We performed laboratory experiments to explore changes in bedform morphodynamics and hyporheic flow following the progressive addition of kaolinite clay to the water column above a mobile sand bed. We characterized these interactions by taking high‐frequency time series measurements of bed topography and freestream clay concentration combined with solute injections and bed sediment cores to characterize subsurface properties. Deposition of initially suspended clay resulted in a decrease of bedform height, celerity, and sediment flux by 14%, 22%, and 29% when 1000 g was accumulated within the bed (equal to clay/sand mass ratio of 0.4% in the bed). The hyporheic exchange flux decreased by almost a factor of 2 for all clay additions, regardless of the amount of clay eventually deposited in the bed. Post experiment sediment cores showed clay accumulation within and below the mobile layer of the bedforms, with the peak concentration occurring at the most frequent bedform scour depth. These results demonstrate the tight coupling between bed sediment morphodynamics, fine particle (clay) deposition, and hyporheic exchange. Suspended and bed load transport rates are diminished by the transfer of suspended load to the sediment via hyporheic exchange. This coupling should be considered when estimating sediment transport rates.

Rivers are a vital part of global ecosystems due to their major role in sediment distribution and cycling of nutrients and carbon (Cole et al., 2007;Tiegs et al., 2019). This is accomplished largely through the interactions between the flow in the stream and the underlying sediment bed. Interactions such as bed motion and water exchange between the stream and the subsurface are influenced by numerous physical properties including stream flow, streambed slope, particle size distribution of the bed, and so on. Biogeochemical processes in streams particularly depend on delivery of nutrients and substrates to microbes that are mostly found in the streambed (

Fine suspended particles are ubiquitous in streams and rivers. Suspended material typically includes sedimentary particles (Wharton et al., 2017), particulate organic matter (Johnson et al., 2018), microplastics (Li et al., 2020, and microbiota such as bacteria, algae, and viruses (Lenaker et al., 2018). Transport and deposition of fine suspended particles play a key role in regulating river-groundwater interactions, river morphodynamics, and hyporheic biogeochemistry (Boano et al., 2014). Clay particle deposition decreases streambed hydraulic conductivity by filling porespace, ultimately clogging the bed, altering patterns of porewater flow, and degrading the benthic and hyporheic ecosystem (Brunke, 1999;Brunke & Gonser, 1997;Fox et al., 2018). Clay in the streambed can also reduce bed sediment motion (Dallmann et al., 2020). The deposition of fine particulate organic matter drives hyporheic metabolism (Newbold et al., 2005) and plays an important role in fluvial carbon cycling (Brunke & Gonser, 1997;Hope et al., 1994). Additionally, fine sediment particles play an important role in the colloid-facilitated transport of sorbed metals (Droppo et al., 2014;Foster & Charlesworth, 1996), as well as the accumulation of contaminants in bed sediment (Arce et al., 2017;Stone & Droppo, 1994). Despite the importance of spatial patterns of particle deposition for hyporheic ecosystems, fluvial biogeochemical processes, and river contamination most studies of riverine fine particles focus on the water column (Drummond et al., 2019;Park & Hunt, 2018;. Considerably less effort has been put into understanding the dynamics

While the ecological significance of hyporheic exchange and fine particle transport in rivers is well established, these processes are generally considered irrelevant to riverbed morphodynamics. We show that coupling between hyporheic exchange, suspended sediment deposition, and sand bedform motion strongly modulates morphodynamics and sorts bed sediments. Hyporheic exchange focuses fine-particle deposition within and below mobile bedforms, which suppresses bed mobility. However, deposited fines are also remobilized by bedform motion, providing a mechanism for segregating coarse and fine particles in the bed. Surprisingly, two distinct end states emerge from the competing interplay of bed stabilization and remobilization: a locked state in which fine particle deposition completely stabilizes the bed, and a dynamic equilibrium in which frequent remobilization sorts the bed and restores mobility. These findings demonstrate the significance of hyporheic exchange to riverbed morphodynamics and clarify how dynamic interactions between coarse and fine particles produce sedimentary patterns commonly found in rivers.