Factors controlling bedform morphodynamics and hyporheic exchange rates in mixed clay-sand beds

Transport of bed and suspended sediment mixtures regulates many critical river system attributes including bedform morphology, nutrient cycling, water quality, and in-stream habitat. In many river systems, fine particles (0.1-100 microns) are ubiquitous within the water column and sediment bed due to bank erosion and substantial inputs from landscape runoff. However, a lack of data on the interactions of fine particles and sediment beds limits our ability to understand the impacts of fine particles on sediment morphodynamics, hyporheic exchange and thus nutrient cycling, and the transport of fine particulate contaminants. To address this knowledge gap, we performed recirculating flume experiments to explore how both bedform morphodynamics and hyporheic exchange rate changed following the progressive addition of kaolinite clay to a mobile sand bed. We characterized the interaction of bed sediment and suspended particle dynamics via high-frequency measurements of bedform morphology and freestream clay concentrations. Below a threshold of total clay mass added to the system, change in bedform morphodynamics is negligible. However, above this threshold of clay mass, bedform height and celerity decreased substantially. We also assessed changes in porewater transport via solute injections performed before and after each clay-addition experiment, and sediment deposition structure via sediment bed cores at the completion of each run. Hyporheic exchange decreased with increased clay mass added to the system. The sediment cores showed that clay accumulated just below the depth of bedform scour. This armoring layer prevents the transport of solutes to the deepest regions of the bed and constrains the amount of sand in the active transport layer, impacting bedform morphodynamics. These experiments demonstrate that bed sediment morphodynamics, fine particle (clay) deposition, and hyporheic exchange are tightly coupled. Studies conducted in environments with substantial fine particles or numerous sources of fine particles must account for changes in bedform morphodynamics and hyporheic exchange to fully understand this dynamic system.