Prof. Shlomi Arnon

Hyporheic exchange is an ecologically important process that controls the exchange of nutrients between the water column and microbe-rich sediment bed in a stream. Fine particle deposition induces clogging and reduces hyporheic exchange and larger stream-groundwater interactions. This phenomenon has primarily been studied with stationary bedforms. We conducted experiments to study the effect of fine suspended particle deposition on clogging and hyporheic exchange with mobile bedforms. Experiments were conducted in a recirculating flume (640 cm X 30 cm) packed with homogeneous sand. This flume has a unique ability to impose up- or down-welling flow through the entire sediment bed. Hyporheic exchange flux was quantified using salt tracer additions, and observed visually by adding a dye tracer added to the overlying water. We also obtained high-frequency timeseries of bedform morphodynamics and water column clay concentration. While clogging occurs close to the surface in stationary streambeds, we observed clogging below the active mobile bed sediment in our experiments. Unlike the case with stationary bedforms, in which clay accumulates primarily on the upstream slope of each bedform and reduces exchange starting from that location, moving bedforms redistribute deposited clay and shift the location of hyporheic inflow as the bedforms propagate downstream. This leads to clay accumulation primarily below the maximum scour depth of the series of bedforms. Additional experiments under gaining and losing flow conditions demonstrate variable clogging patterns. These experimental results represent unique observations of the effects of fine particle deposition on hyporheic exchange dynamics for losing, neutral, and gaining fluxes under mobile bed conditions. These results are important for understanding the multi-scale complexity of hyporheic exchange in rivers with both bedform-induced hyporheic exchange flow and reach-scale patterns of up or down-welling groundwater.