Dynamics of Hyporheic Exchange Flux and Fine Particle Deposition Under Moving Bedforms

Deposition of fine suspended particles can reduce hyporheic exchange flux (HEF), which may result in impairment of various ecological processes. However, the dynamics of fine particle deposition and streambed clogging are still not well understood, especially when the bed is in motion. We conducted flume experiments to study the effects of coupled sand-clay dynamics on HEF with episodic (pulse) inputs of suspended kaolinite. Three experiments with a mobile sand bed and constant discharge were conducted with repeated injection of suspended kaolinite pulses at fixed concentration increments of 0.16, 0.41, and 0.66 g/L, respectively. HEF and participating porewater volume were assessed using tracer tests. Kaolinite deposition rates were inferred from turbidity measurements while deposition patterns were measured using core samples. We found that fine sediment primarily accumulated within a layer below the bedform scour zone, and that this layer was thicker when kaolinite was added in larger pulses. HEF declined linearly over time as kaolinite accumulated in the bed sediment; an increase of 0.1% kaolinite in the bed led to a decrease in HEF of 26.6 cm/day, regardless of the pulse concentration. However, the same kaolinite increase led to a reduction of 50% in participating pore volume for larger kaolinite pulses, versus only 30% for the smallest pulse. These results indicate that clogging occurs not just during and after high-flow events, but also under constant flow conditions in which episodic deposition of suspended clay by HEF leads to the formation of a low-conductivity layer in the bed.