SOLUTE AND FINE SEDIMENT STORAGE AND REMOBILIZATION BY FLOODS IN A SAND-BED STREAM

Streamflow hydraulics, bedload transport, and hyporheic-zone processing of solutes and fine sediments are key drivers of stream ecological processes. The study objective was to determine how peakflows affect transport, storage, and redistribution of energy-rich solutes and organic-rich fine sediments in sand-bed streams. We injected a conservative solute tracer (KBr) and a suspended sediment tracer (fluorescent dye particles with neutral buoyancy that mimicked organic particles) for four hours and tracked the fate of tracers through a 200-m stream reach of Clear Run, NC, an urban, sand-bed stream on the Atlantic coastal plain. We simultaneously measured solute and tracer sediment movement into hyporheic flow paths and in situ dissolved nutrient, ferrous iron, and sulfide profiles in the subsurface, while also observing bedload transport using an overhead camera. At baseflow, solute and fine sediment tracers penetrated the streambed’s sand ripples that were 1.5 cm high by 12 cm long to depths of 12 and 3 cm, respectively. Ripple migration (0.4 cm s^-1) and the resulting turnover of the streambed (2 min^-1) was a primary mechanism of solute exchange within the bed's top 1.5 cm. Hyporheic pumping was the primary control on solute exchange with the bed at slightly greater depths (2 – 12 cm) and on slower timescales (approximately 0.05 min^-1). Although some injected particles were retained in hyporheic flow paths, a much greater percentage were stored in recirculation zones downstream of sand ripples. All measurements continued during a small flood that we initiated one hour after the injection ended by releasing flow from a temporary dam. At its peak the experimental flood raised stage by 14 cm and increased flow by ten times which increased the exchange flux and depth of exchange by hyporheic pumping. Bedload transport also increased due to growth of larger (6 cm high by 75 cm long) and more rapidly migrating (3.4 cm s^-1) bedforms with a faster turnover (2.7 min^-1). Bed disturbance by the flood flushed nearly all of the tracer particles that had been stored behind ripples during baseflow. In summary, bedform dynamics and hyporheic exchange exerted important and overlapping controls on the fate of organic-rich, fine particles and on energy-rich and redox-sensitive solutes in this urban, sand-bed stream.