The effects of discharge and slope on hyporheic flow in step-pool morphologies using cold water as a tracer

The hyporheic zone is a transitional area between surface and groundwater systems that supports both a wide range of species ranging from bacteria to incubating fish embryos. A better understanding of the movement and extent of flows through the subsurface may lead to innovative habitat restoration techniques to make streams more productive and to enhance aquatic stream biology. While the majority of studies thus far have examined the hyporheic zone in low gradient streams with pool-riffle and dune-like morphology, this study has investigated hyporheic flow in a steep stream with step-pool morphology. A series of flume experiments were conducted which examined the effect of discharge and slope on hyporheic flow in step-pool channels using cold water as a tracer. The channel consisted of 3 step-pool units with 64mm sized steps, discharges between 0.3-4.5L/s and slopes of 8% and 4%. The results indicated the water moving along steeper slopes and the moderate discharges produced the deepest flows while downwelling at the base of steps was related to the stream discharge. Results produced by a model using hydraulic head measurements were inconsistent of those produced by temperature sensors. The model suggested hyporheic downwelling flows are largely unaffected by discharge while increases in residence times are related to decreases in discharge. We found the temperature tracer to provide a more accurate representation of hyporheic flows than that derived from piezometric measurements though the former procedure would be difficult to replicate in field experiments. This experiment emphasizes the importance of topography and roughness elements along the channel surface in dictating the locations and magnitude of downwelling and hyporheic exchange.Arts, Faculty ofGeography, Department ofGraduat