Multi-scale streambed topographic and discharge effects on hyporheic exchange at the stream network scale in confined streams

Abstract

Summary The hyporheic zone is the volume of the streambed sediment mostly saturated with stream water. It is the transitional zone between stream and shallow-ground waters and an important ecotone for benthic species, including macro-invertebrates, microorganisms, and some fish species that dwell in the hyporheic zone for parts of their lives. Most hyporheic analyses are limited in scope, performed at the reach scale with hyporheic exchange mainly driven by one mechanism, such as interaction between flow and ripples or dunes. This research investigates hyporheic flow induced by the interaction of flow and streambed topography at the valley-scale under different discharges. We apply a pumping based hyporheic model along a 37 km long reach of the Deadwood River for different flow releases from Deadwood Reservoir and at different discharges of its tributaries. We account for dynamic head variations, induced by interactions of small-scale topography and flow, and piezometric head variations, caused by reach-scale bathymetry–flow interactions. We model the dynamic head variations as those caused by dune-like bedforms and piezometric heads with the water surface elevation predicted with a 1-dimensional, 1D, hydraulic model supported by close-spaced cross-sections extracted every channel width from high-resolution bathymetry. Superposition of these two energy-head components provides the boundary condition at the water–sediment interface for the hyporheic model. Our results show that small- and large-scale streambed features induce fluxes of comparable magnitude but the former and the latter dominate fluxes with short and long residence times, respectively. In our setting, stream discharge and alluvium thickness have limited effects on hyporheic processes including the thermal regime of the hyporheic zone. Bed topography is a strong predictor of hyporheic exchange and the 1D wavelet is a convenient way to describe the bed topography quantitatively. Thus wavelet power could be a good index for hyporheic potential, with areas of high and low wavelet power coinciding with high and low hyporheic fluxes, respectively.