Unveiling surface-subsurface flow interactions of a salmon redd

Female salmonids bury their eggs in streambed gravel by digging a pit where they lay their eggs, which they then cover with gravel from a second pit, forming a rough-surfaced dune-like structure called a redd. The interaction between a redd and the stream flow induces surface water to flow into the sediment, through egg pockets, and reemerge downstream of the crest. These downwelling and upwelling flows form the hyporheic exchange, which is vital for the embryos’ development because it regulates the egg pocket temperature regime and delivers oxygen-rich surface water to the embryos. Here, we experimentally investigate the effects of (1) redd surface roughness and (2) egg pocket presence on redd-induced hyporheic flows under different surface hydraulics by constructing a synthetic redd made of transparent grains that allow us to measure freestream, near-bed, and interstitial flow velocities with non-invasive techniques in a recirculating flume. Results indicate that flow through redds is proportional to the squared Froude number, but surface roughness causes this relation to be more complex. The egg pocket, having larger grains than the surrounding matrix, enhances mechanical dispersion, which increases water mixing within the egg pocket and causes convergence of downwelling flows towards the egg pocket. Overall, these results show that river modeling studies focused on salmonid preservation should consider surface roughness as well as varying hydraulic conductivities within the nesting environments.