Unveiling surface-subsurface flow interactions of a salmon redd

• A 2D synthetic redd is constructed in a large-scale recirculating flume and is made of transparent grains, allowing non-invasive optical measurements of the freestream, near-bed, and interstitial flows. • The subsurface velocities through the redd tailspill scale to the square of the freestream Froude number, i.e., Fr2 , indicating that faster, shallower flows – like those found at pool-riffle transition areas – are more favorable for salmon egg survival because of the increased advection of dissolved oxygen (DO) from the surface water to the eggs laid within the hyporheic zone. • Increased surface roughness along the salmon redd sediment-water interface (SWI) tends to slow down the downwelling subsurface currents, though this roughness may induce near-bed sweep- and ejection-like turbulence that creates a more complex surface-subsurface flow relation. • A zone of increased permeability within the tailspill – analogous to a salmon egg pocket – causes convergence of downwelling hyporheic flows and stronger dispersion within this zone, indicating that spawning activities that disturb the riverbed sediment may benefit egg survival by increasing the deliverance of DO to the egg pocket region. 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.