Time-averaged SPIV data for 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 dune-like structure called a redd, having coarse grains on the surface due to the winnowing away of fine material. The interaction between a redd and the stream flow induces surface water to enter the sediment, flow through the egg pockets, and reemerge downstream of the redd 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. We use non-intrusive optical measurement techniques, including stereo particle image velocimetry (SPIV) and planar laser-induced fluorescence (PLIF), in a large-scale flume by matching the refractive index of the solid grains (made of THV) and the fluid (water mixed with 15% Epsom salt). This allows us to see through the solid matrix of grains and map the flow fields above and within the salmon redd for four flow scenarios, e.g. slow and fast velocities with shallow and deep depths, two redd constructions, e.g. a smooth and rough surface, and with and without an egg pocket. Results indicate that the flow through the redd is approximately proportional to the square of the Froude number for the smooth redd, and more complex for the rough redd because roughness impacts flow characteristics. The egg pocket, having larger grains than the surrounding matrix, enhances mechanical dispersion that increases water mixing within the egg pocket and causes convergence of the downwelling fluxes towards the egg pocket. Article DOI: 10.1016/j.advwatres.2025.104947