The Role of Riverine Bed Roughness, Egg Pocket Location, and Egg Pocket Permeability on Salmonid Redd‐Induced Hyporheic Flows

Salmon spawning activities alter streambed morphology, forming a dune‐shaped egg nest called a redd. The spawning process increases redd sediment hydraulic conductivity, KD, by removing fines, creating an egg pocket with larger sediment grains such that egg pocket hydraulic conductivity, KEP, may be higher than KD. Salmon females may create one or more egg pockets within a single redd. Although the impact of redd shape and KD on redd‐induced hyporheic fluxes has been studied, the effects of streambed roughness, R, egg pocket permeability, and egg pocket location on egg pocket hyporheic fluxes, q‾ep ${\overline{q}}_{ep}$, (downwelling flows from the stoss side of the redd which may enter egg pockets) have not yet been quantified. This study investigates this knowledge gap with a set of numerical simulations supported by flume experiments. We simulated hyporheic flows for five egg pocket locations, five KEP values from 0.0025 to 0.02 m/s, and 12 rough streambed surfaces. Surface roughness was scaled from a measured streambed surface in two ways—only vertically (R1) and both vertically and horizontally (R2)—with scaling coefficients ranging from 0.5 to 3. The measured streambed surface had a median diameter, D50, of 1 cm and a standard deviation (σD) of 0.77 cm. The results indicate that the dimensionless flux into the egg pocket, q‾ep∗=q‾epKD ${\overline{q}}_{ep}^{\ast }=\frac{{\overline{q}}_{ep}}{{K}_{D}}$ increases noticeably with the downstream distance of egg pockets from the redd pit, and less strongly with KEP∗=KEPKD ${{K}_{EP}}^{\ast }=\frac{{K}_{EP}}{{K}_{D}}$. The near‐surface downwelling fluxes significantly increase with R1, but only negligibly with R2, and for deeper egg pockets, q‾ep∗ ${\overline{q}}_{ep}^{\ast }$ is minimally impacted by surface roughness. Our results suggest that the typical simplification of a smooth redd surface with a single redd hydraulic conductivity provides a good representation of the interstitial flow within the redd, and the effects of surface roughness and egg pocket hydraulic conductivity on q‾ep∗ ${\overline{q}}_{ep}^{\ast }$ fall within the uncertainty of the egg pocket location. Plain Language Summary Salmonids lay their eggs in the streambed gravel, forming dune‐shaped egg‐nests, called “redds.” Here we study water movement through the salmon redd, investigating egg pocket permeability, streambed roughness, and egg pocket locations within the redd. This flow is crucial as it delivers oxygenated water and nutrie