Abstract
Abstract Salmon spawning activities alter streambed morphology, forming a dune‐shaped egg nest called a redd. The spawning process increases redd sediment hydraulic conductivity, K D , by removing fines, creating an egg pocket with larger sediment grains such that egg pocket hydraulic conductivity, K EP , may be higher than K D . Salmon females may create one or more egg pockets within a single redd. Although the impact of redd shape and K D 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, , (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 K EP 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 ( R 1 ) and both vertically and horizontally ( R 2 )—with scaling coefficients ranging from 0.5 to 3. The measured streambed surface had a median diameter, D 50, of 1 cm and a standard deviation ( σ D ) of 0.77 cm. The results indicate that the dimensionless flux into the egg pocket, increases noticeably with the downstream distance of egg pockets from the redd pit, and less strongly with . The near‐surface downwelling fluxes significantly increase with R 1 , but only negligibly with R 2 , and for deeper egg pockets, 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 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 nutrients to eggs while removing waste, influencing embryo survival. One key finding is that the egg pocket near the downstream end of the redd receives significantly more water, over five times more than the upstream‐most egg pocket. Interestingly, the presence of additional egg pockets within a redd doesn't substantially alter water flow into any specific egg pocket. While higher permeability in the egg pocket can increase the flow, its location has a more substantial impact; an 800% increase in permeability results in only about a 71% increase in flow into the egg pocket. Streambed roughness is less crucial for egg pockets deeper within the redd compared to the effect of redd's shape and egg pocket location. Our results indicate that treating redd as a single, smooth feature helps to understand the key factors affecting water flow into egg pockets. Information on egg pocket location, permeability, and riverbed roughness helps to better estimate the flow variations.
Key Points The interstitial flow toward the egg pocket increases with the distance from the pit, and toward the tailspill crest Egg pocket's higher hydraulic conductivity leads to a modest increase in interstitial flow toward it Redd shape primarily controls flows near egg pockets, with streambed roughness having minimal impact deeper within the redd