Hydrodynamic Drivers of Juvenile-Salmon Out-Migration in the Sacramento River: Secondary Circulation

Abstract

The entrances to the two lowest-survival migration routes for juvenile Chinook salmon in the tidal Sacramento River are located in the outside of a river bend where secondary circulation occurs. Three-dimensional simulations are conducted, in the Eulerian and Lagrangian frame, to understand tidal and secondary circulation effects on salmon migration route selection within this river reach. Fish were assumed to behave as neutrally-buoyant particles. Findings show that simulated particle entrainment rates into these routes tend to be larger than those expected from flow entrainment calculations alone, particularly during ebb tides, due to several factors. First, the fraction of the flow diverted to these routes tends to be higher in the shallowest layers, as a result of the secondary circulation that develops in the main river. Second, and supporting previous work done at the study site, the secondary circulation acting upstream also causes the surface-biased salmon distribution to be skewed toward the outside of the bend as they approach the entrance to the migration routes. As a result of these effects, the risk of particles being entrained is maximal near the surface, remaining higher than 50% during the course of a tidal cycle.