Route use of emigrating steelhead in a heavily modified river delta.

This article focuses on predicting the routing of juvenile hatchery-reared steelhead (Oncorhynchus mykiss) smolts at two river junctions—the head of Old River and Turner Cut—in California's Sacramento–San Joaquin River Delta ("Delta"), a heavily managed tidal estuary with low smolt survival and large-scale water extraction. Using six years of acoustic telemetry data and hydrodynamic modeling, the study found that localized, subhourly flow conditions at the junctions were stronger predictors of fish route selection than commonly used daily management metrics such as upstream river discharge and water export pumping rates located 20–30 km away. The presence of a temporary rock barrier at the head of Old River significantly increased main-stem San Joaquin River route use, while water export operations had only a small effect on routing compared to local flow dynamics. The findings suggest that water resource managers should prioritize monitoring and managing local hydrodynamic conditions and combine physical barriers with habitat improvements in the interior Delta to optimize migratory survival of steelhead through this complex system.