Trail Bridge Dam is a 100-ft high rockfill embankment dam on the McKenzie River in Oregon, USA operated by Eugene Water & Electric Board for hydropower generation. Currently there is no infrastructure for downstream fish passage at the project. Modifications are being developed to provide downstream fish passage through an existing radial spillway gate. A regulated opening has been proposed to be built in the lower section of the existing radial gate to provide safe downstream fish passage. A 1:12 reduced-scale physical hydraulic model was developed to investigate the hydraulics associated with the proposed changes and to analyze the possible unsteady flow phenomenon and dynamic loads associated with the radial gate and proposed fish passage gate operations. The intent of the physical model study was to evaluate fish passage flows and create a fish-friendly flow path through the spillway and downstream chute while avoiding flow instabilities or possible adverse flow interactions between the radial gate and embedded fish passage gate that could lead to vibrations, structural concerns, or fish injury. Prior to physical modeling, the fish passage gate design was initially developed using computational fluid dynamics (CFD) modeling. The physical model was used to assess and improve the design, with a focus on hydraulics and fish passage safety. Model testing resulted in several geometric changes to the roof, walls, leaf gate, and the outlet chute to improve the hydraulics and provide a smoother flow path into and through the fish passage gate. Considerable attention was given to improving the jet transition onto the spillway surface to achieve the desired jet characteristics. The model was also used to confirm target flow rates could be achieved and assess the impact of debris and potential for vortex formation that may impact the operation of the radial gate and fish passage gate. This paper will present and discuss the details of the model, the model test results, and final configuration with the selected fish passage gate geometry.