Spill is considered one of the safest fish passage strategies. However, fish traveling over a spillway can be exposed to elevated stresses due to deceleration, strain rate, or pressure changes during their impact with the tailrace. Spillway deflectors, installed to minimize gas super saturation, deflect water horizontally instituting an additional momentum change that might further increase stress experience by passing fish. In addition, since deflectors completely change the flow pattern in the tailrace, possible fish migration delay is also of concern. In this study, a CFD model was developed to evaluate the effect of spillway deflectors on fish injury and tailrace retention time. Free surface simulations were performed to obtain the flow field in the spillway face and tailrace. A particle tracking technique was employed and the history of acceleration, strain rate and pressure changes were calculated. Numerical results were correlated with biological data found in the literature to obtain the probability of fish injury. This paper presents details of the numerical model and discusses results obtained in the Hells Canyon Dam spillway for four operational conditions, with and without deflectors. According to the model, the inclusion of deflectors in a 7Q10 flow increases the percent of fish with minor injuries from approximately 5% to 10%. The percent of major injury increases from 1% to 3%. Residence time of particles released from the spillway decreases with spillway flowrate. The residence time of particles from the powerhouse is affected by powerhouse entrainment into the spillway region. A small level of entrainment increases the residence time since particles are pulled to a deep low velocity region in the stilling basin. As the lateral flow increases, some particles from the powerhouse join the high velocity surface jets decreasing their residence time. According to the model, deflectors decrease significantly the residence time in the tailrace.