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Gas bubble disease (GBD) is a condition that adversely affects aquatic animals exposed to supersaturated water. Fish mortality attributed to GBD has been observed in the tailrace of the Colider Dam on the Teles Pires River in Brazil during spillway releases. Gas supersaturation downstream of dams typically is caused by the dissolution of air bubbles entrained at depth in the stilling basin. The most common strategy to reduce total dissolved gas (TDG) concentration, and the risk of GBD, is to install spillway flow deflectors that redirect plunging flows horizontally and thus minimize the bubble transport to a depth at which dissolution is enhanced. This paper presents a numerical study, based on the open-source code OpenFOAM, assessing the design of spillway deflectors at Colider Dam and determining their effectiveness at reducing TDG. A two-fluid flow model was implemented in a free-surface solver to predict spillway jet regimes and TDG distribution for 900 m of the Colider tailrace. Residence time and TDG exposure in the tailrace were estimated using a particle tracking technique. A rigid-lid model was used to predict TDG distribution in the Teles Pires River 10 km downstream of the dam. Reasonable agreement was found between numerical model results and velocity data measured in a 1:120 reduced-scale hydraulic model. Predicted spillway jet regimes were consistent with the deflector performance curve obtained in a 1:60 reduced-scale sectional model. After calibration and validation against TDG field data, numerical simulations were performed to better understand the effect of the deflector curvature radius, length, and elevation on TDG production. Fish mortality or injury due to GBD was not observed in Colider during spill events after deflectors recommended in this study were installed in 2021. Field measurements indicate that TDG reduction by the deflectors is consistent with values predicted during the design of the deflectors. Fish mortality with gas bubble disease symptoms due to elevated total dissolved gas concentrations have been observed downstream of Colider Dam since the first spillway operation in 2018. The most common strategy to reduce TDG production downstream of hydropower dams is to install flow deflectors on the spillway face. This paper presents a numerical study to assess the performance of flow deflectors for Colider Dam. A computational fluid dynamic (CFD) model, comprising the air and water phases, and bubbles in the water phase, was developed to predict spillway jet regimes and TDG concentration. The model included the main features of the dam and a 10-km reach of the Teles Pires River. After calibration and validation, 28 simulations were performed to evaluate and improve the deflector design. A final deflector design was recommended based on TDG production and TDG exposure time. No fish mortality has been observed since the deflector construction was completed in 2021, even under operational conditions fatal to fish in the past. [ABSTRACT FROM AUTHOR]