Colider Dam is an earthfill dam on the Teles Pires River in the northern region of Mato Grosso, Brazil built by Copel Generation and Transmission S.A. from 2011 to 2019. Fish mortality and gas bubble disease symptoms were observed downstream of Colider due to elevated total dissolved gas (TDG) concentrations during spillway releases. Two reduced-scale laboratory models and a three-dimensional computational fluid dynamics (CFD) model were developed to understand the dynamics of spillway jets responsible for gas dissolution and supersaturation. The laboratory models, a 1:120 scale hydraulic model comprising the entire Colider tailrace and a 1:60 scale spillway sectional model, were constructed in the CEHPAR/Universidade Federal do Parana in Curitiba. A three-phase flow CFD model, comprising the air and water phases and bubbles, was implemented in the open-source code OpenFOAM to predict spillway jet regimes and total dissolved gas downstream of the dam. A detached eddy simulation (DES) model was used for turbulence closure. A particle tracking technique was used to estimate fish mechanical injury and TDG exposure. The model includes the main features of Colider: four spillway gates, left and right training walls, stilling basin, powerhouse units and 1 km reach of the Teles Pires River downstream of Colider. CFD model results agreed well with velocity data collected in the 1:120 hydraulic model and deflector performance curves obtained with the 1:60 sectional model. The model was calibrated and validated against TDG field data collected at the dam. After validation, 28 simulations were performed to evaluate and improve the deflector design. This paper will present and discuss details of the models, calibration and validation, and final simulations with the selected deflector design.