The safety of migrating salmon, especially salmonids, in the Pacific Northwest has been a concern for decades. With the advent of fish bypass systems, and safer turbines the focus of salmon safety has turned to total dissolved gases. Produced by entrainment of air into tailrace waters, total dissolved gases (TDG) can cause gas bubble disease, a harmful and potential lethal disease in fish. Avian predators are another danger for migrating salmon. In some areas of the world birds common in the Pacific Northwest can account for as much as 65% of salmon smolt losses. The goal of this thesis is to determine the effects of changing operational conditions at McNary dam on fish exposure to predator habitats and TDG. Computational fluid dynamic models were implemented to predict the hydrodynamics, TDG distribution and inert particle trajectories in the tailrace of McNary dam for varying operational conditions. A 3D volume of fluid (VOF) model was used first to capture the free surface shape in the tailrace. A rigid-lid model was then used to simulate the hydrodynamics and TDG distribution within the tailrace using the free surface shape from the VOF model. This 3D two phase model utilized an anisotropic Reynolds Stress turbulence model. All grids were generated using the commercial Gridgen software. A lagrangian particle tracking model that followed Newton's laws of motion were used to track inert particles throughout the domain. Validation of the model was performed. A grid refinement study with four different refinement levels was performed. Velocities for each grid type were compared against field data taken in 2004, and TDG was compared amongst the four grids. It was determined the medium level of refinement could accurately predict the velocities, and the TDG was relatively independent of grid density; TDG averages at the grid outlets were within 1.435% of one another. The TDG distribution was then compared, using the grid of medium refinement against field data measured in 1997and were between 1.5 and 3% of error depending on the transect. After validation of the model 16 predictive simulations were run with varying levels of total river flow and operational conditions. Tailrace hydrodynamics along with TDG production and distribution were compared for simulations with comparable total river flow rates. Fish trajectories were tracked using the particle tracking model. Inert particles were injected into the domain and properties such as velocity, distance to the shore and depth about each were recorded. Statistics were then generated for the particles based on criteria that defined dangerous predation zones within the tailrace. After completion of the simulations, it was determined that existing operations consistentlyproduced higher levels of TDG due to increased entrainment of the powerhouse flows into the spillway regions. It was also found that with increasing total river flows, TDG levels increased. On average, summer operations had lower TDG than spring due to the lower total river flows. Predation zones were similar for all simulations, but particle statistics varied depending on operational conditions. In general, particles were safer for higher flowrates as fewer low velocity eddies where particles could be trapped formed in simulations with high flowrates.