© 31st IAHR Congress 2005: Water Engineering for the Future, Choices and Challenges. All Rights Reserved.This paper discusses the application of a 3D RANS model as a prediction tool to simulate flow and temperature distributions within the McNary Dam and its forebay situated on the Columbia River. The model contains all the relevant geometrical details of the hydraulic structures and forebay bathymetry over a length of 13,000ft upstream of the dam. The Boussinesq flow module in FLUENT is utilized to obtain the flow hydrodynamics and temperature distributions. The model is first validated using temperature field data supplied by the US Army Corps of Engineers (USACE). It is found that the model is able to accurately predict the salient features of the thermally stratified flow in the forebay for a test case corresponding to strongly stratified conditions. Then several structural alternatives are simulated with the goal of exploring the effect of these changes on the gatewell and fish orifices temperature distributions at the 14 powerhouse units of the dam. The two types of structural alternatives considered include modifications of the intake roof geometry and introduction of a floating vertical barrier (curtain) in the forebay. One of the alternatives involving the introduction of a curtain wall appears to be the best solution to alleviate occurrence of adverse thermal properties within the turbine intakes, gatewells and fish sluiceways among the ones analyzed in the present study.