We have evaluated the ability of the Soil Water Assessment Tool (SWAT) to simulate the hydrologic processes of large scale river basins such as the Missouri River. SWAT is a quasi-physically based, continuous time model that operates on a daily time step and is computationally efficient for applications over large scales and long time periods. Spatial and temporal variability are represented by combining readily available input with automated parameter estimation using a GIS interface, making large scale modeling more feasible. The evaporation, snowmelt, and surface water/ground water interaction components of the model have been enhanced to improve simulation in mountainous terrain resulting in an adequate and representative large scale model. Climate and weather forecasting for global change scenarios are implemented to analyze the immediate and long term impact on the hydrologic processes of large scale river basins. The climate change scenarios include the variation of five climate parameters: precipitation, air temperature, solar radiation, relative humidity, and atmospheric carbon dioxide. Simulating these climate change scenarios provides insight to the impact of global climate change on the hydrologic processes of major rivers.