The new paradigm for the protection of drinking water resources in Ontario is management on a watershed-scale. Decision-making for management at this scale is facilitated using GIS-based water quality models. The evaluation of model predictions in different regions of Ontario is important and significant for improving the tools for water resource management in Ontario watersheds. This thesis is an investigation of the hydrology component of the CANWET water quality model in three Ontario watersheds: Black River, Canagagigue Creek and Fairchild Creek. The focus of this study is establishing the ability of this model to simulate watershed hydrology in Ontario, model limitations, the resolution of data required to support modeling and sources of error in the model results. These goals are achieved by comparing model predictions of stream flow and water balance components to observed data. CANWET was found to satisfactorily predict stream flow at the watershed outlet on an average monthly scale.