Rainfall Runoff modeling is an important tool in the study of water resources and water management of the watersheds. Rainfall runoff models are mainly used for river flow forecasting for the management of the resource and to minimize the ill effects through early warning measures. This paper deals with rainfall runoff modeling of the upper Awash River catchment located in the central part of Ethiopia. The area of the basin is 7615 sq. km. The river is the main tributary of the Koka multi purpose reservoir used for hydropower generation, irrigation and flood control. The Galway Flow Forecasting System, GFFS, was used to model the flow. Various models of GFFS, ranging from simple linear models to SMAR – the conceptual model were used to simulate river flows. Brief description of the models is presented in the paper. Performance comparisons have been made to select models that are more suitable for the catchmenent. It was concluded that in the simulated mode, the SMAR model performs better than the others.