Sediment transport is a worldwide environmental problem that degrades soil productivity, water quality, causes sedimentation to the reservoirs and increases the probability of floods. Gefersa reservoir, one of the surface water supply sources of Addis Ababa city for the last 70 years, face this problem. The reservoir supplies an average of 30,000m3 of treated water per day to the city. Based on the 1979 and 1998 bathymetric surveys, with the assumption of linear yearly siltation rate there is 22,252 m3/year of sediment inflow to the reservoir. In terms of soil loss from the catchment area, this constitutes a loss of 575 tons/km2/year contributed by the catchment area. In this study, Soil and Water Assessment Tool (SWAT) was used to calibrate and validate a hydrologic component and sediment yield of Gefersa watershed. Back-calculation process was carried out to estimate the natural inflow to the Gefersa reservoir. Sensitivity analysis, model calibration and validation were also performed to assess the model performance.