The population growth and construction of hydropower generation dam caused changes in land cover of the OMO River basin, Gilgel Gibe III sub watershed and this changes have impacted on the stream flow of the watershed by changing the magnitude of stream flow. This study is mainly focusing on the assessment of the impacts of the land cover changes on stream flow through satellite Remote Sensing (RS) and Geographic Information System (GIS) integrated with the SWAT model. ArcGIS used to generate land use and cover maps from Land sat TM,ETM+ and Ls8 acquired, respectively, in 1987,2003 and 2013. The land cover maps were generated using the Maximum Likelihood Algorithm of Supervised Classification. The result of this analysis showed that the cultivated land has expanded, forest land decreased, range grass land decreased and water body has increased during the study period of 1987-2013. Using the three generated land cover maps, three SWAT models set up were run to evaluate the impacts the land use and cover changes on stream flow at the watershed. The performance of the SWAT model was evaluated through sensitivity analysis, calibration, and validation. eleven flow parameters were identified to be sensitive for the stream flow of the study area and used for model calibration. The model calibration was carried out using observed stream flow data from 01 January 1989 to 31 December 2000 and a validation period from 01 January 2002 to 31 December 2007. both the calibration and validation results showed good match between measured and simulated stream flow data with the coefficient of determination (R2) of 0.72 and Nash-Sutcliffe efficiency (ENS) of 0.67 for the calibration, and R2 of 0.86 and ENS of 0.6 of the validation period. Because of change in land use land cover mainly decrease in forest land by 24.03 %, increase in cultivated land by 30.146 % and decrease range-grass land by 6.846 % from year 1987 to 2013 stream flow has increased by 37.13 m3/s,60.92 m3/s and 98.05 m3/s from 1987-2003,2003-2013 and 1987-2013 Land use Land cover respectively.rnKey words: Geographic Information system (GIS),Gilgel Gibe III Watershed, Land use and cover change, Remote sensing, Soil and Water Assessment Tool (SWAT),Surface runoff.