objective of this study was to make an impact assessment that Gibe III hydroelectric damrnmay bring on the movement, distribution and quantity of surface water. The hydrologic impactrnassessment of the project during its operation, however, was carried out based on scenarios thatrnare developed taking the maximum power output capacity of the project, i.e. 1,870 MW, intornconsideration. Reservoir rules of two US hydropower projects, namely Trinity and Shasta, werernreferred while developing the scenarios. This is because the reservoir operation rule of the GibernIII hydroelectric project is not available in a way needed for the purpose of this study. SWATrnmodel was used to model the watershed, simulate the flow and approximate the evaporation fromrnthe reservoir. Flow simulation was done twice: without reservoir case and with reservoir case.rnMeteorological data such as daily rainfall, daily maximum and daily minimum temperature andrnrelative humidity; and spatial data such as DEM, soil and land use map of the study area werernused as an input of the model. The meteorological stations considered were Baco, Bonga,rnHossana, Jimma, W/Sodo, Limugenet, Wolliso, Wolkite and Sekoru. In addition to metrologicalrnand spatial data sets, observed flow data of the Great Gibe Near Abelti station were used forrnmodel calibration and validation purposes. According to the result of the study the averagernmonthly flow at the dam site is 1,162m3/sec with the minimum average monthly flow beingrnobserved in January, 324m3/sec, and maximum in August, 2,227m3/sec. The result also showedrnthe yearly average flow portrayed decreasing trend since 1999 and never went above the flowrnvalue of 1,200m3/sec. During a reservoir fill period, a total volume of 2,300 Mm3 water wasrnblocked from joining the downstream flow to fill the reservoir to its minimum operation level;rnwhile a total of 12,300 Mm3 volume of water was retained to fill the reservoir to its live capacity.rnOn average 919,283 m3 of water was estimated to be lost through evaporation during reservoirrnfill to its minimum operation level whereas the evaporative loss was computed to be 7,272,721rnm3 during the reservoir fill to its live capacity. Under both scenario wet season flow withoutrnreservoir was observed to be much higher than the wet season flow with reservoir; whereas dryrnseason flow without reservoir found significantly decreased compared to dry season flow withrnreservoir.rnKeywords: - SWAT, Gibe III, Great Gibe Near Abelti station, Scenarios, Meteorlogical Data,rnSpatial Data