Dams generate man-made lakes, which are used to store water for water Supply, Hydropower Irrigation, transportation, recreation, and other uses. The construction of a dam for water storage may drastically alter the river's hydrodynamics, having direct consequences for the habitat, river morphology, and ecology of the entire river system. Reservoir sedimentation may be complex to handle and it undermines the valuable life of the reservoir. The study area lies 42 kilometers southwest of Bahir Dar and 7 kilometers southwest of Merawi, in the Weast Gojam Zone of the Amhara National Regional State's Mecha Woreda. The study's main objective is to investigate erosion hot spot regions and sediment inflow in the Koga dam reservoir. The Garmin GGPSMAP421s was utilized to record the geographic position of the watercraft when recording each profundity estimation, and was utilized to decide the profundity from the water surface to reservoir to collect bathymetry information and RUSLE model used to identify erosion risk areas. The result appeared that diminish of the storage capacity from plan capacity of 83.1 million cubic meter in 2006, 82.7 in 2012 up to 78.91 million cubic meter in 2020. During the last 11 years operation the outcome revealed that the reservoir's storage capacity fell by 4190018.94 m 3 , close to 5.04 percent, of its overall volume. It showed that Koga average reservoir capacity loss, which is lower than the predicted global rate of reservoir capacity loss (1%), is 0.38. Although the reservoir's design life is 50 years, its usable life is only expected to last 23.5 years. The bathymetry and RUSLE model's annual average sediment yield of 20.24 and 16.52 tons per hectare per year respectively. The result of the model showed that the Rim main watershed is contributing substantial amount of soil loss and the most erosion hotspot portion of the Koga watershed. For reducing silt inflow into the reservoir upper watershed should undergo the same management and water conservation efforts as the lower watershed. Since RUSEL doesn’t consider gully erosion, future studies should be conducted using other technics that can incorporate gully erosion. Key words; Bathymetry, Koga Reservoir, Reservoir Sedimentation, RUSLE model