Construction of dams has offered benefits to the modern world through provision of water for different purposes. However, the consequent termination of flooding and reduction in peak flow affect the existing ecosystem balance and pose a range of threats to the downstream community. Ribb dam, Upper Blue Nile Basin, Ethiopia is intended mainly for irrigation purpose completed in the late 2018. Meanwhile it started impounding Ribb River water, to imbalance and affect the downstream environment specifically Fogera floodplain. Thus the study was focused on evaluating the impacts of Ribb dam on Fogera floodplain inundation area considering two inflow hydrograph scenarios before and after dam operation. Daily rainfall and stream flow data (1995-2017) years representing scenario I was collected from National Meteorological Agency and Abay Basin Authority respectively. To transformation daily point to areal and the daily areal to hourly probable maximum precipitation (PMP) Theissen polygon and (Hershfield, 1965) Methods respectively were applied. Then probable maximum flood (PMF) of 431.6m3/s was computed using HEC-HMS model version 4.2.1 as the model performed respective values of NSE, R2 and RVE 0.0.59, 0.77 and -3.6 for calibration; and 0.56, 0.76 and 5.95 for validation. For scenario II (2019-2063) years operation inflow discharge was collected from Water Works Design & Supervision Enterprise and PMF of 266.3m3/s was fixed from the release. Manipulating xyz data parts surveyed primarily and the remaining from Water Works Design & Supervision Enterprise to (.2dm) format using QGIS plug-In, BASEmesh version 1.4.3 has been done. BASEMENT model version 2.8 was calibrated by manually changing Manning’s roughness coefficient and performed RMSE =0.1427 and the Nash Sutcliffe Efficiency of NSE =0.87. The model produced simulation results of inundation area, mean depth and velocity of 46.5km2, 0.411m and 0.2m/s for scenario I; and 28.6km2, 0.246m and 0.19m/s for scenario II respectively. The drop in PMF has brought 41% reduction in inundation area. Therefore, Fogera floodplain contributed by Ribb River will be severely affected as a result of Ribb dam operation. Henceforth, it is urged to undertake further risk analyses studies considering the flood inundation area reduction on Fogera floodplain resulted from Ribb dam construction.