Since the rivers are the main basic and accessible resource of water for miscellaneous uses, the erosion and Sedimentation condition of rivers are of a great deal of importance. Koga River has a considerable interest because of strategic and environmental conditions regarding its water projects planning and agriculture. The morphological changes due to erosion processes, sedimentation, and Sediment transport affects the hydraulic structures like Intake port and irrigation systems. Even though bathymetric survey was conducted two times so as to check the amount of trapped sediment by the reservoir, prediction of the future sediment distribution pattern using Bathymetric survey is impossible. Thus, the mathematical model HECRAS has been applied to simulate the reservoir bed of Koga Dam. The simulation periods of this research is 9 years from 2009-2017.Intial boundary conditions, flow data, sediment data and geometric data was identified and analyzed to run HEC-RAS model, where as the literature was reviewed to obtain transport functions, sorting method and fall velocity. The model was calibrated and validated using the measured bathymetry data of the year 2012 and 2017 respectively. Model results have R2-values of 0.71 to 0.68 for the years for 2012 and 2017 respectively in river bed profile. The results showed that prediction of bed elevation was in a good agreement with the field measurements and imply that the HEC-RAS 1D model can simulate to an extent satisfactory with quasi unsteady flow. Once the model was validated, prediction of future bathymetric changes was also made through model simulation. For this prediction it was assumed that the existing flow data could be recycled. Therefore, Study of two or more cases with the present model by comparing results and trying to find out under which circumstances the model performs well is interesting to identify some possible scenarios for future studies.