The sudden release from the dam during monsoon season is now becoming a grave concern, especially in the downstream areas. The situation worsens if rainfall occurs at the same time in the lower part of the region as well. Thus, the flow contribution due to rainfall and lateral flow plays a significant role while issuing flood warnings in downstream flood-prone areas. This study presents a coupled reservoir operation and a hydrodynamic model to simulate the hydrograph in the flood-prone region considering downstream flow contribution including baseflows. Standard Operating Policy has been adopted for the reservoir operation model, and a 1D hydrodynamic model has been employed as the flood routing model. The model has been applied at Ranganadi Hydropower Project located in Arunachal Pradesh, India. The flood-prone area is situated around 46 km downstream from the dam site. A Flow Duration Curve (FDC) was prepared for the flood-prone site for the different values of downstream flow contribution. The statistical analysis of the observed dam releases imparts that the releases from the dam contributes around 10% of flow in a year. Thus, from the FDC curve, the flow values with exceedance probability of more than 10% were considered which include Q95, Q70, Q50 and Q30. A flood event of 50 hrs. inflow hydrograph was considered in the analysis. The peak increment factors were calculated by comparing Q95 (considered as the reference downstream flow contribution value) with the other mentioned flow values. The results elucidate that the peak increment factor increases from 0.019, 0.068, 0.13 corresponding to Q70,Q50,and Q30 respectively. This analysis illustrated that for precise flood levels monitoring, it is necessary to incorporate downstream flow contribution.