Surma is a principal distributary of the Barak River, which is the main source of fresh water in South-Eastern region like Sylhet, Sunamganj of Bangladesh Bengal. Every year huge amount of sediment loads are coming from upstream which are settling down on Surma river bed, hindering the safe passage of flow which contributes to the change in hydrodynamic and morphological characteristics of the river. Recently a dredging has been done in Sunamganj area to increase the navigability of the river in that area. In this study work, the hydrodynamic study of the river Surma has been investigated to check the impact of dredging with the help of HEC-RAS. For these flood frequency analysis has been done for ( 5, 10, 25, 50, 100) years return period and comparison of water level, velocity and discharge has been done before and after dredging. Before dredging the water level, flow path was very high, velocity, discharge was low but after dredging water level, flow path. Discharge also decreases after dredging in the 6 stations which are located in the dredged area, the values are 1375.41 m 3/s, 1374.31 m 3/s, 1373.73 m 3/s, 1369.7 m 3/s, 1373.11 m 3/s, 1372.08 m 3/s and before dredging the values are 1669.62 m 3/s, 1665.53 m 3/s, 1664.69 m 3/s, 1658.12 m 3/s, 1624.23 m 3/s, 1383.98 m 3/s. Increment of velocity has also been observed.