The Vam Nao River is located Vietnamese Mekong Delta’s upstream, plays the role of transferring flow from the Tien River to the Hau River and balances the discharge for two of these main branches. Hence, the hydrodynamic regime is very complicated. In 2017, there was a severe riverbank failure, resulting in the loss of many houses and negative impacts on the local people. So, in this research, the TELEMAC model was applied to simulate flow and bed change, combined with field surveys, thereby analyzing the hydrodynamic regime of this river. The model was calibrated using the water level, discharge and suspended load data. The simulation results show that the flow has only one direction from the Tien River to Hau. The maximum velocity in the flood season is 1.66 m/s, the discharge is 8298 m3/s (in 2017), the flow after the confluence appears the vortex. Moreover, the confluence area has a rather high erosion rate, up to 1.2 m/year. According to data collected nearly 30 years, the Vam Nao River’s topographic data signify the riverbed is growing to expand and deepen, over 43 m after the confluence and distributed near the riverbank. With this development trend, it is necessary to continuously monitor to provide early warnings to minimize damages due to the risks of riverbank failure.