Understanding the streamflow hydrodynamics is important for forecasting flooding phenomena in mountainous rivers. Accordingly, based on long‐term runoff observations obtained using diverse runoff techniques (i.e., rating curves and fluvial acoustic tomography (FAT)), the authors (1) evaluated and explored flood phenomena, (2) discussed the effects of dams on flood hydrograph, (3) and revealed backwater effects caused by river channel bends, sand bars, and floodplains. The duration and slope of the falling limb of the hydrograph were directly influenced by the catchment characteristics that affected runoff recession, whereas the behaviour of the rising limb of the hydrograph was directly influenced by rainfall. A weaker correlation between discharge and rainfall in higher flood events indicates that rainfall events were strongly altered by water released from dams in the basin. Additionally, because of the backwater effects, the rate at which the flow velocity and boundary shear stress changed with the increasing discharge was reduced, potentially slowing the pace of erosion. Our findings are relevant for managing mountain rivers, particularly in river bend reaches prone to backwater effects. The FAT method could help increase the ability to monitor short‐term fluctuations in floods and associated hazards. Fluvial acoustic tomography system was used to determine key streamflow characteristics (i.e., rainfall, backwater effects) based on a high temporal resolution of 10 min and long‐term observations of streamflow in a mountainous river.