Abstract: To thoroughly investigate and comprehend the multifaceted response of river morphology and hydrodynamic characteristics to extreme floods in a branching river section, a comprehensive study was conducted on the Qingyuan Lunzhou section, which represents the lower reaches of the Beijiang River. This study employed a combination of field research, riverbed evolution analysis, and numerical simulation methods to systematically examine and elucidate various aspects, including the water flow characteristics, intricate changes in riverbed deformation patterns, sediment particle size distribution dynamics, and overall water level fluctuations during the once-in-a-century flood in the June 2022 flood event. The results indicated that during the “22·6” flood, the main stream of water shifted to the left due to the inertial effect, causing significant damage to the infrastructure in the Lunzhou Island. Moreover, after the water discharged from the canyon, a decrease in flow velocity and sediment carrying capacity led to severe sedimentation in the island head area and the right branch of the river, while the left branch experienced scouring. Additionally, differences in flow velocity between the island head area and the left and right branches resulted in variations in sediment particle size distribution. The sediment particle size in the island head area was notably larger compared to that in the left and right branches of the riverbed. Following the “22·6” flood, the elevation of the riverbed led to a significant increase in water level compared to before the flood. Moreover, the diversion ratio of the left branch increased. Specifically, under an inflow of 5 000 m~3/s, the upstream water level of the island head rose by almost 1m, and the diversion ratio of the left branch augmented by approximately 16% compared to pre-flood conditions.