Abstract: By analyzing the water and sediment conditions and bed degradation and aggradation of the Lower Yellow River from 1950 to 2021,we studied the complex relationship of bankfull discharge and water and sediment discharge,riverbed boundary and base level conditions in different periods.It is found that the bankfull discharge has a good positive and negative relationship with the 4-year-moving-average water discharge and sediment incoming coefficient in the flood season,respectively.They are the main factors of the bankfull discharge.Bankfull discharge is negatively related with the accumulative sedimentation volume per kilometer,and positively relevant to the median diameter of river bed,respectively,which reflects the internal relationship between the bankfull discharge and channel boundary.Bankfull discharge has a good negative relationship with the water level of the same discharge at Lijin and the river length at the estuary,which reflects the effect of the estuary condition.Since the operation of the Xiaolangdi Reservoir,the Lower Yellow River has been obviously coarsened and the scouring efficiency reduced,resulting in a slow increasing rate of bankfull discharge.Overall,the bankfull discharge is well related with different factors in the same period,but it adjusts towards different equilibrium states and even changes in opposite trends in different periods.Sometimes the bankfull discharge “abnormally” increases with the lengthening of the tail river due to the coupling effects of retrogressive and aggressive erosion and deposition processes and the delayed response of river evolution.This shows that the bankfull discharge is not determined by a single factor,but is comprehensively affected the long-term change trends and fluctuation of multiple factors.