In order to investigate the riverbed formation mechanism of alluvial river and the influence of flow and sediment process to alluvial river, it is very necessary to analyze the relationship between the adjustment of channel morphology and the flow and sediment process. The braided reach of Lower Yellow River is chosen as studying objects in this paper, and studying approaches of observed data analysis and numerical modeling are used. The characteristics of sediment scouring and silting under various runoff-sediment conditions are explored. And the relevance between fluvial facies coefficient and characteristic parameter of water and sediment transport is founded. This reflects the regulation mechanism of river system. It is shown that different flow and sediment processes have significantly different effects on adjustment of channel morphology, and the scouring intensity of flood closely related with cross-sectional geometry has stage characteristic. The channel shaped by the larger peak discharge is hard to keep its original sediment-transporting intensity during non-peak flood periods. The cumulative channel-scouring effect has a good response relationship to the flow and sediment process. And only the high-efficient sediment-transporting flood process could shape the most appropriate cross-sectional geometry so as to achieve both the optimum sediment-transporting efficiency and the maximum channel-scouring efficiency. For the different magnitudes of overbank floods, only the flood process (Q _max = 6000 m³/s) could shape the channel morphology with the best cumulative channel-scouring effect, and the corresponding synthetical factor Φ((B/H)(S/Q)^0.5) is always maximum. Therefore, the discharge corresponding to Φ _max could be considered as a regulation index for overbank floods in the Lower Yellow River.