Bankfull discharge (Q_(bf)) of alluvial rivers reflects the hydro-geomorphic coupling at channel-scale. It is closely related with the drainage basin factors. Based on this concept, we have studied the variation in the Q_(bf) of the Inner Mongolia reach of the upper Huanghe River, and elucidated the causes at channel and drainage basin levels. The results show that the dams'regulation changes the flow regime, making the downstream flows more even, greatly reducing the high-flow season discharges and therefore, Q_(bf) of the downstream channel decreased. The enhanced channel sedimentation (S_(dep)) after the Longyangxia Dam construction, especially the sedimentation within the main channel due to the decreased bankfull frequency is also an important factor. The sedimentation has some time-lag or cumulative effect on the variation in Q_(bf), namely, Q_(bf) correlates not only with the S_(dep) of the same year, but also with the moving averages of S_(dep) of the foregoing N-years, and the correlation coefficient (r) reaches the highest at N=4. A multiple regression equation has been established between Q_(bf) and annual discharge, yearly maximum discharge and the foregoing 4-year average S_(dep), indicating that Q_(bf) depends not only on the flow conditions at the same year, but also on the cumulative S_(dep) at a certain time-scale. The existence of a cause-effect chain has been identified, which is generalized as follows: the construction of the Longyangxia Dam the increased July-October C/Q (C: suspended sediment concentration; Q: water discharge) the enhanced S_(dep)the decreased Q_(bf). Thus, by means of increasing the July-October flow release from the Longyangxia Reservoir, the S_(dep) can be reduced and the Q_(bf) increased. Therefore, the increased flood-risk due to the reduced Q_(bf) may be mitigated. As well, it has been demonstrated that the decrease in Q_(bf) occurs as the outcome of the changing drainage basin factors. A multiple regression equation was established, which indicates that the increased climatic warm and dry trend, the water diversion ratio and the total capacity of the reservoirs and the decreased natural runoff coefficient are the causes for the decreased Q_(bf) in the upper Huanghe River.