中文摘要：

冲积河流平滩流量既反映了河道尺度上的水文-地貌耦合关系,又与流域因素密切相关。从这一概念出发,以黄河上游内蒙古河段为例,研究了平滩流量的变化,并在流域层面与河道层面上对其成因进行了研究。结果表明：上游水库调节改变了出库流量过程,使汛期流量大幅度减小,泄流过程均匀化,因而平滩流量减小。在龙羊峡水库修建后洪水流量减小导致漫滩机遇降低的情形下,泥沙淤在主槽内,也是平滩流量减小的重要原因。泥沙冲淤对于平滩流量的影响存在着某种累积或滞后效应,巴彦高勒平滩流量Qbf,BY不仅与当年的冲淤量有关,还与以前冲淤量有关,前4 a平均淤积量对平滩流量的影响最大。建立了平滩流量与前4 a累计淤积量、年均流量、年最大日流量的多元回归方程,该方程表明平滩流量既与当年的流量特征相联系,也与一定时间尺度上河道冲淤造成的后果相联系,反映了某种水文地貌耦合关系。研究还表明,存在着以下因果关系链：龙羊峡水库修建→汛期来沙系数增大→河道淤积加强→平滩流量减小。因此,通过改变龙羊峡水库的运用方式,增大汛期下泄流量,可以减小河道淤积、增大平滩流量,从而增大下泄洪水的能力,降低凌汛期间的防洪压力。平滩流量的变化是流域因素变化的结果,所建立的多元回归方程表明,在50 a的时间尺度上,暖干化指标的增大、引水率的增大、水库总库容的增大和天然径流系数的减小,是黄河上游平滩流量减小的原因。

英文摘要：

Bankfull discharge（Qbf） 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 Qbf 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, Qbfof the downstream channel decreased. The enhanced channel sedimentation（Sdep） 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 Qbf, namely, Qbfcorrelates not only with the Sdepof the same year, but also with the moving averages of Sdepof the foregoing N-years, and the correlation coefficient（r） reaches the highest at N=4. A multiple regression equation has been established between Qbfand annual discharge, yearly maximum discharge and the foregoing 4-year average Sdep, indicating that Qbf depends not only on the flow conditions at the same year, but also on the cumulative Sdepat 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 Sdep→the decreased Qbf. Thus, by means of increasing the July-October flow release from the Longyangxia Reservoir, the Sdepcan be reduced and the Qbfincreased. Therefore, the increased flood-risk due to the reduced Qbfmay be mitigated. As well, it has been demonstrated that the decrease in Qbf 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 wa