黄河河口延伸与下游淤积关系研究中的问题分析

阐明了黄河河口延伸与下游淤积关系问题的实质,指出了黄河河口延伸与下游淤积关系对时间尺度的依赖性,进而深入探讨了以前研究这一问题所用论据的合理性题。分析结果揭示,那种认为黄河下游淤积由河口延伸控制和河口延伸作用局限于近口河段的说法都缺乏令人信服的证据。

Relationship Between Mouth Extension and Aggradation of the Lower Yellow River

With a high sediment load, the lower Yellow River has been aggrading rapidly for a long time.The causes of the aggradation became a hot topic among Chinese scholars in recent decades.Opposite opinions on the issue were raised.Some attributed the aggradation mainly to base level control, and some others associated it mainly with watershed control.Changes of base level control in the lower Yellow River are caused by the extension of the river mouth.This study reveals that different opinions on the issue were advanced because the issue was investigated on different time scales and some impertinent methods and evidence were employed by some previous studies.A certain time scale is necessary for studying this issue because there exist complex and constant changes in upriver inputs, mouth extension, and other factors influencing the sedimentation in the lower Yellow River with time as well as the hysteresis of channel adjustment of the river to these changes.A period of tens to hundreds of years seems to be a proper and practical time scale on which to discuss the issue.In order to back up the argument that the aggradation associated with base level control dominates in the lower Yellow River, some previous studies try to prove that the slope of the lower Yellow River is large enough for transporting the sediment inputs if the river mouth does not extend or that the aggradation in the river takes the form of headward aggradation.However, the author finds that all the evidence provided by those studies for this argument is untenable.The change in water level or deposits thickness along the river in a period is among the evidence.In piece of telling and concrete evidence, it has a good performance in determining the effects of base level control on aggradation on many other rivers, but the existence of great yearly variations of upriver inputs and concomitance of aggradation due to upstream and downstream controls in the lower Yellow River make it unsuitable in this case.As a fact, this evidence was also used by some other studies to claim the dominance of aggradation due to upstream control.Moreover, through flume experiments or calculation based on hydraulic models some studies disclose that the slope of the river is much lower than the graded slope under the current upstream inputs even if the river mouth does not extend.The methods and evidence used by studies to support the argument that the effect of base level control on aggradation has been limited in a reach close to the river mouth are also examined and disproved.Thus, both the arguments that the aggradation in the lower Yellow River has been dominated by the base level control and that the effect of mouth extension has been limited in a reach close to the river mouth for all time scales are not acceptable.