关键词: 黄河河床纵剖面, 加积作用, 沿程淤积, 溯源淤积, 准平原, 老年期河流

Abstract:

A number of research approaches, such as digital analysis, relationship between river mouth extension and channel evolution, sedimentary analysis,channel topographic map comparison, and hydrologic regime research, have been presented in studying the adjustment processes of the river bed longitudinal profile morphology of the lower Yellow River.Theoretically,it is basically concerned about the evaluation if the role of downward and headward silt-clearing.Comparing the river bed longitudinal profile of different years in terms of the data of water level corresponding to the discharge of 3000 m 3 /s, Zhang Ren(1985) concluded that the longitudinal profile of the lower Yellow River had been elevated during the 1930s to the 1970s.By means of digital analysis, Jia Shao-feng et al.(1992) have theoretically demonstrated that the retrogressive aggradation due to continual base-level rise was the basic reason for the uniformly elevated channel bed of the lower reaches of the river. On the basis of the change of flood-plain height and the associated river mouth extension, Wang Kai-chen(1982) has also indicated that the river bed longitudinal profile at downstreams was uniformly elevated. Pang Jia-zheng et al.(1982) and Xian Jian-heng et al.(1980) have shown that the main controls on the lower Yellow River's longitudinal profile were channel extension and course changes, and that the effect of the headward degradation and aggradation declined gradually from dowmstreams to upstreams and finally was over in Luokou.By the statistical analysis of 5-year water level change with a discharge of 3000 m 3 /s, Yin Xua-liang(1991) has shown that annual water level change was different and degradation and aggradation should have been caused by downward factors.Wei He-long et al.(1996) have indicated that the downward degradation and aggradation of the lower Yellow River was principally related to the variations of discharge and the silt content and that the headward degradation and aggradation was resulted mainly from base-level change at downstreams which was basically due to change of river length.To sum up, although for the lower Yellow River a number of research methods were concerned with the self-adjustment of the river bed longitudinal profile morphology, the morphological indexes, gradient and concavity had been paid little attention to. This paper calculated the values of gradient and concavity indexes for the long-term and long-distance river bed longitudinal profile of the lower Yellow River using the least square method and the parabolic partial differential equation. Minor variations in both the gradient index and the concavity index indicate that the entire lower Yellow River has almost uniformly elevated its river bed over a long period of time in the self-adjustment of the river bed longitudinal profile morphology, and that the lower Yellow River has become an aggradational peneplane reach. The nearly uniform elevation of the lower Yellow River may have resulted from downward sedimentation and headward sedimentation caused by relative base-level rise in the river mouth. The aggradational process had introduced underground streams to run on the surface, in such a case, the entire lower Yellow River became an old-stage stream.The old-stage stream implies that the channel development may have achieved a critical stage which heavy aggradation and channel decline may have occured. On the other hand, the capability against historical floods may have greatly decreased, and disasters may have frequently happened.

Key words: river bed longitudinal profile, aggradation, downward sedimentation, peneplane, headward sedimentation, old-stage stream