黄河上游流域河道形态特征及裂点成因

Morphological Characteristics and Knickpoint Causes of Channels in the Upper Yellow River Basin,China

活动造山带的地形地貌是构造运动与气候变化相互作用的产物,伴随着河流动力侵蚀模型发展与地形数据集精度提升,对河流开展非均衡形态的相关分析可获取丰富的构造变形、侵蚀沉积及气候变化的信息。以30 m分辨率的数字高程模型为基础,提取了黄河上游流域(两湖源区至兰州河段)的河流网络,并采用积分法分段计算出精确的河道陡峭指数,据此识别出裂点的位置,实现了对裂点的量化表征。通过对河道陡峭指数及裂点分布特征的研究,发现其与地形分布具有较大的相关性。两湖源区至若尔盖盆地河段地形较为平缓,河流纵剖面整体为上凹型且鲜有裂点发育,因而地形状态相对稳定;而若尔盖盆地至兰州河段地形起伏度较大,河流纵剖面大多为上凸型,并且具有更大的陡峭指数与密集的裂点带,表明该区域隆升速率的差异性与侵蚀基准面的多期改变。另外,通过对研究区内裂点的成因分析,发现流域内裂点成因复杂且有较多巨型裂点,断裂活动、构造隆升、岩性差异、气候条件、地质灾害及冰川作用等均可导致河流纵剖面发生变化以及裂点产生,裂点性质、大小及河流纵剖面形态可有效反映裂点形成原因,这为进一步了解活动造山带的演化提供了思路与理论依据。

The topography of active orogenic zones is the result of the interaction between tectonic movements and climate change.As the most sensitive and active unit in the evolution of the landscape,rivers contain abundant information of tectonic deformation,erosion-deposition and climate change.With the development of stream power incision model(SPIM)and the improvement of the accuracy of topographic data sets,the extraction and analysis of non-equilibrium morphological information in river longitudinal profiles have become one of the vital tools in the study of the geomorphological evolution of active orogenic zones.Thus,a digital elevation model with 30 m resolution was used as the basis for extracting the channel network in the upper Yellow River basin(from Zhaling Lake and Eling Lake to Lanzhou),calculating the accurate channel steepness index using the integral method,identifying the location of knickpoints,and achieving the quantification of knickpoints.Through the analysis of channel steepness indices and the distribution of knickpoints,it is found that they are highly correlated with topographic features.For instance,the river longitudinal profiles from Zhaling Lake and Eling Lake to Ruo erge Basin are overall upward-concave curves with few knickpoints,so the terrain of this part has a relatively stable state,whereas the river longitudinal profiles from Ruo erge Basin to Lanzhou show upward-convex curves,so the terrain is the state of momentary change,and the greater steepness index and denser knickpoint zones indicate variability in the rate of uplift and multi-period changes in the erosional datum.In addition,it can be found that the causes of knickpoints within the basin are complex and substantial.Fracture activity,tectonic uplift,lithological differences,climatic condition,geological hazards and glaciation all lead to changes in the river longitudinal profiles and the creation of knickpoints,and the morphology of the knickpoints differs from one cause to another,which provides more ideas and theoretical basis for further understanding of the evolution of active orogenic zones.