关键词: 河流陡峭系数, 裂点, 河流纵剖面, 石羊河流域, 祁连山东段

Abstract: The river system is very sensitive to landscape fluctuations and the pattern of drainage contains the past and present tectonic information and can record the latest even tiny change along the orogenic belt system. Therefore, fluvial geomorphology is always used to describe the shapes of river channels and recognize the different segments of active faults. Qualitative and quantitative geomorphic analyses can provide useful information on detecting active tectonic features and the influence of landscape change and evolution. Quantitative analysis such as analysis of river longitudinal profile and geomorphic indices can help researchers evaluate the relative level of tectonic activity and characterize the geomorphic features of landscape quantitatively.
Our study focuses on the geomorphic analysis of Shiyanghe River Basin which is located in the eastern part of Qilianshan Mountains. The tectonic deformation is very strong since late Cenozoic, and Quaternary active thrust faults, strike-slip faults and active folds are distributed all over the region, indicating that the whole region is suffering from crustal shortening and sinistral shear. In this region, the latest tectonic deformation and tectonic activities have been recorded by its fluvial system.
Based on GIS spatial analysis technology, the longitudinal profiles of seven tributaries(including Gulanghe River, Huangyanghe River, Jintahe River, Zamuhe River, Xiyinghe River, Dongdahe River and Xidahe River)in the Shiyanghe River Basin are extracted by using digital elevation models(DEM)and Matlab script. In channel longitudinal profiles, most tributaries in Shiyanghe River Basin exhibit an increased channel gradient in their midstream and downstream area. This pattern is consistent with the models of transient channel profile which suggests an increase in rock uplift rate or base level fall. The longitudinal profiles of seven tributaries are analyzed synthetically by using the method of bedrock channel erosion model, and the concavity(θ), steepness index(k_sn), as well as the knickpoints information(including distribution, elevation, distance from mouth and drainage area)of seven tributaries are obtained. The result shows that each of the tributaries in the Shiyanghe River Basin at least has one major knickpoint. The comprehensive study of the longitudinal profiles, knickpoints and the lithology of the river basin show that the Gulanghe River, Jintahe River, Zamuhe River, Xiyinghe River, Dongdahe River and Xidahe River all have ‘slope-break’ knickpoints, which suggest that they are in a transient state. The knickpoints represent a transient response to the dynamic surface uplift since late Cenozoic. Therefore, we can conclude that the evolution of fluvial geomorphology in eastern Qilian Mountains is mainly related to tectonic activities. Channel segments upstream of knickpoints exhibit lower concavities(mean θ is 0.458±0.053)and higher channel steepness indices(mean k_sn is 129.09±1.82). In contrast, lower channel segments are more complanate(mean k_sn is 68.162±0.821)and exhibit a higher concavity(mean θ is 0.831±0.147). The distribution of concavity is related to the erosion rate, thence, we can infer that the higher value of concavity in downstream area indicates the higher erosion rate. Because the different steepness index(k_s)and the concavity(θ)below and above the reach of knickpoints indicate that they have different development trends in different channel segments, and the distribution of knickpoints represent the evolution process of the longitudinal section of the tributaries. Using the concavity value of the knickpoint, each lower reach longitudinal profile of tributary is fitted. According to the fitted result, the calculated approximate average erosion volume of the Shiyanghe River Basin is 488m since it formed, and the average erosion volume of the six tributaries, which originated in Gulang nappe, is 508.5m. The total amount of erosion is positively correlated with rock uplift when a river is in transient state. Thence, it concludes that the Gulang nappe has experienced a strong uplift. Furthermore, we obtained the spatial distribution of k_sn values of the whole fluvial system in the Shiyanghe River Basin from calculating and interpolating the k_sn values, and combined the geomorphic parameters results to analyze the tectonic significance of the Shiyanghe River Basin synthetically. The spatial distribution of k_sn values of the Shiyanghe River Basin represent the accommodation of geomorphic landscape to the tectonic force and the manner of channels responding to tectonic forces. In this study, most of the channel gradients obtained from midstream are higher than upstream and downstream and k_sn values in downstream reaches is less than 60m^0.9, the high k_sn values are in the Gulang nappe, reaching over 1 400m^0.9, which indicates that the Gulang nappe has experienced uplift since the Quaternary. Therefore, we conclude that the regional difference of the k_sn is mainly controlled by the uplift rate of bedrocks.
Based on the comprehensive analysis of geomorphic parameters and tectonic background, we conclude that the geomorphic evolution of the Shiyanghe River Basin is in a non-equilibrium state, and the tectonic deformation is the main factor affecting the geomorphic evolution of the eastern Qilianshan Mountains and controlling the present tectonic pattern, geomorphic development and evolution history of the study area.
According to the river longitudinal profiles and modeling analysis, this study indicates that the quantitative geomorphic analyses can provide useful and effective information on detecting active tectonic features and the influence of landscape change and evolution, and the geomorphic indices are useful and appropriate tools to analyze the coupling of tectonic and geomorphy.

Key words: channel steepness index, knickpoint, longitudinal profile, the Shiyanghe River Basin, eastern Qilianshan Mountains