Characteristics Of River Longitudinal Profiles And The Drainage Evolution Along The Zoulangnan Shan

The interactions among tectonics,climates and landforms are greatly focused by recent researches.In tectonic active orogenic belts,river landforms are usually recorded abundant information about tectonic activities and climate changes,and thus draw great efforts to study the characteristics of river longitudinal profile and their relationships with external factors.In the last30 years,studies on the longitudinal profiles induced the development from qualitative state to semi-quantitative and quantitative state.However,due to the complexities and diversities of factors?such as tectonic activity,climate change,lithology,glaciation and river evolution,etc.?affecting the characteristics of river longitudinal profile,how to effectively distinguish the impact by these factors on river longitudinal profile forms is still an unsolved problem,and is also the base for quantitatively understand the river longitudinal profile.The Zoulangnan Shan,at the central portion of the Qilian Shan mountain,has been suffering intense tectonic activities,glaciations and significant adjustment of river drainage system.Meanwhile,the river channels are mostly bed-rock or mixed bedrock-alluvial channels.These properties along the Zoulangnan Shan supply a wonderful object for us to study the characteristics of river longitudinal profile and the influencing factors.This study will enrich the quantitative study of the river longitudinal profile,and also will provide a new method for the study on the river evolution in the upstream of the Heihe River.In this study we used SRTM DEM as data source to analysis the geomorphology in Zoulangnan Shan at a large scale by ArcGIS,we extracted geomorphological parameters,including slope,local relief,swath profile and HI values.On the basis of geomorphological analysis,we selected several rivers originating from the Zoulangnan Shan to extract the longitudinal profile by the Matlab script,we obtained a series of parameters used to describe the morphological characteristics of longitudinal profiles,such as k_sn,concavity index,and the position of kinckpoint.Finally,we combined regional structure,lithology,and climate data to discuss the results of geomorphological and longitudinal profiles analysis,then the effects of tectonic,lithology,and climatic factors upon the longitudinal profile were discussed separately.Combined with the research status of glaciers in the region and the field work,we further explored the role of glacier evolution and water system reorganization upon the longitudinal profile.The results show that:?1?Widespread knickpoints at high altitude?>3600m?on the disequilibrium longitudinal profiles along the Zoulangnan Shan,divide the longitudinal profiles into two sections,above the knickpoint the longitudinal profiles are gentle and below the knickpoints are steeper.The steepness index of the upper reaches is relatively low with slight changes.The analysis on the knickpoints indicates that the formation of the knickpoint at high altitudes is not controlled by lithology,climate and/or tectonics,and the knickpoint at lower altitude?<3600m?may reflect the time or spatial differences in the tectonic uplift rate,but these the high altitude knickpoint may be formed by factors other than tectonic activities.Much evidence indicates that the high altitude knickpoint of the Zoulangnan Shan mainly reflects the boundary between residual glacier valleys and fluvial channels.The result suggests that the inheritance landform by ancient glaciation should be paid more attention while analyzing the knickpoint locating at high altitude.?2?The longitudinal profile of the main stream of the Heihe River is far from equilibrium,which shows an upward convex shape with multiple knickpoints.The results of geomorphological analysis show that the geomorphological features of the different reaches of the Heihe River are quite different.The valley of east-west flowing river is broad with low slope and low local relief.In the midstream of the river,the flow direction changes greatly and the valleys become narrower with high slope and local relief.Based on the analysis of the geomorphology of the Heihe River Basin and the results of the longitudinal section of the river,we proposed that the paleo-Heihe River once flowed along the east-west flow path?the channel of current Ganzhou River and the Babao River?.Afterwards,the river in front of the Zoulangnan Shan captured the originally east-west Heihe River,which led the Heihe River's flow into the Hexi Corridor northwardly.At present,the upper reaches of Heihe River may be in the adjustment phase of the river capture event.?3?There are obvious differences in the morphology of the longitudinal profiles of the rivers on the southern and northern sides of the Zoulangnan Shan.The profiles of the south-side rivers are relatively slow while the profiles of north-side rivers are relatively steep.After excluding the effect of regional lithology and tectonic upon the longitudinal profile,the obvious differences of river longitudinal profiles on both sides of the Zoulangnan Shan are mainly affected by the different base level.The south-side rivers are controlled by the higher base level,the relative decline of the base level is not obvious and incision rates are small,thus the longitudinal profiles are relatively gentle.On the other hand,the north-side rivers have the Hexi Corridor as the base level,and the base level due to continuous uplift of the Zoulangnan Shan relative continues to decline,the rivers cut rapidly thus the longitudinal profiles are steeper.