Spatial Distribution Of The Knickpoint In The Upper Reaches Of Minjiang River

As a proof of the unbalanced river,knickpoint is often used to indicate changes in tectonical,climatic,and lithological factors in the longitudinal profile of the river.The upriver basin of the Minjiang River is located on the eastern edge of the Qinghai-Tibet Plateau.Since the Quaternary,there have been strong tectonic activities.And glaciers developed in the upper reaches.All this things provides good conditions for the formation of knickpoint.In addition,most of the rivers in this area are bedrock channels,which is convenient for the extraction and analysis of the characteristics of knickpoint using the stream-power incision model.Based on this,the analysis of the causes of knickpoints in the upper reaches of the Minjiang River can not only find the specific control factors of the knickpoints,but also help to understand the specific interactions between tectonic,climate and landforms.In this paper,based on ALOS DEM data,we utilize ArcGIS to analyze the geomorphology of upriver basin of the Minjiang River.Several geomorphic parameters including terrain slope and local relief are extracted,thus the regional geomorphic features can be obtained.Based on the analysis,we select the tributaries below level 5 in the area.Using a Matlab script program to extract the longitudinal section of river,the spatial distribution of knickpoint and a distribution of ksn are obtained.Finally,by analyzing distributed characteristics of knickpoint on the longitudinal profile of the river,and integrating lithology,climate and tectonics data,we investigate their effects on the formation and development of knickpoints.The conclusions are summarized as followings:(1)Generally,the tributaries located in upriver basin of the Minjiang River appear unbalanced states with knickpoints.According to the morphological characteristics of longitudinal profile of the river,the knickpoint can be classified as two types.One is accompanied by a knickzone below,which is characterized as a step-pool system.The other without knickzone,which divide the longitudinal section as smooth upriver and steep downstream.At an altitude of 3,400 meters,the knickpoints show different distribution characteristics in the flat.Above 3,400 meters,the knickpoints are usually located in the area with low terrain relief;while for the knickpoints below 3,400 meters,most of them lie in areas with large terrain relief and steep watercourses.(2)The generation of knickpoints in upriver basin of the Minjiang River is under the combined influences of lithology,climate and tectonics.The knickpoints produced by the differences in lithology mainly are mainly located in the boundaries between granite and the other stratums.However,not all rivers flowing through the granite bring about knickpoints on the boundaries,which indicate that for different zones,the dominant generation of knickpoints transform from lithology to the other factors.The knickpoints generated by climate changes are mainly influenced by glaciers.Some high-altitude knickpoints in upriver basin of the Minjiang River reflect the boundaries between the glacial landform and the river lanform.And the other high-altitude knickpoints characterized by continuous,slight scarp may indicate the landforms of glacial erosion and deposition in glacial valleys.Knickpoints that are layered may be affected by regional uplift or climate.The knickpoints formed in the same period are at similar altitudes.The stepped knickpoints appearing between steep slopes and gentle slopes are mainly caused by seismic landslides,which is typical step-pool systems.