Comparability in geomorphic evolution of the three rivers in the northern Longitudinal Range-Gorge Region： Evidence of statistic relationships between geomorphic parameters and analyses of cause of formation

For discussing the evolution characteristics and correlation of the tributaries of the Nujiang, Lancang and Jinsha rivers in the northern longitudinal rang-gorge region of northwestern Yunnan, the relationships between some geomorphic parameters of the total, first- and second-order tributaries whose catchment areas are larger than 100 km(2) in the three rivers of the study area are analyzed and a series of correlation formulae are proposed. Results indicate that the corresponding order tributary systems of the three rivers have very high comparability especially for the first-order tributaries. The statistical relationships between catchment area and channel length of the corresponding order tributary systems indicate that the Nujiang and Lancang rivers have more commonalities than the Jinsha River, being very evident in the relationships between the total- and first-order tributaries. The relationships between catchment area and channel gradient also indicate that there is more high comparability between the Nujiang and Lancang rivers, being very evident in the relationships of the total-order tributaries. The causes leading to the high comparability between the three rivers are as follows. The geological tectonic comparability in the longitudinal rang-gorge region causes the comparability of landform and the comparability of land surface condition, thus causing the comparability of the geomorphic evolution in the study area. The Nujiang and Lancang rivers in the study area are located at the margin of Eurasia Plate. They are strongly controlled by the collision zone of the India and Eurasia plates. The Jinsha River in the study area is relatively far from the collision zone. It is controlled by the collision zone in relative low degree and its geomorphic evolution is relatively free. Furthermore, relative infirm runoff because of the relative arid climate in the Jinsha River reach led to the relatively low ability in alteration of the landform systems of the river. So, except some evident commonness, there are a few differences between the Jinsha and Nujiang or Lancang rivers.