Ongoing westward migration of drainage divides in eastern Tibet, quantified from topographic analysis

Landscape evolution is controlled by tectonic strain, bedrock lithology, and climatic conditions, and is expressed in the spatial and temporal variations of river channel networks. In response to tectonic and climatic disturbance, the channels and drainage divides of river networks shift both laterally and vertically to achieve a steady state. Several metrics are available to assess the nature of river network disequilibrium, from which the direction of drainage divide migration can be interpreted. However, to link this information to other observational, theoretical, and experimental data requires knowledge of the rate of migration, which is still lacking. Here, we develop a modified method based on Gilbert metrics to calculate the transient direction and rate of drainage divide migration from topography. By choosing a high base level, we obtained linear or quasi-linear χ-plots for rivers on both sides of the drainage divide, and the elevation-χ gradient is proportional to the average normalized steepness index (ksn). In turn, the velocity of divide migration can be quantified theoretically from the cross-divide comparison of χ. We applied this method to eastern Tibet and obtained a uniform, westward migration pattern for 29 points along two drainage divides with rates of between 0.02 and 0.66 mm/yr, which is consistent with capture events affecting the major rivers of the region. The directions and rates of drainage divide migration as informed by the topographic analysis method developed in this study provides a quantitative insight into the ongoing river network re-organization in eastern Tibet. This insight further illustrates the importance of plateau growth and its eastwards expansion, driven by interplay between tectonics and climate.