Eastward drainage-divide migrations driven by the spatial variations in precipitation and tectonic uplift contribute to the formation of the Parallel Rivers in the Hengduan Mountains, Southeastern Tibet

In the Hengduan Mountains (Southeastern Tibet), the south-draining Nmai Hka (Upper Irrawaddy), Nu (Upper Salween), Lancang (Upper Mekong), and Jinsha (Upper Yangtze) Rivers flow parallel and in proximity to each other, thus forming rather a unique geomorphic configuration (i.e., the Parallel Rivers). It has been suggested that this configuration is an outcome of tectonic shortening that “squeezed” the rivers together. Although the tectonic and surface processes in this region have been extensively studied, the processes that led to this configuration remain controversial. Here, we investigate the potential role of drainage-divide migration in forming the Parallel Rivers. We demonstrate, based on geomorphologic analysis of divide stability between the parallel rivers, that while the two western divides (Nmai-Nu, Nu-Lancang) are likely migrating eastward, the eastern divide (Lancang-Jinsha) is roughly stable. This pattern suggests that the formation of the Parallel Rivers is associated with an abutment of the migrating divides against the fixed one. GPS and precipitation data combined with prior research suggest that the migrations of the two western divides are driven by the spatial variations in precipitation and rock uplift. Concurrently the stability of the eastern divide is influenced by a combination of climatic and tectonic factors, as well as a historical river capture event. This implies that the formation of the Parallel Rivers has not necessarily been an outcome of east-west tectonic shortening but has been influenced by the spatial variations in precipitation and tectonic uplift.