Drainage basin reorganization in the Qilian Shan, NE Tibet

Drainage divides, the backbone of terrestrial landscapes, are dynamic features in geomorphic systems, migrating laterally in response to differential erosion rates and slopes across drainage divides. Divide migration is of wide interest because it changes the topology of river networks, and therefore holds great significance for sediment routing, landscape evolution and biotic evolution. In this study, we report new data combined with previously published ¹⁰Be-derived basin-wide erosion rates in the same region. Our erosion rate database covers a large geographic area in the Qilian Shan, pertaining to drainage basins that define three main drainage divides of this region: the divides between the Hei River valley and the Hexi Corridor, the Hei River valley and the Datong River valley, and the Datong River valley and the basins draining to Qinghai Lake. We mapped large differences in erosion rate across the three main drainage divides. The large contrast in erosion rates across the drainage divides (𝚫E) indicates the divides are migrating towards the plateau interior. This is consistent with our measurement of cross-divide differences in channel-head 𝝌 values (𝚫𝝌), which imply that the divides should be migrating in the same direction as that implied by the 𝚫E data in 15 of the 16 cross-divide pairs. Our measurements also reveal a positive correlation between the magnitudes of 𝚫E and 𝚫𝝌. This is consistent with observations of 𝚫E and 𝚫𝝌 in the Appalachians and the Ozarks, whose 𝚫E vs. 𝚫𝝌 regressions have similar slopes as those in our Qilian Shan data, but intercepts that are lower by 1-2 orders of magnitude. We show that these patterns are consistent with the transient behavior of 𝚫E and 𝚫𝝌 in 1D landscape evolution simulations that start out of equilibrium and approach steady state. These observations provide empirical support for the hypothesis that larger cross-divide differences in 𝚫𝝌 reflect faster divide migration. Our erosion rate measurements provide the first direct measurements for longstanding hypotheses about drainage reorganization in the Qilian Shan, and add a new dimension to our understanding of the evolution of topography and sediment sources in the Qilian Shan.