High Sensitivity and Fast Valley Landscape Response to Climate Change in the Lower Reaches of the Jinsha River, Southeastern Qinghai-Tibetan Plateau, Field Characteristics and Numerical Modeling

The Jinsha River, the upper reaches of the Yangtze River, winds and incises through the eastern margin of the Qinghai-Tibetan Plateau, where Asian monsoon climate prevails. Along the Jinsha River, extensive and thick Quaternary valley fillings and multiple river terraces are developed. However, their specific relations to climate change and tectonics remain elusive. In this study, we conducted field investigation, chronological studies and numerical modeling in its lower reaches. Our study indicates that the extensive valley fillings in the Wudongde-Menggu segment were formed in cold-arid climate with Optically Stimulated Luminescence (OSL) ages of ~23-19 ka B.P., coinciding with the timing of the Last Glacial Maximum (LGM). We suggest that the post-LGM warming and humidifying, by restrained sediment supply while enhanced discharge, switched the valley from prevailing aggradation into incision. Multiple subsequent fill-cut terraces, postdating the LGM, were formed and nested in the valley fillings. OSL data indicate that the first to third terrace sediments were formed at ~10 ka B.P., ~11.5-12.5 ka B.P. and ~13.5 ka B.P., respectively. Our study suggests that these terraces are a sensitive record of millennial and sub-millennial cold-arid climate perturbations superimposed over the post-LGM deglaciation. Our modelling highlights a high sensitivity of valley aggradation and incision to cold-arid and warm-wet climate change, as well as a fast valley landscape response in the Jinsha River characterized by high sediment transport influx. We propose that valley landscape and terraces serve as a timely record for climate change in certain rivers with low response time threshold characterized by high sediment transport influx.