Differential terrace configurations in the Upper Yangtze River: Evaluating distinct intensifies of external perturbation and their impact on river behaviors

Abstract

The interplay of tectonics, climate and topography shapes the river dynamics that give rise to terrace formation in mountainous regions. Unraveling the distinct influences of these factors on fluvial processes is a complex challenge due to their intricate interactions. By combining field observations with optically stimulated luminescence (OSL) dating, we have discerned and examined the sedimentary and geomorphological attributes of terraces, as well as the distinctive event sediments linked to recurrent mass movement across four varied reaches of the Upper Yangtze River. Each reach exhibits distinct tectonic settings, such as subsidence depressions, uplift gorges, or relict planation topography, which have imparted varied responses of river processes since the penultimate glacial period. Evidence from all study reaches indicates the presence of two cycles of fluvial aggradation-incision, yet the differentiated tectonic activities, precipitation patterns, and topographic reliefs have elicited divergent river behaviors across different reaches. In the subsidence depression, a single set of terraces has been formed, whereas in the most upstream region, characterized by relict planation topography, two superimposed sets of terraces have developed. In contrast, two separate sets of terraces have emerged in the uplifted gorge since the post-penultimate glacial period. The striking differences in terrace configurations across the valley reaches suggest that the magnitude of environment perturbations has exceeded critical thresholds of the different equilibrium conditions. One transition in equilibrium conditions relates to the minimum riverbed sediment thickness required to supply abrasive agents necessary for riverbed armoring. Another transition triggers a shift from fluvial-dominated processes to those dominated by extreme events, with critical sediment inputs modulating this transition, particularly within the gorge setting.