Surface Roughness and Chemical Weathering of Fluvial Geomorphic Surfaces: A Case Study from Qingyi River, Eastern Tibetan Plateau

River terraces and alluvial fans preserve important records of landscape evolution, yet establishing their relative chronology is often difficult where suitable materials for numerical dating are scarce. This study explores whether geomorphic-surface roughness and sediment chemical weathering can be used together to reconstruct the long-term evolution of fluvial surfaces across extended Quaternary timescales in the Qingyi River basin on the eastern margin of the Tibetan Plateau. We combine digital-elevation-model-based roughness analysis with major-element geochemistry and previously established age constraints for a sequence of river terraces and paleo-alluvial fans developed under humid and strongly erosive conditions. The results show that younger surfaces generally retain smoother depositional morphologies, whereas older surfaces display greater topographic variability caused by progressive runoff erosion, gully incision, and surface fragmentation. At the same time, older deposits record stronger chemical alteration, expressed by increasing depletion of mobile elements and enrichment of relatively immobile components. When evaluated against surface age, both roughness and the Chemical Index of Alteration follow nonlinear, progressively saturating trajectories. These trends indicate rapid geomorphic and geochemical modification during the early stages of exposure, followed by a gradual decline in the rate of change as weatherable minerals are depleted and landscape dissection approaches an advanced state. Neither proxy shows a simple correspondence with glacial–interglacial cycles. Instead, their long-term age-dependent trends reflect the cumulative effects of physical erosion and chemical weathering acting over the duration of surface exposure, while climate modulates the rates and relative intensities of these processes. The parallel evolution of roughness and weathering therefore provides complementary evidence for surface age and preservation. When evaluated against independently constrained surface ages, their joint use provides complementary evidence for the relative correlation of terraces and alluvial fans, especially in humid mountain-front settings where suitable materials for numerical dating are limited.