Spatial and temporal variations in erosion rate in a landslide-dominated basin, the Lao-Nung River, SW Taiwan

Abstract As an island with high tectonic uplift and a humid climate with frequent typhoons, landsliding is one of the primary processes that shapes the landscape in Taiwan. To investigate the impact of landslides on long-term erosion in a landslide-dominated setting, we analyzed the spatial and temporal variations in the erosion rate in the Lao-Nung basin, southwestern Taiwan. In this basin, a series of well-surveyed and dated fluvial and landslide fan terraces are distributed along its main stem, providing an ideal setting to reconstruct paleo-erosion rates using in situ 10 Be concentrations in quartz. The results reveal relatively steady long-term erosion rates averaging ~ 0.7 mm/yr but with notable temporal and spatial variations. A reduced erosion rate around ~ 12 ka likely reflects cold and dry climatic conditions, as supported by pollen and organic matter records in lake sediments. In contrast, an elevated rate around ~ 6 ka corresponds to a warm and wet climate and increased landslide activity, evidenced by the widespread formation of landslide fan terraces. However, the absence of such terraces in the upstream sampling area suggests the higher erosion rate may also reflect spatial differences in uplift, as indicated by increasing normalized channel steepness index upstream. Despite extensive recent landsliding, modern erosion rates remain relatively uniform along the main stem except in the upstream sections, suggesting that much of the landslide-derived sediments remain trapped in the upstream area rather than being efficiently transported downstream.