Late Pleistocene Aggradation and Holocene Surface Reworking of Fluvial Terraces in the glacially overprinted Western Higher Himalaya

Abstract. Himalayan rivers are thought to undergo continuous adjustment in response to climatic variability, with changes in monsoon intensity and glacial extent playing a particularly important role in the Higher Himalaya. At the same time, short-term processes such as landslides, debris flows, and floods exert significant influence on both river dynamics and landscape morphology. We study a ~60 km long reach of the Ravi River in the western Higher Himalaya and a ~20 km reach of its main tributary, the Suil River, extending into the Sub-Himalaya. Using 12 new luminescence ages, 36 cosmogenic nuclide (¹⁰Be and ²⁶Al) samples, remote sensing analysis, and detailed field observation, we reconstruct the formation of fluvial terraces and constrain the timing of subsequent erosion processes. We identify 10 recurring terrace levels, reaching elevations of up to ~225 m above the present rivers, with most terrace surfaces capped by a colluvial layer. The luminescence chronology documents repeated phases of fluvial aggradation, followed by major incision periods during the Late Pleistocene (~30–13 ky). In contrast, the cosmogenic surface exposure ages do not reveal terrace abandonment ages, but indicate distinct, Late-to-Mid Holocene reworking (~7–2 ky) on all but two terrace surfaces. Our topographic analysis suggests that up to ~50% of the Ravi and Suil catchment areas may have been influenced by glacial coverage during the Late Pleistocene, likely contributing significantly to sediment aggradation during this time. The Holocene surface exposure ages, in contrast, are interpreted to reflect post-depositional modification by mass wasting processes (e.g., landslides or debris flow) and, in some cases, by potentially vigorous glacial lake outburst floods.