Impacts of glacier retreat, greening, and a changing sediment supply on debris flows in contrasting monsoon‐dominated Himalayan climates

Summary Sediment transport in high mountain environments is affected by the interplay of hydro‐meteorological dynamics, erosion processes, and climate‐driven landscape change, such as glacier retreat and mountain greening. Mountain catchments in High Mountain Asia (HMA) are prone to debris flow hazard due to the steep terrain, intense monsoon precipitation, and rapid cryosphere degradation. Catchments susceptible to debris flow events have previously been classified as having either transport‐limited or supply‐limited sediment regimes. However, the contribution of glacier meltwater, precipitation extremes, changing vegetation cover, and sediment availability, on debris flow activity remains poorly quantified across environments in the region. Here, we modified the Sediment Cascade (SedCas) model to simulate water balance components and subsequent sediment transport and debris flow events of a conceptual debris‐flow catchment in two monsoon‐dominated regions of the Central Himalaya: Langtang (humid, monsoon‐dominated) and Mustang (semi‐arid, rain‐shadow), Nepal. We consider a total of 55 scenarios reflecting transitions from glaciated to vegetated terrain and a set of sediment supply scenarios and intra‐annual erosion patterns. Our results show that glacier melt more strongly affects runoff and debris flow magnitude in drier, continental climate, while increases in vegetation and a reduction in glacier cover substantially decrease potential sediment supply and shift debris flow events to later in the monsoon season. The number of annual debris flow events remains relatively stable across scenarios, but their magnitude and seasonality are altered by both sediment recharge (i.e., sediment supply recovery) timing and land cover evolution. The modeling experiments show that as the landscape changes (through glacier retreat and vegetation succession), transport‐limited and supply‐limited conditions become more similar, suggesting that future debris flows may become smaller. These findings highlight the importance of hydro‐geomorphic feedbacks under rapid environmental change to understand sediment‐related hazards in high‐relief Asian mountain regions.