Channel debris from landslides serves as the primary material source for debris flows in the arid Daheba Basin, Northeast marginal Tibet Plateau

Debris flows have increased in frequency within the arid Daheba Basin on the northeastern Tibetan Plateau, but their sediment sources remain poorly quantified. Using high-resolution UAV-derived DEMs from 51 small catchments, this study evaluates the relative contributions of landslide-derived and channel-derived sediment in controlling debris-flow fan magnitude, and quantifies sediment supply during the 2023 rainy season using DEM differencing. A total of 766 landslides occurred predominantly on slopes of 40°-50° and southeast-southwest aspects, generating 36.17 × 10⁴ m³ of material. Gully heads exhibit exceptionally lower landscape dissection thresholds compared with loess and Quaternary regions in China, indicating high susceptibility to failure under intensified runoff. The results show that Landslide area-volume scaling exponent (b) varies with hillslope geometry (K_u): b > 1.3 for K_u < 8 and generally b < 1.3 for K_u > 8, indicating more complete scar evacuation upslope and partial erosion downslope. Despite the abundance of landslides, their contribution to debris flow fan magnitude is minor (< 25%), with channel debris dominating (> 75%). DEM differencing of a small catchment before and after the 2023 rainy season further reveals that sediment supply originates primarily from the main channel (60.6%) and tributaries (23.3%), with smaller contributions from channel banks (6.8%) and channel heads (9.2%). Tributaries exhibit the greatest mean erosion depth (4.2 m), exceeding that of the main channel (3.8 m). These findings demonstrate that debris-flow material supply in the Daheba Basin is transport-limited and controlled mainly by fluvial entrainment rather than slope failures. Climatic warming and wetting may enhance slope instability, but sediment mobilization is dominantly governed by runoff-driven channel erosion. This study underscores the importance of prioritizing channel sediment dynamics in debris flow hazards assessments for arid regions of the Tibetan plateau.