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 degrees-50 degrees and southeast-southwest aspects, generating 36.17 x 10(4) m(3) 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.