Spatial Shift of Dominant Controls and Development Patterns of Tributary Mouth Bars in the Xiaolangdi Reservoir

Tributary mouth bars (TMBs) at confluences in the Xiaolangdi Reservoir have encroached on effective storage, threatening reservoir safety. This study selected four tributaries (Zhenshui River, Dongyang River, Xiyang River, Yanxi River) with a continuous distance‐to‐dam gradient and no independent inflow or sediment supply. Using long‐term (2000–2024) data, we analysed TMB evolution and its dominant controls. Results show a longitudinal gradient in TMB height. The near‐dam Zhenshui River experienced five stages, with a peak TMB height of 11.2 m. The far‐dam Yanxi River developed only low bars (< 1 m), which disappeared after 2019. The delta apex advanced from ~74 (2001) to ~4.6 km (2020), with a temporary stable period at ~16 km during 2011–2017, then retreated to ~11.4 km by 2024. This migration modulated the longitudinal gradient. Dominant controls shift systematically along the reservoir. For Zhenshui River, the incoming sediment coefficient and mainstream grain size show positive effects, while the sediment release ratio shows a negative effect. In the middle reach, water level amplitude shows a negative effect and mainstream grain size a positive effect on Dongyang River; for Xiyang River, both the incoming sediment coefficient and mainstream grain size have positive effects. For Yanxi River, the incoming sediment coefficient becomes negative, whereas the positive effect of mainstream grain size persists. Model validation shows good agreement between measured and predicted values. By comparing tributaries with no sediment supply of their own, this study reveals the spatial differentiation of TMB development and the spatial shift of dominant controls, providing a basis for managing tributary deposition in sediment‐laden reservoirs.