[Objective] The cascade reservoirs in the Lower Jinsha River Basin are important hydropower bases in southwestern China and are crucial for the rational development and utilization of regional water resources. Systematic research on the hydrochemical composition of typical river waters in the basin and their rock weathering processes is conducted to provide theoretical support for evaluating the geochemical cycle in the Lower Jinsha River cascade reservoirs. [Methods] Three main tributaries of Xiangjiaba Reservoir (Zhongdu River, Xining River, and Dawenxi River) were selected as the research objects, and 69 river water samples were collected in May 2024 during the period of rapid algal growth in the reservoir. The hydrochemical composition was analyzed, and the spatial distribution characteristics of major ions in the river water and their sources were examined. The potential sources of river water ions were analyzed using ion ratios, principal component analysis, and ion mass balance methods. The contribution of different sources to the major ion concentration was calculated. [Results] The river water in the three tributaries of Xiangjiaba Reservoir was weakly alkaline, with small spatial differences in pH values. The average values were 8.07, 8.06, and 7.78, respectively. The major cations in the river water were Ca²⁺ and Na⁺, while the major anions were {\mathrm{SO}}_4^{2-} and {\mathrm{HCO}}_3^{-}. In the Zhongdu River, Xining River, and Dawenxi River, the concentrations of Ca²⁺, Na⁺, {\mathrm{SO}}_4^{2-} and {\mathrm{HCO}}_3^{-} accounted for 89%, 89%, and 88% of the total ion concentration, respectively. The hydrochemical type of the three tributaries was Ca-Na-HCO₃-SO₄, which was very similar to the river water in the Jinsha River Basin but showed significant differences compared with the mainstream of the Yangtze River, the Yarlung Zangbo River, the Xijiang River, and the Danjiangkou Reservoir. The major ions in river water of the three tributaries were influenced by multiple factors, with potential sources including atmospheric inputs, human activities, and rock weathering (such as carbonate dissolution and silicate weathering). The contribution of atmospheric inputs to ion concentrations in the river water showed little spatial difference, with an average of 12.6%. Rock weathering had a greater influence on the major ions of the Zhongdu River than on those of the Xining and Dawenxi Rivers, while human activities had a greater impact on the ion concentrations of the Xining River. [Conclusions] According to the mass balance equation, the contributions of atmospheric inputs to the ion composition of the Zhongdu River, Dawenxi River, and Xining River are 12.2%, 12.9%, and 12.7%, respectively. The contributions of silicate dissolution to the ions in the three tributaries are 31.2%, 27.3%, and 26.8%, respectively. The contributions of carbonate dissolution are 16.1%, 17.2%, and 18.1%, respectively. The proportions of river water ions derived from human activities are 40.6%, 42.5%, and 42.4% for the Zhongdu River, Dawenxi River, and Xining River, respectively. The research results provide guidance for understanding the hydrochemical characteristics and chemical weathering processes of cascade reservoirs in the Lower Jinsha River Basin.