Attribution of Thermal Regime Changes in the Upper Yangtze River: Disentangling Discharge- and Temperature-Mediated Pathways

River water temperature (RWT) in the upper Yangtze River has been substantially altered by intensive reservoir regulation and climate change, yet the respective contributions of these drivers and their interactions are not fully understood. Here, we developed an attribution framework that integrates a watershed hydrological model, a hydrothermal dynamics model and multiple linear regression models to reconstruct RWT under a range of counterfactual scenarios. This framework provided estimates of driver contributions and elucidated the discharge- and temperature-mediated pathways through which RWT was modified. Results showed that climate change made the largest contribution to annual mean warming, accounting for approximately 49–56% of the observed increases. In contrast, dams dominated seasonal RWT changes. At the Three Gorges Reservoir (TGR) inlet, upstream cascade dams (CDs) cooled RWT by 0.77 °C in spring and warmed it by 0.74–1.02 °C in autumn and winter. At the TGR outlet, the TGR cooled RWT by 3.00 °C in spring and warmed it by 1.58–2.64 °C in autumn and winter. Pathway analysis further revealed two contrasting mechanisms through which upstream CDs regulate the TGR thermal effect: CD-induced discharge alterations generally attenuated the TGR’s thermal effect, whereas CD-induced changes in TGR inflow temperature generally amplified it.