Influence of Dams and River Regulation on the Isotopic Composition of a Major Sub‐Tropical River Basin

ABSTRACT Cascade dam construction has altered river systems, impacted hydrological processes, and river water isotopic compositions. However, specific impacts of reservoir operations on riverine hydrological processes remain insufficiently understood. This study investigates cascade dam influences on Zijiang River (ZJR) hydrological processes by analysing the isotopic composition of river water during summer (August 2023) and winter (January 2024). The results indicate that: (1) The δ 2 H and δ 18 O values of the ZJR exhibit a clear seasonal pattern, with more positive values in winter and more negative values in summer, alongside progressive downstream enrichment in 2 H and 18 O relative to the headwaters. (2) In the headwater region, δ 18 O values are negatively correlated with precipitation amount and positively correlated with evapotranspiration, reflecting natural watershed conditions. In the middle reach, where a reservoir is located, the effects of the reservoir begin to emerge, as precipitation effects weaken and evaporation‐induced enrichment in heavy isotopes ( 2 H and 18 O) becomes more pronounced. Cascade dam regulation extends water residence time, enhancing enrichment of 2 H and 18 O through evaporation, resulting in a positive correlation between δ 18 O values and both precipitation amount and evapotranspiration. (3) In summer, the largest reservoir within the ZJR, located in the middle reaches, retains precipitation from April to June that has more positive δ 18 O values and enhances evaporative enrichment in 18 O and 2 H through prolonged water retention, leading to more positive isotopic values in the reservoir's river water. In winter, elevated reservoir water levels and extended water mixing substantially dampen isotopic fluctuations, leading to more stable downstream isotopic compositions. Our results underscore the significant role of cascade dams in modifying hydrological dynamics and water isotope patterns.