Human perturbations reshape hydrological responses in riverine systems: Insights from a reach-level quantification framework to the Pearl River Basin

• Static assessments substantially underestimated river confinements after 2000. • Longitudinal confinement shifts downstream, and lateral confinement shifts inland. • Dam regulation and urbanization exhibit opposite hydrological consequences. • When both confinements act, dams reduce runoff while urbanization increases variability. • Heightened flood risks emerge in the PRD and rapidly urbanizing towns in Guangxi. • Ecological fragility increases in heavily regulated reaches (e.g., Nanpan River). Human perturbations greatly threaten the health of riverine systems, producing ecological and hydrological consequences that challenge progress toward the Sustainable Development Goals. Yet existing quantifications of these perturbations remain coarse and static, limiting the assessment of their fine-scale impacts on streamflow responses. This study proposes a dynamic reach-level framework to track human-perturbed precipitation–streamflow (P–Q) relationships. By mapping 35 years of dam and urban datasets (1985–2019) onto 17,329 river reaches in the Pearl River Basin, we quantify longitudinal and lateral confinements using annually varying Degree of Regulation (DOR) and urbanized floodplain percentage (URB). We then classify reaches by their perturbation levels and evaluate how these confinements alter streamflow response intensity (conversion efficiency) and response variability (ratio of Q to P variability). Our results show that past static assessments substantially underestimated confinement levels, with longitudinal underestimation in the main channel ranging from 4.07 % to 18.80 %, and lateral underestimation in the Pearl River Delta ranging from 3.9 % to 16.96 %. These gaps arise from intensified damming shifting downstream and urbanization expanding inland. Where both confinements intensify, hydrological responses are characterized by reduced conversion efficiency due to damming and increased variability linked to urbanization, demonstrating that variability changes are more sensitive to lateral confinement. These results delineate regions with elevated flood sensitivity in rapidly urbanizing lowlands and emerging ecological fragility in heavily regulated uplands, which can inform adaptive, sustainability-oriented river management strategies.