The codes and the dataset used for the following article : 'Flow–Ecology Relationships in Atlantic Salmon Index Rivers of Quebec, Canada.' Environmental-flow assessments often rely primarily on hydrological information, emphasizing the need to identify ecologically relevant flow components for aquatic species and life stages. Atlantic salmon (Salmo salar) provide a relevant case, as some populations in Canada are monitored through index rivers that offer long-term records of population dynamics. Yet ecological observations rarely coincide with continuous hydrological records, limiting assessment of life-stage responses to flow magnitude, timing, and variability. We investigated flow–ecology relationships using long-term ecological records and historical hydrological conditions reconstructed with deterministic hydrological modeling for two near-pristine Atlantic salmon index rivers in Quebec, Canada. The results confirmed life-stage- and season-dependent relationships: 1) smolt migration was linked mainly to spring flow dynamics; 2) in-river survival to winter and early-spring conditions; 3) smolt body length to seasonal flow variability and summer flows; and 4) adult upstream migration to summer–autumn flow dynamics. Panel Data Analysis (PDA) provided the inferential framework and further showed that the source of hydrological variation underlying these relationships differed among ecological indicators. Specifically, smolt migration and in-river survival were associated with overall variation across rivers and years, whereas smolt body length was associated mainly with temporal changes within each river and adult migration with hydrological differences between rivers within the same year. These PDA results were further supported by complementary river-specific support vector regression (SVR) models, which generally yielded consistent relationship directions. These findings demonstrate the importance of life-stage-specific and seasonal flow requirements and support integrating ecological evidence into environmental-flow assessment, conservation, and management.