Global disparities in river flow seasonality across major river basins

River flow seasonality (RFS) governs water availability for ecosystems and societies, but its response to future climate change remains poorly quantified at global scales. Here, we assess historical and future RFS trends across twenty major river basins using observational data (G-RUN Ensemble) and multi-model hydrological projections (RCP2.6 and RCP6.0). Our analysis shows declining historical RFS in snow-dominated high-latitude basins, contrasting with increases in tropical and temperate regions. Future projections show amplified river basin divergence: RFS weakens further in northern high latitudes but intensifies elsewhere, with RCP6.0 exacerbating these trends. Notably, over 30% of densely populated basins face co-occurring RFS intensification and population growth, compounding water stress. Our basin-scale analysis identifies hotspots where climate-driven RFS shifts intersect with rising water demand, providing actionable insights for adaptation planning.