Steep sustainability challenges in transboundary basins worldwide.

Transboundary hydrological basins span international borders and are essential to global water systems, human development, and environmental sustainability. Nearly 40 % of the world's population lives within these basins, which supply critical resources such as freshwater, food, energy, and biodiversity. Yet their sustainability remains poorly understood, as existing assessments often overlook the unique social, environmental, and political complexities of transboundary basins. This study addresses that gap by developing and applying a systematic framework to assess Sustainable Development Goals (SDGs) progress across 310 transboundary basins worldwide. Here we show that transboundary basins score significantly lower on average SDGs achievement (an SDG Index score of 42 on a scale of 0-100) compared to national averages (a score of 67), with considerable variation between regions. We identify four distinct types of transboundary basins in terms of SDGs achievement and associated challenges. We also show that progress on a specific set of goals can drive broader sustainability within each basin type. Notably, achieving clean water (SDG 6), sustainable economic growth (SDG 8), and healthy livelihoods (SDG 3) is linked to overall SDGs success in 38 % of transboundary basins worldwide. Our results highlight the importance of basin-level analysis for revealing sustainability patterns overlooked by national assessments. This framework can inform future basin research and support policy development in transboundary regions.