Hotspots and Blind Spots in Global River Temperature Science: Implications for Evidencing Climate Adaptation

Rising river water temperatures (T_w) pose threats to freshwater ecosystems, water quality and water-dependent sectors. Yet the evidence needed to support adaptation to higher T_w remains uneven. This systematic review synthesises 529 studies published between 1990 and 2025 to assess how T_w science has developed and where it can most effectively support adaptation. We show that the field has expanded substantially, but remains concentrated in high-income, temperate regions, with the United States, Canada and the United Kingdom accounting for approximately 70% of all studies. Much of the research is local or catchment-scale, based on daily data, and oriented towards empirical modelling, ecological responses and broad thermal-risk screening. These studies provide a solid foundation for understanding river thermal regimes, particularly in data-rich temperate systems, but their geographic bias limits transferability to under-represented regions, river types, sectors and decision contexts. The current evidence base is strongest for ecological and cooling-oriented adaptation, but weaker where decisions apply to event-scale risk assessment, water-quality thresholds, operational triggers, or integration with urban, agricultural and infrastructure pressures. We assert that the main challenge is not simply a shortage of T_w studies, but a mismatch between a historically hydro-ecological evidence base and the wider adaptation decisions now required under climate change. Future research should therefore prioritise transferable monitoring and modelling frameworks, event-relevant metrics, stronger integration with water-quality indicators and clearer translation of T_w evidence into sector-specific adaptation guidance.