Freshwater salinization is an increasingly widespread environmental challenge, driven by climate change, hydrological alteration, land-use intensification, and growing human demands on water resources. Rising salt concentrations are reported across rivers and streams worldwide, with significant consequences for aquatic ecosystems, water quality, and the sustainability of freshwater services. In Europe, salinization is of particular concern given extensive river regulation, high population density, and the connectivity of transboundary river networks. Both the European Water Framework Directive (WFD) and the United Nations Sustainable Development Goal (SDG) indicator 6.3.2 explicitly require the assessment of salinity and the establishment of criteria for good ecological status. However, few European countries have developed salinity thresholds, and their ecological rationale is often unclear, highlighting the urgent need for a continent-scale understanding of the extent, drivers, and impacts of freshwater salinization to inform robust criteria.
Using a pan-European modelling approach, we show that freshwater salinization is already pervasive. Of the 35 countries with available data, 28 contain severely salinized rivers or streams, and an estimated 14–26% of the European river network exhibits some degree of salinization. We confirm several well-known hotspots—including the Ebro basin in Spain, the Meurthe in France, Alpine streams in Italy, and coastal rivers in Belgium, the Netherlands, and Denmark—while also identifying previously unreported regions with elevated salinity. Observed levels are sufficiently high to restructure aquatic communities, reduce biodiversity, disrupt ecosystem functioning, and increase the likelihood of harmful algal blooms and fish mortality. In many locations, salinity exceeds recommended limits for drinking water, irrigation, and industrial use.
Our results indicate that vulnerability to salinization is shaped by natural factors such as soil composition, geology, and rock weathering, but human pressures—including urbanization and agriculture—are strong predictors of elevated salt concentrations. Ion ratios further suggest that evaporation is an important driver, particularly in western Europe. As freshwater salinization intensifies under future climate and land-use change, these findings underscore the need for coordinated, transboundary management and the development of scientifically robust, ecologically meaningful salinization criteria to support WFD implementation and progress towards SDG 6.3.2, preventing future ecological and societal impacts.