Rivers of salt: the extent of salinization in European running waters.

Freshwater salinization is emerging as a widespread but poorly documented challenge across Europe. Using random forest models applied to river-reach scale data, this study provides a continent-wide quantification of salinization and identifies its key drivers. Of the 35 countries with available data, we demonstrate that 28 contained rivers where salinity more than doubled relative to baseline, with increases reaching up to 11,377 µS/cm. Across the European river network, 14% showed salinity increases of up to 100% and a further 26% up to 50%. The analysis revealed previously underreported hotspots, notably catchments in Romania and Hungary. More than 23,000 km of river network, around 2% of the total, exceeded 1 mS/cm, surpassing international standards for aquatic life protection and raising the risk of altered community structure, disrupted ecosystem function, harmful algal blooms, and fish mortality. Salinity exceeded drinking water limits in 1% of the network and irrigation and industrial limits in 3%. Salinization is driven by natural factors, such as evaporation, soil composition, and rock weathering, and by human pressures, highlighting the need for further research and Europe-wide salinization standards aligned with the Water Framework Directive.