Long-Term Evolution of Salt-Wedge Intrusion in the Ebro River Under Changing Hydrological Conditions

The Ebro River — one of the major Mediterranean rivers of the Iberian Peninsula — has undergone substantial hydrological alterations over the past century, driven by natural variability and intensified by anthropogenic interventions in the upstream catchment, including regulated flows and extensive agricultural water extraction. A key manifestation of these changes in the lower river reach is the persistent intrusion of a salt wedge, which under low-flow conditions can extend up to ~30 km inland from the river mouth. This study provides a century-scale assessment of salt-wedge dynamics in the Ebro River, combining historical observations, hydrological records, and numerical modelling from 1916 to 2025. A high-resolution hydrodynamic model was developed to simulate steady-state salt intrusion under a wide range of river discharge scenarios and the micro-tidal forcing characteristic of the river mouth. The model, validated against in-situ salinity and current measurements, accurately reproduces the formation, structure, and upstream migration of the salt wedge, and underscores the strong influence of riverbed bathymetry on penetration length.

Model results reveal a robust inverse relationship between river discharge and salt-wedge intrusion. In a long-term context, the analysis shows that both the frequency and the spatial extent of salt intrusion have increased by roughly 400% over the past century, primarily due to the progressive reduction of annual freshwater inputs. These findings advance understanding of the long-term evolution of estuarine salinization in Mediterranean rivers and highlight the need to integrate historical datasets, field observations, and scenario-based numerical modelling to support adaptive water-resource management and enhance coastal-zone resilience under ongoing climate and anthropogenic pressures.