Impact of dams on river regime and extreme flow events in MIÑO–SIL river basin (NW of the IBERIAN peninsula)

River regulation is a major driver of hydrological change, yet its long-term effects relative to climate and land-use variability remain poorly constrained in many river basins. In the Miño–Sil River Basin (NW Iberian Peninsula), this study evaluates the respective roles of precipitation variability, land-use change, and dam regulation on river flow regimes and extreme flood events using hydro-meteorological records spanning 1944–2023. Two contrasting periods were analysed: a quasi-natural phase prior to large-scale regulation (1944–1959) and a fully regulated phase (1995–2023), based on observations from key gauging stations in regulated and unregulated river sections. Results indicate that land use and basin-scale precipitation remained relatively stable over the study period, whereas dam regulation caused substantial modifications to river flow. In regulated reaches, reservoirs reduced winter discharges and enhanced summer flows, producing a clear seasonal redistribution of streamflow and a marked decoupling between discharge and precipitation. Unregulated sections preserved a natural flow regime closely linked to rainfall variability. Analysis of historical extreme events further shows that flood attenuation strongly depends on reservoir storage conditions prior to an event, leading to contrasting flood responses under similar precipitation forcing. These results demonstrate that reservoir operation fundamentally reshapes river regimes and modulates extreme flows beyond climatic controls. Accounting for dam management is therefore essential for flood risk assessment and climate-adaptive water management in regulated river basins. • Dam regulation is the dominant driver of long-term streamflow change in the Miño–Sil River Basin. • Reservoir operation redistributes seasonal flows, reducing winter discharge and enhancing summer flows. • River regulation decouples streamflow from precipitation at monthly timescales. • Flood mitigation effectiveness depends critically on pre-event reservoir storage conditions. • Dams substantially reduce flood severity under comparable precipitation forcing.