Impact of River Floods on Tidal Dynamics in the Guadalquivir Estuary (SW, Iberian Peninsula): Insights from 30 Years of Observations

Estuaries respond nonlinearly to variations in river discharge, yet the role of flood persistence in controlling estuarine hydrodynamics remains poorly quantified. This study analyzes three decades of hourly river discharge and water level records to examine how severe floods modify tidal dynamics in the Guadalquivir estuary, a tide-dominated, human-regulated system in southwest Iberian Peninsula. Flood events were objectively classified using k-means clustering based on duration and discharged volume, identifying three recurrent types: prolonged fluvial-dominated (Type I), short-duration (Type II), and moderate-duration floods (Type III). Hydrodynamic responses were characterized using anomalies in low- and high-tide peaks across eight tide gauges and wavelet spectral analysis. Results show that each flood type generates a distinct tidal response. Type I events trigger a persistent regime shift in which the estuary becomes fluvially dominated, strongly damping semidiurnal tidal constituents and enhancing subtidal variability at fortnightly-monthly scales in the upper estuary. Type II events induce short-lived semidiurnal damping followed by post-flood tidal intensification. Type III events generate system-wide water level increases and transient tidal damping without long-term spectral restructuring. These contrasting signatures demonstrate that flood persistence and magnitude jointly control the estuarine tidal response. Although physical drivers cannot be identified directly from water level observations, the spatial and spectral patterns are consistent with changes in bottom friction, likely associated with sediment resuspension and export. Overall, these findings show that flood persistence is as critical as flood magnitude in shaping tidal dynamics, providing a framework for anticipating hydrodynamic and morphodynamical evolution in regulated, low freshwater inflow estuaries.