Dual evolution of tides in a turbid macrotidal estuary at decadal timescales

Abstract. Changes in bathymetry or in hydrological cycle can affect tides in estuaries, resulting in complex associated hydro-morpho- sedimentary feedbacks, and historical in-situ data is often insufficient to study them. The Gironde estuary, located on the Atlantic coast of France, has a large in-situ dataset covering the estuary–tidal river continuum and has undergone predominantly natural morphological changes over the last half of the past century. This study quantifies the spatial and temporal variations in tides along this continuum based on water-level measurements from nine tidal gauges and bathymetry data for selected years between 1953–2018, together with the evolution of river discharge and bottom friction. The results distinguish two zones. In the lower estuary (from the mouth up to km 50), the annual tidal range is driven by the lunar nodal cycle, which modulates the M2 tidal constituent. In the upper estuary (km 60 to 70), the tidal range increased (+40 cm on 1982–2000), primarily due to the morphological evolution, notably the strong erosion that occurred between 1994 and 2000. The Garonne tidal river (km 70 to 95) shows a similar but stronger response than the upper estuary (+80 cm on 1971–2000), further modulated by the seasonal changes of the riverbed roughness caused by the estuarine turbidity maximum (ETM). This research reveals the predominant role of river discharge in a feedback loop linking the ETM — and thus bottom friction — with morphology and tidal dynamics, with implications for risk management like flooding in the context of the future changes to the hydrological cycle.