Tidal rivers and estuaries may experience high levels of suspended particulate matter (SPM), which impacts water quality and ecosystem functioning. The processes controlling the development of estuarine turbidity maxima (ETM) are fairly well understood. However, predicting the maximum SPM concentration in an estuary based on aggregated parameters (estuarine dimensions, river discharge, tidal range) remains, up to now, impossible without extensive in-situ measurements and/or numerical models. This study introduces an approach that links the strength of the ETM to the tidal, river, and morphological characteristics of a system. Using in-situ data from contrasting meso- to macro-tidal estuaries, we found a consistent pattern of maximum SPM concentrations within a two-dimensional parameter space. The resulting turbidity diagram reveals a high SPM hotspot in estuaries with specific forcing conditions, corresponding to intermediate relative tidal amplitudes and freshwater Froude numbers. This multi-site research advances our predictions of ETM intensity in tide-dominated estuaries, offering a straightforward method to explore potential turbidity trajectories under various human pressures.

Most estuaries display strong decadal changes in morphology, bottom friction and river flow due to human activities and climate change. These changes can in turn modify tide propagation and flow. In macrotidal and highly turbid estuaries characterized by strong seasonal shifts in the Estuarine Turbidity Maximum, these hydro-morpho-sedimentary changes may be relevant even at seasonal time scales. In this study, we evaluate the amplification, damping, distortion, and asymmetry of tidal waves in the hyper-turbid Gironde-Garonne and Loire estuaries in relation to hydrological, sedimentary and morphological seasonal changes. For this purpose, a non-stationary harmonic analysis based on S-TIDE was applied to 5-year times series of water level from 10 (Gironde) and 14 (Loire) stations. River discharge strongly influences tidal range, overtides generation/damping, and therefore tidal deformation in both estuaries. For example, in the Gironde-Garonne (Loire) estuary, the M2 tide peak increased by 20% (6%) and shifted upstream by 74 km (43.5 km) from high to low river discharge periods, while M4 tidal peak increased by 21% (21%) and shifted upstream by 31 km (16 km). A river flow threshold for the maximum tidal distortion was observed in the upstream reaches of both estuaries: e.g., 1500 m3/s at 136 km from the mouth in the Gironde-Garonne estuary, and 500 m3/s at 94 Km from the mouth in the Loire estuary. The influence of annual and seasonal changes of bottom roughness on tidal range was highlighted in the upper reaches of both estuaries. For example, in the Garonne tidal river, the tidal range can increase by 25 cm after the ETM installation near Bordeaux Harbour, for the same (average) river flow. Morphological changes seem to have a relatively lower impact on tides in the Garonne tidal river at these time scales. Future analytical and numerical simulations of tide propagation will provide further insight into the relative role of the different morpho-sedimentary factors on tide propagation in both estuaries.