the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Oct 2026
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.
Status: open (until 19 Nov 2026)
- RC1: 'Comment on egusphere-2026-5034', Anonymous Referee #1, 22 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5034', Anonymous Referee #2, 25 Oct 2026
reply
This study uses water-level records from nine tide gauges, bathymetry, river discharge, and ETM presence data in the Gironde Estuary–Garonne tidal river system to analyze multidecadal changes in tidal range, amplification factor, and tidal asymmetry over 1953–2017. It further distinguishes oceanic control in the lower estuary, morphological control in the upper estuary, and ETM/discharge control in the GTR. The discussion also addresses the important issue of morphodynamic–tidal–sediment feedbacks. However, several aspects would benefit from further strengthening. The comments below are offered for the authors’ consideration, and I recommend major revision.
Major comments
1. The relationship with Jalón-Rojas et al. (2018) should be clarified more explicitly. Jalón-Rojas et al. (2018) already reported increases in tidal range and the relative phase difference in the GTR, discharge modulation, morphological changes, reduced friction by mobile mud/ETM, and a feedback loop. I suggest stating more directly in the Introduction and Discussion what the novel contribution of the present manuscript is relative to the 2018 paper.
2. The tidal asymmetry indicators could be made more complete. The manuscript mainly uses the phase direction. Adding the distortion amplitude ratio would make the conclusion that “asymmetry increased” more complete.
3. The hypothesized morphology–tide–sediment feedback in Section 5.4 is interesting and consistent with recent observational and conceptual studies in strongly tidal, highly dynamic bed systems. The authors may be interested in two recent studies. Xie et al. (2022, CATENA, https://doi.org/10.1016/j.catena.2022.106131) developed a coupled river–tide–morphology conceptual model for the Qiantang Estuary and discussed the interactions among river discharge, tides, and bed morphology across multiple timescales. Xie et al. (2026, JGR-Oceans, https://doi.org/10.1029/2026JC024508) further developed a mechanistic framework for this morphodynamic feedback loop based on high-frequency suspended sediment concentration observations, showing a cycle in which high river discharge scours the riverbed, the resulting deepening enhances tidal/bore energy, and this in turn drives resuspension and deposition; that study also proposed a mechanism by which morphological changes affect the estuarine turbidity maximum. These two studies may serve as useful references for further refining the presentation of the feedback mechanism, and may help place the Gironde feedback hypothesis in the broader context of macrotidal estuaries.
4. The attribution “primarily due to morphology” could be expressed more cautiously. The upper estuary shows a tidal-range increase of +40 cm during 1982–2000 and the GTR shows +80 cm during 1971–2000, concurrent with erosion during 1994–2000; however, river discharge also decreased significantly during the same period. The authors acknowledge in Section 5.2 that observations cannot distinguish among the various factors, yet the conclusion still uses “primarily.”
5. The conclusions regarding the nodal cycle in the lower estuary should be interpreted more cautiously. The Lomb–Scargle spectrum of the continuous monthly mean tidal range at Le Verdon shows a peak at 17–19 years with power >0.45. However, I suggest clarifying whether the series was detrended and reporting the significance level and confidence interval. Data at the other stations are discontinuous, and inferring that the entire lower 50 km is controlled by the nodal cycle based only on a “similar pattern” is not yet sufficiently supported. I suggest reporting further details of the spectral analysis and phase consistency, and distinguishing between the regional sea-level signal and in-estuary tidal propagation.
Minor comments
1. The descriptions of years and stations should be unified. Section 3.1.1 lists years such as 1963, 1971, 1982, 1994, 2000, and 2007, whereas the Results section and Table 2/Figure 2 also include 1953, 2017, and other years. I suggest that the data section consistently list the stations actually available in each year, data gaps, and digitized years, and that the tables and figures be made consistent. This would help readers assess the comparability of tidal parameters across different decades and better understand the role of years such as 1953 and 2017 in the continuum analysis.
2. The analysis selects only the low-flow season from June to October and removes days with daily discharge >2000 m³/s, while the manuscript also discusses flood-period erosion, interannual tidal-range changes, and wet/dry-year contrasts. I suggest briefly explaining the possible influence of this filtering on the “interannual tidal range” results and on comparisons with the full-year results of Jalón-Rojas et al. (2018).
3. The harmonic analysis uses a five-month window and about 83 constituents performed without applying nodal corrections, while Section 5.1 discusses the influence of the lunar nodal cycle on tidal range in the lower estuary. I suggest that the authors clarify whether the absence of nodal correction affects the interannual comparison of M2 amplitude, particularly for the interpretation of the nodal cycle in the lower estuary.
Citation: https://doi.org/10.5194/egusphere-2026-5034-RC2
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Please find my detailed comments in the attached file.