To What Extent Multidecadal Changes in Morphology and Fluvial Discharge Impact Tide in a Convergent (Turbid) Tidal River

Abstract

Abstract Understanding nonstationary tides in tidal rivers is a major contemporary challenge. In particular, the response of river tides to natural developments in the estuary remains poorly investigated. This study analyzes the evolution of tidal characteristics over the last six decades in the Garonne Tidal River (GTR, SW France), in order to explore the effect of natural and human‐induced morphological and hydrological changes on river tides. The tidal Garonne is an excellent example, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. Tidal range (TR), distortion (A M4 /A M2 ), and asymmetry direction (2ϕ M2 ‐ϕ M4 ) were calculated at four locations from the water level time series of 1953, 1971, 1982, 1994, 2005, and 2014. The annual time series of M 2 and M 4 amplitudes and phases were obtained through complex demodulation. Results reveal that both TR and 2ϕ M2 ‐ϕ M4 have increased since the 1950s. River flow modulates TR and A M4 /A M2 significantly. A long‐term decrease in summer discharges from 200 ± 50 to 100 ± 50 m 3 s −1 would increase TR by +11.5% in the upper GTR. Natural morphological changes amplified TR and 2ϕ M2 ‐ϕ M4 by up to +12–15% between 1953 and 2014. TR and 2ϕ M2 ‐ϕ M4 doubled in the regions affected by gravel extraction between 1953 and 1971. Further, the persistence of mobile mud in the GTR increased TR seasonally but also interannually (by up to +16% in winter and spring of dry years). The potential impact of these changes on suspended sediments is discussed, revealing complex feedback between the evolution of hydrology, morphology, tides, and sediment trapping.