River-tide dynamics remain poorly understood, in part because conventional harmonic analysis (HA) does not cope effectively with nonstationary signals. To explore nonstationary behavior of river tides and the modulation effects of river discharge, this work analyzes tidal signals in the Yangtze River estuary using both HA in a nonstationary mode and continuous wavelet transforms (CWT). The Yangtze is an excellent natural laboratory to analyze river tides because of its high and variable flow, its length, and the fact that there are do dams or reflecting barriers within the tidal part of the system. Analysis of tidal frequencies by CWT and analysis of subtidal water level and tidal ranges reveal a broad range of subtidal variations over fortnightly, monthly, semiannual, and annual frequencies driven by subtidal variations in friction and by variable river discharges. We employ HA in a nonstationary mode (NSHA) by segregating data within defined flow ranges into separate analyses. NSHA quantifies the decay of the principal tides and the modulation of M 4 tide with increasing river discharges. M 4 amplitudes decrease far upriver (landward portion of the estuary) and conversely increase close to the ocean as river discharge increases. The fortnightly frequencies reach an amplitude maximum upriver of that for over tide frequencies, due to the longer wavelength of the fortnightly constituents. These methods and findings should be applicable to large tidal rivers globally and have broad implications regarding management of navigation channels and ecosystems in tidal rivers.

Numerous research efforts have been devoted to understanding estuarine morphodynamics under tidal forcing. However, the impact of river discharge on estuarine morphodynamics is insufficiently examined. Inspired by the Yangtze Estuary, this work explores the morphodynamic impact of river discharge in a 560 km long tidal basin based on a 1-D model (Delft3D). The model considers total load sediment transport and employs a morphodynamic updating scheme to achieve long-term morphodynamic evolution. We analyze the role of Stokes drift, tidal asymmetry, and river discharge in generating tidal residual sediment transport. Model results suggest that morphodynamic equilibrium is approached within millennia by vanishing spatial gradients of tidal residual sediment transport. We find that the interaction between ebb-directed Stokes return flow/river flow with tides is an important mechanism that flushes river-supplied sediment seaward. Increasing river discharge does not induce continuously eroded or accreted equilibrium bed profiles because of the balance between riverine sediment supply and sediment flushing to the sea. An intermediate threshold river discharge can be defined which leads to a deepest equilibrium bed profile. As a result, the shape (concavity or convexity) of the equilibrium bed profiles will adapt with the magnitude of river discharge. Overall, this study reveals the significant role of river discharge in controlling estuarine morphodynamics by supplying sediment and reinforcing ebb-directed residual sediment transport.

This study focuses on suspended sediments and in situ flocculation in the Yangtze River, with the goal of improving our understanding of the relationship between freshwater and estuarine flocculation. A field survey with state-of-the-art instruments was carried out in January 2008 in the reach from downstream of the Three Gorges Dam to the estuary. The data show that in situ floc mean diameters range from 22 to 182 μm in the river, whereas the median dispersed grain sizes are 4.4-11.4 μm. This demonstrates that flocculation is an important process during the transport of suspended sediments along the river. The flocculation characteristics, suspended sediment concentration and dispersed grain sizes all vary longitudinally in the main stream of the Yangtze River. Biochemical factors are likely be more significant in the freshwater flocculation than in the estuary, where hydrodynamics and biochemical factors are both important. Flocculation is found in the freshwater river, in the estuary and in coastal waters, which indicates that dynamic break-up/reflocculation processes take place during the suspended sediment transport. The freshwater flocs may behave as parent flocs to the estuarine flocculation. This study enhances our understanding of flocculation from estuarine and coastal areas to fresh river systems and provides insights into the effects of input of riverine flocs to the estuarine flocculation and into the sources and fate of flocs.