Evolution of tidal reaches of the Yangtze River related to reduced non-uniform sediment after the impoundment of Three Gorges Reservoir

The sharp reduction in non-uniform sediment, composed of particles of varying sizes, caused by climate change and human activities, has triggered significant evolution in the tidal reaches of the Yangtze River. Clarifying the intrinsic link between these changes is crucial for addressing water sustainability challenges, particularly the prolonged low-sediment-laden flow that may persist for hundreds of years. Based on over 70 years of observational data, this study analyzed the spatiotemporal variation of non-uniform sediment entering the tidal reaches of the Yangtze River and established its relationship with tidal reach evolution. The results show that non-uniform sediment entering these reaches has steadily declined over the past 70 years, with a sharp reduction of more than 70 % and coarsening of over 46 % following the impoundment of the Three Gorges Reservoir, approximately 20 % of which was eroded from the riverbed from Hankou to Datong. This reduction triggered severe erosion in the tidal reaches, primarily within low-water channels. The amount of sediment with particle sizes greater than 0.125 mm showed a strong correlation ($R^2$ > 0.8) with riverbed erosion on a large spatial scale. In the future, improved research on topography and sediment measurement technologies, along with the combined impacts of other human activities and oceanic processes, will support more accurate quantification of the evolution triggered by sediment reduction. These findings offer valuable insights for mitigating adverse effects and promoting sustainable development in the tidal reaches of the Yangtze River following the operation of upstream cascade reservoirs.