Changjiang Delta in the Anthropocene: Multi-scale hydro-morphodynamics and management challenges

The Changjiang Delta (CD) is one of well-studied large deltas of critical socio-economical and ecological importance regionally and global representativeness. Cumulated field data and numerical modeling has facilitated scientific understanding of its hydro-morphodynamics at multiple spatial and time scales, but the changing boundary forcing conditions and increasing anthropogenic influences pose management challenges requiring integrated knowledge. Here we provide a comprehensive synthesis of the multi-scale deltaic hydromorphodynamics, discuss their relevance and management perspectives in a global context, and identify knowledge gaps for future study. The CD is classified as a river-tide mixed-energy, muddy and highly turbid, fluvio-deltaic composite system involving large-scale land-ocean interacted processes. Its hydro-morphodynamic evolution exhibits profound temporal variations at the fortnightly, seasonal, and inter-annual time scales, and strong spatial variability between tidal river and tidal estuary, and between different distributary channels. As the river-borne sediment has declined >70%, the deltaic morphodynamic adaptation lags behind sediment decline because sediment redistribution within the delta emerges to play a role in sustaining tidal flat accretion. However, the deltaic channels have become narrower, deepened and growingly constrained under cumulated human activities, e.g., extensive embankment and construction of jetties and groins, possibly initiating a decrease in morphodynamic activities and sediment trapping efficiency. Overall, the CD undergoes transitions from net sedimentation and naturally slow morphodynamic adaptation to erosion and human-driven radical adjustment. A shift in management priority from delta development to ecosystem conservation provides an opportunity for restoring the resilience to flooding and erosion hazards. The lessons and identified knowledge gaps inform study and management of worldwide estuaries and deltas undergoing intensified human interferences.