Highly accelerated siltation of abandoned distributary channel in the Yangtze Delta under everchanging social-ecological dynamics

Abstract

After the construction of more than 50,000 dams, the Yangtze (Changjiang) Delta has been evaluated as the highest vulnerable delta in response to anthropogenic climate change, but most of existent assessments did not take adaptive measures and ecosystem feedbacks into consideration, leading to counterintuitive results. In this study, annual to decadal morphological changes in the North Branch were specifically investigated by using time-series satellite images from 1984 to 2018, and controlling factors and mechanism were analyzed in the context of complex human-earth interactions. The North Branch has naturally evolved from a main distributary channel before the 1840s to the present tide-dominated estuary, infilled continuously with a huge amount of sediments. Annually 102 ± 7 Mt. sediment (1958–2009) was estimated to deposit in the North Branch, mostly sourced from erosion of the lower and outer estuarine channel. Recent land reclamation projects have further accelerated channel accretion by promoting tidal sandbars accretion and secondary channels siltation. Totally, 196 km2 of intertidal wetland has been reclaimed in the period 1978–2018. Lavish land reclamation was carried out in the 1990s and 2000s to meet for boosting socioeconomic development in Shanghai and abide the law “no net loss in farmland”, producing sharp decrease of saltmarsh and mudflat area. The overexploitation of wetland resources is very short in the Holocene delta life cycle. It is fortunate to see gradually renewing saltmarsh after the new policies have recently been strictly implemented by controlling land reclamation to meet the target of ecological civilization. In the secular human-earth duel, the recent technical and economic development gives human superiority to reduce exposure to natural hazards, but may induce ecological degradation and consequently adverse feedback to coastal societies. Therefore, we stress again to balance sustainable development between human society and coastal ecosystem, coining a new civilization in the Anthropocene.