An examination of estuary stability in response to human interventions in the South Branch of the Yangtze (Changjiang) estuary, China

Estuary morphologies are dynamic systems, and their stabilities are dependent on various forcing conditions, including tides, waves, and fluvial inputs. However, during the past half century, massive anthropogenic interventions have occurred in many estuaries around the world, resulting in substantial changes in morphologies. Here, we examine such changes in the Yangtze estuary to study decadal morphological stability under anthropogenic disturbances using an entropy-based approach. Using a numerical model, the influence of bathymetric changes and sea-level rise on the variations in energy within the South Branch was examined. An analysis of the spatiotemporal bathymetric variations suggested that the South Branch can be subdivided into three segments of the lower, middle and upper reaches. The changes in these three segments relative to a theoretical equilibrium state were used to investigate and attribute the causes of change. It was found that (1) reclamation works in the South Branch during the last half century, primarily the Xuliujing reclamations (before 1980s) in the upper reach and the Changxing Island expansion (Qingcaosha Reservoir project, 2002–2007) in the lower reach, moved the system away from equilibrium by 2.5–3% in total, although the natural evolution between 1987 and 1997 restored some of the lost efficiency; (2) before large-scale reclamations, river flooding disturbed the system away from equilibrium by 3–6% in 1958, but this was mitigated by 1–2% due to the reclamation works that constrained the channel and deepened the subtidal area; (3) an entropy-based analysis suggested that the Xuliujing reclamation introduced a river constraint that influenced the reach ~20 km downstream, and by enclosing the Qingcaosha Reservoir, a tidal constraint was introduced that influenced the reach ~30 km upstream; and (4) morphological adjustment within the South Branch (a form of self-organization) has enabled the system to adjust to the imposed changes toward a new dynamic equilibrium, consistent with the prevailing constraints and forcing conditions. The results of this study demonstrate a method to determine estuary stability in the context of human interventions, and this method may be relevant to other estuaries subject to large-scale changes.