Mega Estuarine Constructions Modulate the Changjiang River Plume Extension in Adjacent Seas

Mega constructions have been built in many river estuaries, but their environmental consequences in the adjacent coastal oceans are not well understood. This issue was addressed with an example of the Changjiang River Estuary, where massive navigation and reclamation constructions were recently built on the tidal flats that separate the three major outlets. Based on numerical model twin validations against data from eight cruises and the scenario simulations, we found that the estuarine constructions have profoundly affected the plume extension and the associated physical stratification in a surprisingly large area. Before the constructions were built, the water masses in the two southern outlets were transported across the tidal flats to the northern outlet by the lateral tidal residual transports. However, this lateral exchange was gradually blocked as the constructions were built, so the freshwater in the two southern outlets now exits the river mouth directly. The overall effect is that the northeastward offshore plume extension has been weakened, whereas the southward along-shelf plume extension has been strengthened. The ecological consequences of this shift in plume regime are speculated. Previously, the Three Gorges Dam (TGD) was thought to be responsible for some offshore environmental changes by modulating the river plume, but our results show that its effects on the river plume characteristics are secondary. The TGD and the mega estuarine constructions were built during a similar period; hence, their influences need to be reconsidered.