A River That Builds Islands
The Yangtze River, the longest river in Asia, carries a tremendous amount of sediment from the interior of China to the sea. Historically, this sediment load averaged nearly 500 million tonnes annually. As the river slows to meet the East China Sea near Nantong, it deposits this material, creating new land in a process called progradation. This constant deposition builds sandbars and mudflats that gradually coalesce. The world's largest estuarine alluvial island, Chongming Island, was formed this way, beginning as separate sandbars around 618 AD and merging over centuries. Today, the island covers an area of over 1,200 square kilometers.
This process of land creation is so rapid that the delta front can advance by several meters each year. The low-lying terrain, with an average elevation of 3 to 5 meters, provides ample space for the delta to expand. The interaction between the massive river discharge and the ocean's tidal currents shapes the complex system of channels, shoals, and islands in the estuary. This dynamic environment shows immense geological forces operating on a human timescale.
An Engineered Waterway
The natural tendency of the estuary to build up land creates a constant challenge for navigation. The ports of Nantong and Shanghai are important economic hubs, and maintaining deep-water shipping channels is a constant task. Underwater surveys are used to map the shifting sand shoals and identify areas where sediment is accumulating. To counteract this deposition, extensive dredging operations are required. For example, the approach channel to the Lvsi area of Nantong Port was designed with a bottom elevation of -11.3 meters to accommodate 50,000-tonne bulk freighters. This involves removing millions of cubic meters of sediment annually.
Human activity has also altered the sediment supply itself. The construction of over 50,000 dams in the Yangtze basin, most notably the Three Gorges Dam which began operation in 2003, has drastically reduced the amount of sediment reaching the estuary. Before the dam, the annual sediment load at Yichang station was over 400 million tonnes; after the dam's impoundment, this fell to just 67 million tonnes. Some measurements show a reduction of up to 92.7% at control stations. This has shifted some parts of the estuary from a state of net deposition to one of net erosion for the first time in recorded history, a change that scientists are monitoring closely. Land reclamation projects also alter local hydrodynamics and directly changing the coastline.