A river that builds land
The Yangtze River Delta is a landscape built by water and sediment. As the third-longest river in the world, the Yangtze carries a tremendous sedimentary load from the interior of China to the sea. Historically, this discharge was estimated at over 470 million tons annually. This constant deposition of silt and mud has been pushing the coastline of the East China Sea further eastward for millennia. Around 7,000 years ago, as sea levels stabilized after the last glacial period, the delta began its steady advance.
The rate of this natural land creation, or progradation, has not been constant. About 2,000 years ago, the process accelerated dramatically. The rate of delta growth jumped from an average of 38 meters per year to as much as 80 meters per year. This change is attributed to increased human activity in the river basin, such as deforestation for agriculture, which led to greater soil erosion and more sediment being washed into the river. The result is a vast, fertile plain that today is one of the most densely populated and economically active regions on Earth.
Engineering a new coast
Humans benefited from the river's gift of land; they actively engineered it. Beginning as early as the fifth century BCE, and intensifying during the Tang and Song dynasties, farmers and officials began constructing dikes, or seawalls, to enclose tidal flats. This practice created polders, known in Chinese as weitian. A dike would be built on the tidal flats, and the area behind it would rapidly fill with the river's sediment, raising it above sea level. Once filled, the new land could be drained and converted into highly productive rice paddies.
This process has continued into the modern era with industrial-scale projects. The city of Shanghai, on the delta, has added over 580 square kilometers of land since 1985. One example is Nanhui New City (formerly Lingang), a massive urban development built on land reclaimed from Hangzhou Bay. Between 2002 and 2006 alone, over 133 square kilometers of new land were created for this project.
This centuries-long process of land creation is now under threat. The construction of tens of thousands of dams throughout the Yangtze basin, most notably the Three Gorges Dam which became operational in 2003, has drastically cut the amount of sediment reaching the delta. The sediment load measured downstream from the dam dropped by as much as 84% compared to pre-dam levels. With less sediment arriving to replenish it, the delta's growth has slowed, and some areas now face the threat of coastal erosion.