The sea north of the Yangtze's mouth is shallow because of silt deposited by the river, while the waters to the south are deeper and dotted with islands.

Consequently, ships coming from southern coastal provinces like Fujian and Guangdong had to transfer their cargo to ships suitable for the waters to the north and vice versa, Xue explains.

Lindsay's report on his findings contributed to Shanghai's inclusion in the Treaty of Nanking that the British forced China to sign in 1842.

This opened the city to merchants from Britain and, later, other Western nations, such as France, Germany and the United States.