Rivers have always been China's main method of transport. To link up the predominantly east-west system of rivers, canals were first built in the 5th century BC. Rulers of the rival states were able to set vast numbers of labourers on to ambitious canal-building projects, such as that linking the valleys of the lower Yangzi and Yellow River.
Both had large populations constantly devastated by floods and famines. Being able to transport food to them and their army garrisons by canal became important for political and military control.
In 486 BC the king of Wu built the Han Gou canal as a supply route for his troops fighting in the north.
One of the biggest canal projects was in Qin to bring water to an entire province. The agricultural prosperity this brought helped its king successfully to attack and defeat the other six rival states and become the first emperor of all China.
Some of these 2,500 year-old canal systems can still be seen today.
