The Sui dynasty (581–618 ce) had harnessed the vast numbers of workers under its control and used them to connect existing natural tributaries into a navigable waterway between the two great rivers. This tied the northern and southern Han to each other more closely than ever before. It took several million slaves five years to do the work, but the ancient problem of how to move supplies south to north had been solved – but not the problem which exists to this day, that of flooding. The
