<P> The Grand Canal at this time was not a continuous, man - made canal but a collection of often non-contiguous artificial cuts and canalised or natural rivers . </P> <P> Although the Tang dynasty (618--907) capital at Chang'an was the most thriving metropolis of China in its day, it was the city of Yangzhou--in proximity to the Grand Canal--that was the economic hub of the Tang era . Besides being the headquarters for the government salt monopoly and the largest pre-modern industrial production center of the empire, Yangzhou was also the geographical midpoint along the north - south trade axis, and so became the major center for southern goods shipped north . One of the greatest benefits of the canal system in the Tang dynasty--and subsequent dynasties--was that it reduced the cost of shipping grain that had been collected in taxes from the Yangtze River Delta to northern China . Minor additions to the canal were made after the Sui period to cut down on travel time, but overall no fundamental differences existed between the Sui Grand Canal and the Tang Grand Canal . </P> <P> By the year 735, it was recorded that about 149,685,400 kilograms (165,000 short tons) of grain were shipped annually along the canal . The Tang government oversaw canal lock efficiency and built granaries along the route in case a flood or other disaster impeded the path of shipment . To ensure smooth travel of grain shipments, Transport Commissioner Liu Yan (in office from 763 to 779) had special river barge ships designed and constructed to fit the depths of each section of the entire canal . </P> <P> After the An Shi Rebellion (755--763), the economy of northern China was greatly damaged and never recovered due to wars and to constant flooding of the Yellow River . Such a case occurred in the year 858 when an enormous flood along the Grand Canal inundated thousands of acres of farmland and killed tens of thousands of people in the North China Plain . Such an unfortunate event could reduce the legitimacy of a ruling dynasty by causing others to perceive it as having lost the Mandate of Heaven; this was a good reason for dynastic authorities to maintain a smooth and efficient canal system . </P>

How did the completion of the grand canal impact china