Although the Tang Dynasty (618每907) capital at Chang'an was the most thriving metropolis of China in its day, it was the city of Yangzhou〞in close proximity to the Grand Canal〞that was the economic hub during 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 taxed grain from the Yangtze River Delta to northern China. Minor additions to the canal were made since the Sui to cut down on travel time, but overall no fundamental differences existed between the Sui and Tang Grand Canal.
By the year 735 it was recorded that about 149,685,400 kg (165,000 t) of grain was shipped annually along the canal. The Tang government oversaw canal lock efficiency and built granaries along route in case a flood or other disaster impeded the path of shipment. To ensure smooth travel of grain shipments, the Transport Commissioner Liu Yan (in office from 763每779) had special river barge ships designed and constructed to fit the depths of each section of the entire canal.
After the An Shi Rebellion (755每763), the economy of north China was greatly damaged and never recovered due to wars and to constant floodings 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 ill-fated event reduced the legitimacy of a ruling dynasty and those who perceived it as losing the Mandate of Heaven; this was a good reason for dynastic authorities to maintain a smooth and efficient-running canal system.
The invention of the water-level-adjusting pound lock in the 10th century was done in response to the necessity of greater safety for the travel of barge ships along rougher waters of the Grand Canal.The city of Kaifeng grew to be a major hub, later becoming the capital of the Song Dynasty (960每1279). Although the Tang and Song dynasty international seaports〞the greatest being Guangzhou and Quanzhou, respectively〞and the maritime foreign trade brought merchants great fortune, it was the Grand Canal within China that spurred the greatest amount of economic activity and commercial profit During the Song and earlier periods, barge ships occasionally crashed and wrecked along the Shanyang Yundao section of the Grand Canal while passing the double slipways, and more often than not they were robbed of the tax grain by local bandits. This prompted Qiao Weiyo, an Assistant Commissioner of Transport for Huainan, to invent a double-gate system known as the pound lock in the year 984. This allowed ships to wait within a gated space while the water could be drained to appropriate levels, while the Chinese also built roofed hangers over the space to add further protection for the ships.
Much of the Grand Canal south of the Yellow River was ruined for several years after 1128, when Du Chong decided to break the dykes and dams holding back the waters of the Yellow River in order to decimate the oncoming Jurchen invaders. The Jurchen Jin Dynasty continually battled with the Song in the region between the Huai River and Yellow River; this warfare led to the dilapidation of the canal until the Mongols invaded in the 13th century and began necessary repairs.
During the Mongol Yuan Dynasty (1271每1368), the capital of China was moved to Beijing, eliminating the need for the canal arm flowing west to Kaifeng or Luoyang. A summit section was dug across the foothills of the Shandong massif during the 1280s, shortening the overall length by as much as 700 km (making the total length about 1800 km) and linking Hangzhou and Beijing in a direct north-south waterway for the first time. Just like the Song and Jin era, the canal fell into disuse and dilapidation during the Yuan Dynasty's decline.