Since antiquity the channel of the Huang Ho (Yellow River) in its lower reaches has shifted unpredictably. Its outlet to the sea was sometimes to the north of the coast, sometimes in the middle, and sometimes to the south, and the northernmost and southernmost points were practically 600 kilometres apart. In A.D. 11-1048, the period including the T'ang dynasty, the Huang Ho generally emptied itself into the sea within the area administratively called Shantung or, in other words, by following what can be termed the middle route, the course of the river in approaching the sea was similar to the course of the same section of the river today, though, it was a little to the north of the present course. This section of the river flowed past a total of 14 chou and 38 hsien (see Map). During this period the Huang Ho was stable in its course and it was just because of this stability that the river was scarcely mentioned in the annals of the Tang dynasty. In the T'ang as in the preceding Han period, the Huang Ho was to some extent serviceable in both irrigation and navigation, especially in the transportation of foodstuffs. At that time, the foodstuffs produced in Kuanchung Area were far from sufficient to meet the demand at the capital city of Changan and the supply had to depend on eastern China, especially on the southeastern area drained by the Chang Chiang (Yangtze River) and the Huai Ho. The transportation of foodstuffs was made possible by connecting the main channel of the Huang Ho with the flanking canals, including the famous Pien Ho. The Pien Ho, a long canal created simply by interconnecting the natural water courses on the southeast side of the old huge alluvial fan of the Huang Ho, was the most important of all during the T'ang period. After flowing past Kaifeng, the Pien Ho swerved to flow southeast and passed a total of 26 cities, covering a distance of 800 li (or 380 kilometres) and it was shorter than the Old Pien Ho of the Sui period. The Old Pien Ho continued to exist after the New Pien Ho was open to traffic, and down to the middle years of the Northern Sung dynasty it was still partly navigable. Evidence of this is found in Sung poetry and prose. Though still partly navigable, the Old Pien Ho was long shunned by government boats because it was longer and circuitous. The initial slope of the old alluvial fan of the Huang Ho was roughly 1:6,000, and a large number of sluices had to be built; consequently, the canal was not deep enough for large-sized vessels. It was mainly through the buoyancy of water that flat-bottomed boats could sail on the canal to carry "taxation rice" from the Chiang-Huai Area to meet the demand of the political centre in the northwest. In a precious painting named Ch'ingmingshanghot'u dating from Sung we can see dozens of such wide dwarfted canal boats. Loaded with much of the muddy water of the Huang Ho, the Pien canal became easily silted and constant dredging was thus necessary. During the T'ang dynasty, it was customary to dredge the canal once a year and the practice was kept even in the early years of Northern Sung. Later, because of the negligence in attending to this important task, the bed of Pien Ho rose higher than the confining banks. In 713-755 when the T'ang Empire was in the apex of its glory, the foodstuffs carriedover the Pien Ho were in excess of a million shih every year, and in 744 the all-time high was four million shih. The foodstuff traffic over the Pien Ho was an impetus to the economic development of the Chiang-Huai Area. As a result, population swelled, in one vast area embracing what is today's Yangtze Delta, the population increased by six times in the span of 150 years. The economic boom of the Chiang-Huai Area also helped the growth of the transportation over the Pien canal. At the junctions of overland routes and waterways appeared several big cities, the best known of which were Yangchou (揚州) and Ch'uchou (楚州). The water volume of the Pien Ho depended directly on the flow of the Huang Ho. In summer and autumn, as the Pien Ho carried the largest volume of water that it could carry, floods were not infrequent. In spring and winter, the canal flowed low and navigation was hazardous. It was therefore a custom to dredge the canal in the low-water season. After the revolt engineered by An Lu-shan and Shih Szu-ming, the Pien Ho was clogged for practically 18 years. Though at a later time the condition was improved, the T'ang Empire was already on the verge of decline; the largest quantity of foodstuffs ever transported was barely 1,100,000 shih and the lowest figure was only 500,000 shih, much less than the amount recorded in the best years of T'ang China. Down to the last years of the T'ang Empire, the wealthy Chiang-Huai Area was actually controlled by the powerful warlords, and the Pien Ho was again closed to traffic. In 897, through the ravages of war, the lower reaches of the canal deteriorated and degenerated into marshes and its function as a foodstuffs-carrying waterway was totally lost. It seems that the rise and fall of the Pien canal is chronologically identical with that of the T'ang Empire.