Natural Barriers. Communication networks via rivers and mountain passes were scarce in China. While the natural lines of communication that went from east to west were only partly usable, there was an almost total absence of links directly connecting North China and South China. From about the area of Kaifeng, navigation down the Yellow River was seriously thwarted by flooding and constant changes of the river's course. Further upstream, the large rocks located at the Sanmen gorge often hindered the progress of grain-laden barges. Because of heavy silt the Wei River was unserviceable for all ships except shallow-draft vessels. Mountain ranges blocked the path of travelers attempting to transport their merchandise from the Upper Yangzi (Yangtze) River valley to North China. Since the Han dynasty (206 B.C.E.-220 C.E.), the imperial governments had tried to use tributaries of the Han and Wei rivers to establish a...