 The establishment of irrigation systems (the most famous one is Dujiangyan, northwest of Chengdu, built during the Warring States period) made agriculture very stable and productive.The Qin and Han empires were actively engaged in the agricultural colonization of the Yangtze lowlands, maintaining a system of dikes to protect farmland from seasonal floods.By the Song dynasty, the area along the Yangtze had become among the wealthiest and most developed parts of the country, especially in the lower reaches of the river.