The river originates in a glacier in the Dangla mountains on the eastern part of the Tibetan plateau. It then runs through the eastern part of Qinghai, turning southward down a deep valley at the border of Sichuan and Tibet to reach Yunnan. In the course of this valley, the river's elevation drops from above 5000 m to less than 1000 m. It enters the basin of Sichuan at Yibin. While in the Sichuan basin it receives several mighty tributaries, increasing its water volume significantly. The river then cuts through Mount Wushan bordering Chongqing and Hubei to create the famous Three Gorges. Eastward of the Three Gorges, Yichang is the first city on the Yangtze Plain. After entering Hubei, the Yangtze receives more water from thousands of lakes. The largest of these lakes is Dongting Lake, which is located on the border of Hunan and Hubei provinces, and is the outlet for most of the rivers in Hunan. At Wuhan it receives its biggest tributary, the Han River, bringing water from its northern basin as far as Shanxi. At the northern tip of Jiangxi, Lake Boyang, the biggest lake in China, merges into the river. The river then runs through Anhui and Jiangsu provinces, receiving more water from innumerable smaller lakes and rivers, and finally reaches the East China Sea at Shanghai. Four of China's five main freshwater lakes contribute their waters to the Yangtze River. Traditionally, the upstream part of the Yangtze River refers to the section from Yibin to Yichang; the middle part refers to the section from Yichang to Hukou, where Boyang Lake meets the river; the downstream part is from Hukou to Shanghai.
Flooding along the river has been a major problem. The rainy season in China is May and June in areas south of Yangtze River, and July and August in areas north of river. The huge river system receives water both from southern and northern flanks, which causes its flood season to extend from May to August. Meanwhile, the relatively dense population and rich cities along the river make the floods more deadly and costly. The most recent major floods were the 1998 Yangtze River Floods, but more disastrous were the 1954 Yangtze river floods, killing around 30,000 people. Other severe floods included those of 1911 which killed around 100,000, 1931 (145,000 dead), and 1935 (142,000 dead).
