Sediment-laden flood is the main reason for the natural evolution of rivers and lakes. Flood can not only shape rivers, but also form lakes and wetlands in the downstream depressions. More than 2,000 years ago, the Yangtze River flooded into Yunmengze, a huge lake wetland with an area of more than 7,300 square kilometers, which is now Jianghan Plain. Because people's productivity was low at that time, they were unable to build flood control reservoirs, dikes and other projects, and Yunmengze played a role in regulating and storing floods. Because of the small population at that time, the contradiction between land and flood was not prominent, so the flood was not obvious. After that, a large amount of sediment brought by the Yangtze River flood led to the gradual siltation of Yunmengze and the shallowing of lakes, which provided conditions for people to build dikes and farmland, and Yunmengze gradually became smaller until it died out.

Flood is a natural phenomenon, and there must be a way out. The process of Yunmengze becoming smaller and disappearing is exactly the process of Dongting Lake becoming bigger. During the Shang and Zhou Dynasties more than two thousand years ago, Dongting Lake was just a small lake in the southwest of Yueyang. Xiang, Zi, Yuan and Li meet in Dongting Plain and flow into the Yangtze River respectively. At this time, the location of the lake area is high, and it is difficult for the Yangtze River to enter the lake. After the Three Kingdoms period, Dongting Lake gradually subsided and expanded, Yunmengze silted up and gradually built dikes. The Jingkou and Cangkou streams of the Yangtze River meet and begin to enter the Dongting Lake Plain. Xiang, Zi, Yuan and Li have entered the lake. The Yangtze River passes through the lake, and its banks are connected with the lake, but its scope is only in the Dongting Lake area. Around A.D. 1 154, the area of Dongting Lake has reached 6 000km2. 1524, Haokou, the last cave in Jiangbei, was blocked, and Jingjiang levee was fully connected. Jiangbei Lake was separated from the Yangtze River, and Yunmengze's storage function completely disappeared. The completion of Jingjiang levee has created a rich Jianghan plain, but at the same time it has further raised the flood level of the Yangtze River. At this time, the flood has the opportunity to enter the Dongting Lake area with higher terrain, so that it will gradually replace Yunmengze and become the main lake for flood storage in the middle reaches of the Yangtze River. During the Ming and Qing Dynasties and the flood period, its lake area was about 6 270km2, which was about 2.3 times that of the present lakes. It became the largest freshwater lake in China, known as the "800-mile Dongting", and then it began to deposit and its area decreased. 1852 and 1870 floods caused the breach of the lotus pond and Songzi, which led to the situation that the lake crossed, the lotus pond and Songzi entered the lake respectively, and the Jingjiang branch flowed into the lake sharply, which directly affected the re-evolution of Dongting Lake. At the end of the Qing Dynasty and the beginning of the Republic of China, due to the large flow and sediment transport of Ouchi River and Songzi River, the sediment entering Jingjiang River increased sharply, and the silt continent expanded rapidly, which greatly reduced the lake surface, and the lake surface was about 90% of the previous one. From the early years of the Republic of China to more than 30 years after the founding of the People's Republic of China, sediment deposition was more rapid, the dikes in Gaozhou were more abundant than before, and the lake surface was relatively reduced, accounting for about 72% of the lake surface in the middle of Qing Dynasty. Due to continuous siltation and blind reclamation, Dongting Lake is obviously divided into three parts: east, west and south. In the 40 years since the founding of the People's Republic of China, Dongting Lake has been managed in a planned way, dikes have been reinforced, spillways have been arranged, and dangerous work has been reduced. At the same time, some new flood storage and detention areas and reclamation areas have been built, and lake branches have been reclaimed and flood skimming projects have been carried out, which have changed the agricultural production in the lake area and greatly reduced the area of the lake area. The natural lake area is about 44% of the lake area in the middle of Qing Dynasty. Today's Dongting Lake has been divided into East Dongting Lake. Small lakes such as South Dongting Lake, Muping Lake and Qixing Lake. According to statistics, the lake area 1949 was 4 350km2, which decreased to 2 740km2 in 1977 and 2 69 1km2 in 1983, ranking second in China.

Judging from the evolution of Yunmengze, Dongting Lake and the Yangtze River, the two lakes played an important role in flood control in the middle reaches of the Yangtze River. Without the regulation of the two lakes, the middle reaches of the Yangtze River will not only be flooded, but also inevitably lead to the diversion of the Yangtze River.