Hongze Lake is the fourth largest freshwater lake in China. It is located in the lower reaches of Huaihe River in the west of Jiangsu Province, west of the central part of the North Jiangsu Plain, within the territory of Huaian and Suqian. With a geographical location of 33º06’-33º40’N, 118º10’-118º52’E, it serves as a junction between the middle and lower reaches of the Huaihe River. It used to be a small group of shallow lakes, and known as Fuling Lake in ancient times. Since the Han Dynasties, it was renamed as Pofu Pond. In the Sui Dynasty, it was termed as Hongze Estuary. In the Tang Dynasty, it acquired the name Hongze Lake. After 1128, the Yellow River moved south, flew through the Sishui River, and entered the sea by capturing the downstream river channel of the Huaihe River beneath Huaiyin. The Huaihe River lost the sea channel, so water was retained in the east of Xuyi. The original small lake was expanded into Hongze Lake. With a vast lake surface, abundant resources and a long history, Hongze Lake is not only a large reservoir and shipping hub at the Huaihe River Basin, but also a production base for fishery products, specialties and livestock. It has long been praised as “producing 10 litre of gold within a day”. 

The formation of Hongze Lake can be ascribed to three factors. First of all, the depression caused by crustal fracture is a natural factor contributing to the formation of Hongze Lake. The embryo originated from the small group of lakes before the Tang and Song Dynasties, which was mainly composed of Fuling Lake, Pofu Creek, Nidun Lake and Wanjia Lake, etc. Secondly, the capture of the Huai River by the Yellow River is an objective factor for the rudiment of Hongze Lake. In the 5th year of Shaoxi's reign in the Song Dynasty (1194 A.D.), the Yellow River breached the dike at Yangwu, and was divided into two branches at Liangshan Marsh, i.e., north and south branches. The south branch joined the Sishui River and flowed south into Huai River in the south. This was the beginning of the diversion of the Yellow River. Till the 5th year of Xianfeng’s reign in the Qing Dynasty (1855 A.D.), the Yellow River moved north and entered the sea via Lijin County, and the Yellow River had captured the Huai River for nearly 700 years. As the Yellow River was commanding, flew back into the Huai River and became confluent with the Huai River, the flow augmented and the water level rose, connecting lakes, marshes and depressions, such as Fuling Lake and Pofu Pond, into a single lake. Third, the great construction of Gaojia Weir (Hongze Lake Embankment) is a human factor and a decisive factor leading to the complete formation of Hongze Lake.