A basin for a giant
Dongting Lake is China's second-largest freshwater lake, but this ranking is part of its story. Its area and volume are in constant flux, governed by a dramatic seasonal flood pulse. In the dry season, from October to April, the lake's surface area is typically around 2,820 square kilometers. During the summer monsoon season, between June and October, the lake swells to absorb floodwaters from the Yangtze River, expanding its surface area to as much as 20,000 square kilometers. This eightfold increase in size is a result of its unique relationship with the Yangtze, for which it is a retention basin.
four major rivers: the Xiang, Zi, Yuan, and Li—flow into Dongting from the south and west, but its primary water exchange is with the Yangtze. During the summer floods, up to two-fifths of the Yangtze's water can pour into the lake, raising its level by as much as 15 meters. This inflow carries enormous quantities of nutrient-rich sediment, which historically made the surrounding lands exceptionally fertile. This process of expansion and contraction drives the entire ecosystem, dictating the life cycles of the flora and fauna that depend on it. Historically, the lake was much larger, known as the "Eight-hundred-li-Dongting," but centuries of sedimentation and land reclamation for agriculture have reduced its permanent size.
An ecological clock
The annual flood pulse is a precise timetable for hundreds of thousands of migratory birds. As the floodwaters begin to recede in autumn, vast, shallow mudflats are exposed. These newly revealed areas have plenty of food, including mollusks and the tubers of aquatic plants, creating an ideal foraging ground for waterfowl. In February 2022, a record-high 404,000 waterfowl from 74 species were recorded at the lake.
The East Asian-Australasian Flyway is a major migration route, and Dongting Lake is a wintering site. Birds like the Siberian crane (Grus leucogeranus), swan goose (Anser cygnoides), and bean goose arrive to feed on the post-flood bounty. The receding water also concentrates the lake's 114 fish species into smaller pools, making them easy prey for other birds and the lake's most famous mammal, the critically endangered Yangtze finless porpoise (Neophocaena asiaeorientalis). The entire ecosystem follows this aquatic rhythm, a process that has been altered in recent decades by the construction of dams, including the Three Gorges Dam, which has changed the timing and volume of the flood pulse.