The lake that keeps taking villages

Stand on the old wall at Yueyang in March and Dongting Lake is a wide sheet of water with grass banks; come back in August and the far shore may have disappeared. The lake is the flood basin of the Yangtze River: between July and September, flood water from the Yangtze pours in and can swell it from a normal 2,820 square kilometres to as much as 20,000, carrying vast amounts of water and sediment from the Chang Jiang. Four great rivers of Hunan also empty into it — the Xiang from the south, the Zi from the southwest, the Yuan from the west and the Li from the north — along with smaller streams, the most famous being the Miluo. All that water has one way out: a natural channel about 14 kilometres long and on average a kilometre wide, past Chenglingji and back into the Yangtze. The two provinces on its shores are named for where they sit relative to this water: Hubei means north of the lake, Hunan south of the lake. Because the lake answers to rain and river flow, its level can swing by nearly thirteen metres in a single year, and its surface shrink from thousands of square kilometres in flood to a few hundred in the dry season. It is less a fixed lake than a slow-moving argument between rivers, silt and the plain.

The lake's history is a story of displacement in both directions. Under the Han dynasty the Yangtze's floods spread into the Yunmeng Marsh far to the north of today's lake; as embankments rose along the river's north bank, the water that had gathered there was diverted south into Dongting, so the lake's great size was, increasingly, the work of human hands. Farmers followed the silt in. For centuries this was China's largest freshwater lake, praised as eight hundred li of Dongting, and its shallows were reclaimed field by field: by the nineteenth century much of the shallow water had been destroyed to create farmland, and a further round of wetland drainage after 1949 took most of what remained. Chinese records put the flood-season surface at about 6,000 square kilometres in the Qing dynasty, falling to 2,625 by 1983. The shrinking lake broke into named pieces — East, West and South Dongting, Datong Lake, Muping Lake, Qili Lake — as diked polders closed off its arms, and each polder held fields, and people farming them. After the Yangtze flood of 1998 came the order to reverse course: the 4350 project, meant to return the lake to 4,350 square kilometres of high-water surface by 2015 by moving people out of the diked fields and into planned lakeside cities. It was never fully carried out. The people, the dike lines and the water are still negotiating.

Managing the lake is now a permanent public works. Dikes and polder embankments still ring its farmed arms, and at flood alert the gates and channels decide which fields go under first. Protection covers a growing share of what is left: the lake has been a Ramsar wetland site since 1992, the East Dongting reserve became a national nature reserve in 1994, and the waters winter more than 130 species of birds. One animal ties the lake's health to the whole Yangtze system: the finless porpoise, of which perhaps 1,400 survive, several hundred of them in Poyang and Dongting. Recovery is uneven. In the spring drought of 2010 the measured water area fell from about 1,649 square kilometres to 382, and grassland grew where the lake had been. In 2007, when flood water was released from the Three Gorges Dam, an estimated two billion reed voles fled the lake's islands into farm villages, damaging crops and dikes until some two million of them were killed. A joint Chinese-Norwegian biodiversity restoration project began in 2005. The oldest hazard is quieter: schistosomiasis parasites still cycle through the shallows, and health authorities tell lakeside residents to avoid direct contact with the water itself.