Lake Dongting, in northeastern Hunan, is the second largest freshwater lake of China, with an extensive catchment area including the whole of Hunan and parts of the neighboring provinces, Hubei, Sichuan and Guizhou. It serves as a great retarding basin for the Chang Jiang (Yangtse River), from which flood water pours into the lake in July-September. The inflow from the Chang Jiang amounts to more than a half of the total water influx to L. Dongting, and carries with it a tremendous sediment load of 140,000,000 m³ yr^-1 on average.

One hundred and fifty years ago, the lake was much bigger (6,200 km²) than it is now (2,740 km²), but the rapid progress of sedimentation and land reclamation works have since reduced its size, resulting in the formation of three more or less separate basins, West, South and East Dongting. In the summer flooding season, however, the three lakes unite into a single water body of about 3,900 km² surface area. The water from the Chang Jiang flows mainly into the West and East Lake through three channels, and is drained from the northeastern corner of East Lake at Yueyang directly into the Chang Jiang again. The annual range of water level fluctuation in normal years ranges between 6.5 m (W. Lake) and 17.8 m (E. Lake).

The greatest environmental problem for L. Dongting is the rapid sedimentation at a rate of ca. 5 cm yr^-1 on a whole lake average) that causes trouble for surface transportation by boats, diminishes the lake's capacity for water storage and flood control, and affects fishery production through changes in the aquatic ecosystem. The Provincial Ordinance for Environmental Protection formulated in 1981, therefore, allows for no further reclamation work in or around the lake.

The heavy application of pesticides to rice fields as well as industrial development in recent years has caused local contamination of rivers and small attached lakes with BHC, heavy metals and organic wastes, but the main lake as a whole maintains fairly good water quality, with a mesotrophic nutrient level.