Changes in water types under the regulated mode of water level in Three Gorges Reservoir, China

The creation of a reservoir is often the most serious impact of damming on upstream river ecosystems, which may cause the dramatic change of water types. A giant subtropical reservoir (Three Gorges Reservoir, TGR) formed by the construction of Three Gorges Dam (TGD) has experienced three impoundment stages. According to the combined classification criterion of reservoir water quality management, a study of monthly change of water types was conducted in the mainstream and Xiangxi Bay of TGR from Jun. 2003 to Dec. 2009. Due to the rise of water level from the first stage to the last stage of impoundment, dramatic changes of water types were found during non-flood season for the mainstream, and during most months of the year for Xiangxi Bay. Under the planned mode of water operation, the mainstream would still be characterized by full mixing during the season of algal growth, which prevents the development of algal blooms, even though it experiences extreme drought events. However, Xiangxi Bay is characterized as a stable system similar to natural lakes (e.g., thermal stratification), which may contribute to the development of algal blooms, especially in years of extreme drought.