River–Lake System Connectivity Effectively Reduced the Salinity of Lake Water in Bosten Lake, Northwest China

High salinity in water constitutes a serious problem for the aquatic environment management of Bosten Lake. Weak water exchange and water movement are the essential factors for the high total dissolved solids (TDS) content of lake water. To improve the water quality of Bosten Lake, a river–lake system connectivity project (water diversion) was introduced starting at the end of 2018, which diverted fresh water from the Kaidu River and the Huangshuigou River to Bosten Lake. In this study, the effect and its mechanism of water diversion on the TDS content of Bosten Lake were evaluated using continuous-field monitoring data. The results showed that the water diversion effectively reduced the TDS content of the lake water and changed Bosten Lake from a brackish lake back to a freshwater lake. Water diversion also improved the spatial distribution of TDS content. One year, two years, and three years after the implementation of the water diversion project, the TDS content of northern, southern, and eastern lake water significantly decreased by more than 20%, 25%, and 30%, respectively. Our study demonstrated that water diversion significantly increased the annual endogenous TDS pollutant amount discharged from the lake and reduced and homogenized the TDS content of the whole lake. TDS content reduction of the lake was realized by water diversion accelerating water movement and water exchange in the lake, especially for the northern, southern, and eastern waters. Therefore, water diversion could be used as an effective measure for water quality improvement in semi-closed inland lakes in arid areas.