A continental re-plumbing
The Hanjiang River Interbasin Transfer is the official name for the Middle Route of China's immense South-to-North Water Diversion Project. This route is a 1,432-kilometer-long engineered artery, designed to move freshwater from the Danjiangkou Reservoir on the Han River to the arid North China Plain, supplying cities like Beijing. Construction on this massive undertaking began in 2003, with the first water deliveries commencing in late 2014. The project's primary goal is to transport 9.5 billion cubic meters of water each year to alleviate chronic water shortages in the north.
To make this possible, engineers undertook a dam modification. The Danjiangkou Dam, first built between 1958 and 1973, was heightened by 14.6 meters. This process, completed between 2005 and 2009, raised the dam's crest elevation from 162 meters to 176.6 meters. The height increase expanded the reservoir's storage capacity by 11.6 billion cubic meters, bringing the total capacity to 29.05 billion cubic meters. The project necessitated the relocation of approximately 345,000 people to accommodate the expanded reservoir and canal routes.
A landscape-scale experiment
Before the first sluice gate opened, hydrological models predicted that diverting such a large volume of water would have significant consequences for the downstream portion of the Hanjiang River. These models forecasted a major reduction in river flow, altering the river's ability to dilute pollutants and transport sediment.
Post-operational monitoring has confirmed these predictions. Studies show the diversion has decreased the monthly flow of the Hanjiang downstream by amounts ranging from 4% to over 74%. This reduced flow diminishes the river's self-purification capacity. Consequently, concentrations of pollutants like total phosphorus and ammonia nitrogen have become a concern, creating conditions that could favor the growth of algae. The diversion also alters sediment transport, which can affect the riverbed's structure and the long-term health of the river's ecosystem. The project is a real-time test of scientific models, demonstrating on a grand scale how inter-basin water transfers affect the donor river system.