Nevertheless, the workforce managed to reverse the original south-to-north flow of the Dongjiang by raising the water level in the upstream section by 46 meters and transferring the water by canals and pipelines.

And in March 1965, Hong Kong started drinking fresh water from the mainland.

The project provided 68.2 million cubic meters of water that year. It could be also regarded as China's earliest water rights trade, according to Long Liangqing, manager of the Guangdong Yue Gang Water Supply Company.

And it has flowed ever since, even through the chaotic years of 1966-1976, when China's Cultural Revolution stalled the economy. An accumulative 22.35 billion cubic meters of water has reached Hong Kong through the project to date.

Some 7.6 billion yuan has been spent over the years on upgrading the infrastructure. Its annual water supply to Hong Kong now stands at 2.4 billion cubic meters, meaning 80 percent of the special administrative region's drinking water comes from the Dongjiang River, according to Long.

The 1.1 billion cubic meters of water now stored by Hong Kong means it will avoid serious drought in future, he adds.

The water is exceptionally clean too. As China has industrialized, factories have spewed pollution into rivers nationwide. But the Dongjiang River's long-shore cities like Heyuan and Huizhou chose moderate industrial and economic development.

Lai Shouxiong, deputy director of Heyuan's water resource bureau, says the city, under-developed by the standards of the Pearl River Delta, has refused over 60 billion yuan in investment by more than 500 industrial projects which would have threatened water pollution.

The city has also spent 70 million yuan on sewage treatment. "There are 400 million people relying on the river for safe water," Lai says. "This is less about money and more about social responsibility."

For 50 years, the quality of water from the project has been officially rated above average, flowing through four pumping stations. The Yue Gang Water Supply Company tests its water for 109 indexes.

"Most of the detection equipment has been imported," says Lin Qing, director of the company's water environment monitoring center. "Science has played an important role in safeguarding water quality."

"The age of huge workforces of people people such as Li Jianhui being recruited for public service projects has gone," Lin says. "But the sustainable supply of high-quality water to Hong Kong will never stop."