A portion of China's massive South-to-North Water Diversion Project has started to supply water.

Shandong province will get about 1,200 million cubic feet of water in the first use of the project's east route, officials said, China Daily reported Tuesday.

The three-route project, expected to cost $81 billion, is considered the biggest engineering endeavor in Chinese history, and involves a mix of canals, tunnels and aqueducts spanning thousands of miles. It is designed to rely entirely on gravity to transfer 1,582 billion cubic feet of water annually from the country's water-rich south to the arid north, including Beijing.

The east route's more than $8.2 billion first phase was completed in March, China Daily reported.

Water diversion for that phase started last month, bringing water from the Yangtze River in Jiangsu province to Shandong along the Beijing-Hangzhou Grand Canal. It is expected to supply as much as 310 billion cubic feet of water annually to Jiangsu, Anhui and Shandong provinces, benefiting about 100 million people, the project office said.

"The amount of water supplied will be adjusted annually according to the shortages in those provinces as well as the inflow in the route's upper reaches," China Daily quoted an unnamed project official as saying.

Shandong, for example, experiences an annual water shortage of 141 billion to 176 billion cubic feet, the province's department of water resources said.

Although China has 20 percent of the world's population, it has just 7 percent of the world's freshwater supply.

China's top leaders Sunday praised the progress of the water diversion project and called for continued efforts to ensure the success of ongoing construction.

"We should strengthen management and keep working to ensure steady progress on the project and stable water quality," President Xi Jinping said.

Environmentalists have opposed the project, citing possible regional climate changes, environmental damage, impact on agriculture and disruption caused by massive relocation efforts.

International Rivers says an estimated 330,000 people recently were relocated for the expansion of the Danjiangkou reservoir, which had marked the beginning of the project's middle route.

The water project's construction committee said Monday a key section, nearly 380 miles, of the middle route has been completed, China's state-run news agency Xinhua reported.

The entire middle route, scheduled for completion by the end of this year, will supply water beginning in 2014 for 19 major cities and more than 100 smaller towns in northern China.