BBC Beijing correspondent
After 10 years of construction the Chinese authorities are closing the sluice gates on the massive Three Gorges dam to allow its reservoir to start filling up.
Much criticism has come from inside China
The $25bn project is the largest hydro-electric power project in the world, but years of controversy have surrounded its building.
The Three Gorges dam looms 300 metres above the Yangtze River in central China.
Its construction won't be completed until the end of the decade, but on Sunday the water level in its reservoir will be allowed to start rising and in the coming years the dam will create a lake 600 kilometres long.
More than 600,000 people have already been resettled from areas that will be flooded. Entire towns will disappear.
China's Communist Party leaders have portrayed the dam as a triumph of engineering and of socialism.
They say it will mean an end to catastrophic flooding along the Yangtze and it will provide hydroelectric power to a thirsty Chinese economy.
Hundreds of thousands have had to find new homes
But the dam's detractors are many and vocal. Environmentalists warn of serious damage to the eco-system; engineers have expressed worries about construction standards, and the project has been dogged by allegations of corruption.
Unusually, much of this criticism has come from inside China; the project has been a lightning rod for dissent here.
But as the sluice gates close and the water level starts to rise, China's Communist Party will point to this colossal construction effort as evidence of its technological prowess and its ability to engineer change in China.