When rivers are dammed, it affects the amount of water available to the surrounding area as well as the ecosystems both up and downstream from the dam. These changes can destroy wetlands and further threaten already endangered species of plants and animals. The dams themselves require infrastructure that can harm the environment. The larger the power plant the more effect the dam has on the surrounding area and the larger the area it affects.
A power plant is never too big and powerful until there is a spill and contamination ensues. Radiation is a strong wake up call for any country with such a powerful facility as Three Gorges Dam. Hopefully there are strong, rigorous standards that they are made to follow or there will be a cataclysmic occurrence sooner than later.
There are a number of things that can go wrong with huge power plants. Firstly, they concentrate pollution within a particular area and cause considerable damage to the local people and the environment. Secondly, if they are to suffer a meltdown or a fault of some kind the amount of damage that they can cause to the local can be catastrophic. In addition to this, the utilities system can be over reliant on these power plants and if they experience problems they can cause widespread and long lasting power shortages.
The power plant uses hydroelectricity to supply power to a large number of people in China, which has more people then almost any other country. The biggest risk is that the power plant will fail and there will not be enough backup power to prevent a major power outage in China.