Massive dams are a terrible idea in any context. They aren't really a good way of generating electrical power, just a way of advertising political and economic power. They're things that governments build as monuments to themselves. They accomplish nothing that a larger number of smaller dams couldn't achieve, other than feeding politicians' egos and pocketbooks. And they do incalculable damage to the surrounding environment -- not to mention the disruption and relocation of surrounding populations and the irrecoverable loss if historical or archaeological sites are flooded.
It's already virtually a given that China's massive Three Gorges Dam -- whose construction process has been plagued with corruption and shortcuts that render it far less safe and structurally sound than the government insists on pretending it is -- is doomed to fail and cause a disaster of monumental proportions, probably within our lifetimes. It's already created a human disaster through the forced relocation and disenfranchisement of millions of residents of the area to be flooded, without sufficient effort being made to find them new homes (again due to rampant corruption, with money meant for that process going into local officials' pockets along with the money meant for constructing the thing safely). All because of human hubris and the naive belief that bigger is better. The last thing we should do is make the same stupid, tragic mistake.