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Hydro-electric isn't a green solution, especially if you have displaced a significant amount of trees (which release methane as they rot in the reservoir for the first few years, probably polluting more per MW than a coal plant would for a time) and converted a wetland into farmland (wetlands filter water, farmland, unless it's just organic, tends to pollute waterways and watertables.) To power those 70,000,000 people, that hydro-electric dam has to be considerably larger than Three Gorges. It would be extremely expensive, unless your city is sitting on a lot of oil or a cheap labour pool.