If you have Hoover Dam, you dont' need to build any type of power plant (hydro, power, nuclear, solar)
You can only build hydro plants in cities near rivers.
My advice: Don't build hydro plants. They are too expensive and hard to build in all your cities. However, DO build the Hoover Dam. You can build it anywhere. And because it replaces all power facilities with hydro power, you get a boost in production, a reduction in pollution, a big economic bonus (since you don't have to pay upkeep on power facilities), and a big savings of industry (since you don't have to waste time building several hydro plants). Later on pollution will occur in your cities despite the presence of hydro power. If so make sure you have a mass transit and/or a solar plant.
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