Just to clarify, a city must be on a river tile in order to build a hydro plant or the Three Gorges Dam (TGD). The river can't just be within the city's workable area, and lakes/seas/oceans don't count. Gotta be a river, gotta be touching it. There is a similar restriction on The Space Elevator; it cannot be built in a city with a lattitude of greater than 30 degrees (north or south).
If you plan on building TGD - which is a really good idea if you have lots of advanced cities on the same continent - I recommend doing a couple things:
1) Make a dash for Plastics as soon as you reach the industrial era (Steel > Steam Power > Railroad > Combustion > Assembly Line > Industrialism > Plastics). You want to get there first because TGD is an expensive wonder and you don't want to let someone else get a head start.
2) Pick an industrialized city on a river tile and start preparing it to build TGD early. You want to make sure it has high production boosted by a factory and other +hammers buildings, and it should be happy and healthy so that you won't have any unpleasant interruptions halfway through.
Finally, if you have a great engineer sitting around, TGD might be a good place to use him.
