More well-known in the US of A, perhaps. More noticeable historically, one can give it that (it did predate most of the other mega-dams in the world by a few years). But I think if you were to take a poll outside the US, you'd find more people aware of the three Gorges project than people aware of the Hoover Dam. It's simply been much more of a "hot topic" in recent years.
That said, I'm in agreement with whoever suggested a "mega-dam" small wonder rather than (or in addition to) the Hoover a great wonder. Simply put, there is a missing step in the order of scale : the small hydro dams in the game that power up a city, and then the FIRST mega-dam - but nothing to represent all the OTHER large-scale projects that came afterward - Grand Coulee in the US, Krasnoyarsk in Russia, La Grande 2 and Churchill Falls in Canada, Itaipu in Brasil, Guri in Venezuela, and a few others. And those are the ones whch dwarf Hoover, with twice the output and more (Itaipu has over four time the output of Hoover).
Of course, these super-dam small wonders would not have to be as powerful as Hoover - which should keep its Great Wonder status, I think - but they should be there, and be quite above standard dams in terms of power level.