The turbines at Hoover Dam have a nameplate capacity of 2.08 GW, making it one of the largest hydro plants in the country and equal to nuclear and big coal plants. The Dam certainly is immense, 650 feet long and 229 feet tall. Lake Mead stretches for 110 miles behind it.
Who are the principal contractors for [Hoover Dam] energy?
The States of Arizona and Nevada; City of Los Angeles; Southern California Edison Co.; Metropolitan Water District of Southern California; California cities of Glendale, Burbank, Pasadena, Riverside, Azusa, Anaheim, Banning, Colton, and Vernon; and the city of Boulder City, Nevada.
Interesting, isn’t it, that much of the power goes to southern California. Power (and water) in the US west is like that. It often comes from hundreds of miles away.
Hoover Dam has all the cheery ambience of a grumpy military encampment. Armed guards inspect your vehicle before you enter. Signs in the parking lot warn that video cameras are everywhere, specify what size packs can be carried, and sternly admonish that no knifes are allowed. This presumably to prevent someone with a Swiss Army Knife  from hijacking the dam. Yes, I understand the need for security but it was way too militarized for me, as increasingly are many of our public facilities.
Hydro does indeed create clean energy but at the cost of flooding huge swaths of land. However the resultant lakes can also become popular recreation areas as Lake Mead has. After several decades though, lakes behind dams start to silting up. Lake Mead “lost about 15 percent of its capacity between 1936 and 1964 due to silting–more than five million acre-feet.” For this reason, big hydro is viewed suspiciously by some renewable energy advocates. As for building new big hydro plants, this is probably a moot question, as most of the good locations are already taken.
