IBM runs a software training event in Las Vegas every year and after the festivities I drove out to Hoover Dam with my friend Elan from high school.
I was there 30 years ago and remembered most things but being there and seeing the awesome scale of this thing is stunning.. The single road crossing the river is narrow and windy on both sides and goes right across the top of the dam. There's a lot of truck traffic and with heightened security at such a sensitive site it just became unworkable. They are building a new road and bridge which will go straight across the valley, parallel to the dam but roughly over the far end of the powerhouses.
The Colorado River was dammed in the '30s for two purposes, first for flood control downstream, second for electricity generation.
The dam paid for its construction and ongoing maintenance by selling the generated power. When we were there only 2 of the 17 turbines spinning which is obviously not peak generation, but only 2/17ths of the water flow capacity was needed downstream. If they run all turbines all the time the lake would eventually drain so there is a tradeoff.