I remember a speculative documentary series called "Life After Humans"" which tried to imagine what would happen to civilization if humans all just suddenly disappeared one day.

They suggested that Hoover Dam could keep producing electricity unmanned for weeks or months. In fact, I think they said the first thing to cause a failure there would be an invasive mollusk species (barnacles, maybe?) in the Colorado River that grows in the channels of the dam that have to be constantly scraped out.