Film Noir, Historical Film, Crime Film
Chinatown takes all the film noir tropes and gives them an extra turn of the screw.
Usually, in film noir, there might be some sexual corruption brewing—like adultery in Double Indemnity. In this case, it's incest, which is way more disturbing.
Film noirs also often involve a complicated plot, like in The Big Sleep, but Chinatown amps this up too, using a plot that's extremely intricate but also grounded in historical fact.
It depicts the bygone 1930s private-eye era of the film noir, but changes it in subtle ways—Jake Gittes might seem cynical on the surface, but he's actually expressing real social concerns in trying to unravel this murder and water theft. It retains the inner darkness of the genre, while depicting a sun-drenched California landscape in living color—not the black and white palette of old school film noirs.
The movie is also historical because it's based on true events—although it takes significant liberties. There really was a struggle for control of water rights in Southern California called the "Water Wars" and there really was a dam collapse that killed numerous people through flooding. And the man who designed that dam, William Mulholland, had a name very similar to Hollis Mulwray.
But Chinatown changes history around—the "Water Wars" happened in the 1920s not the 1930s—and uses that history to tell its story. But this film isn't supposed to be a straightforward treatment of history, any more than, say, Citizen Kane is a biopic about William Randolph Hearst.
Finally, the movie is a crime film because it deals with…crime. (Surprise, surprise.) It was meant to revitalize the crime and detective genre and give it new life—which it did—but now Chinatown itself seems like a classic example of the genre, rather than a new kid on the block.
But the crimes Chinatown deals with go beyond murder to deal with greater issues of political corruption and the betrayal of the public—heavier subject matter than your typical stab-happy crime film deals with.