Criminals usually want to steal suitcases with mysterious glowing objects in them (Pulp Fiction) or expensive bird statues (The Maltese Falcon) or money from a crashed drug smuggling plane (A Simple Plan)—things like that.
But in Chinatown, the bad guys are stealing water. Yeah. Two hydrogens and an oxygen. Big deal, right?
Water is sort of an unusual thing to steal. It's a natural element—it's already free in nature. But when you live in a desert community in California, the need for water can become desperate, especially when you consider that, in Chinatown, L.A.'s not just in the state of California—it's also a state of drought.
Hollis Mulwray wanted to make sure that the water would be publicly owned, but Noah Cross, his dastardly partner, wanted to continue to own the water company privately—that way, he'd be able to make a profit and, maybe more importantly, maintain power and control over the city via the water supply.
Cross's trying to dominate a crucial natural resource, because if you can control one thing that's totally necessary for life (like water) naturally you can control everything else, outstripping or buying out the power of the mayor and others.
Just check out this exchange:
GITTES: I want to know what you're worth—over ten million?
CROSS: Oh, my, yes.
GITTES: Then why are you doing it? How much better can you eat? What can you buy that you can't already afford?
CROSS: The future, Mr. Gittes—the future.
Noah ultimately wants more than money—he wants power, power, power. His desire for possessing something that isn't rightfully his is mirrored by the incest he commits, and by his attempt to gain possession of the daughter/granddaughter born from that act of incest. They're both grotesque and illegal uses of power over others.
The screenwriter Robert Towne originally wanted to write a trilogy—Chinatown would've been the first installment—all involving the struggle for natural resources. Chinatown was about the water wars, while the second movie The Two Jakes involved the oil industry.
But The Two Jakes was a major failure, thus destroying plans for the third movie, Gittes vs. Gittes, which would've been about land. (Source)
And that, in a word, sucks. Who would have thought that the follow-up to what people cite as "an essential screenplay every aspiring screenwriter must read" and the third best screenplay ever written would have flopped harder than a fish in the dry L.A. river?