Jim nailed it. Water has been the story of the west for pretty much the entire history of American settlement there, although the general populace is still not quite aware of this (even here in the west). Controlling the flow of the waterways and subsequent fights over access to the water there pervades every phase of the build up of the American west.
Humans can't survive without water, and where we are, it just doesn't fall from the sky all that much. What little does fall collects in streams and rivers in the places it falls, and we take that water and divert it for our uses. This works well for small numbers of people, but as the numbers increase, the water available does not. It just doesn't scale. It doesn't help that in addition to the scads of people out here, we're filling up the empty land with farming and ranching, which are both big water consumers.
The once mighty Colorado River now doesn't even empty to the ocean. It's literally sucked dry by the states that it passes through, and ends up a muddy trickle by the time it leaves Arizona. I got to drive past the Colorado River delta in Mexico a couple of years ago, and it's just salty mudflats now.
Changes in water usage patterns along with waste treatment options (like toilet to tap) can buy us some time, but the growth rate is still unsustainable. One might see that as a self-limiting problem, since scarcity of water will just cause the growth to stop. That might be true if it just organically stopped everywhere at the same time. But, if you get some areas that keep growing out of proportion to other areas, the water wars that Jim alludes to will ultimately result in some area no longer getting enough water to support their existing population. I think that's the start of a death spiral for whatever town or county or state that ends up on the losing side of that battle.