The urbanization of the southwestern United States is a modern legacy of Manifest Destiny, that nation-shaping vision of an America settled from sea to shining sea. But there was one problem: westward expansion was predicated on adequate water supplies.

That water was secured through a century of massive government dam-building and irrigation projects, diverting every major and minor water flow in the west. Those days are over. The greatest engineering project in human history has run its course.

"The systems we have built are unsustainable without fundamental change," wrote Peter Gleick, president of the Pacific Institute for Studies in Development, Environment, and Security, in a Dec. 14 Proceedings of the National Academy of Sciences article. "The 20th century approaches used to deal with water challenges are now failing, and new thinking and management approaches are needed."

Gleick's is one of eight new PNAS papers on the future of water in the southwestern United States. The region has become a giant laboratory for arid regions around the world, where limited water resources and growing populations are on a collision course.

The analysis contained in the papers is sobering, but they reinforce what experts have said for years: If people want to live in the desert, they can't avoid its environmental realities.

Image: Grand Coulee Dam./Bureau of the Interior.