Not surprisingly, as the drought deepened – and the system on the U.S. side of the border got tighter and tighter – less and less spring floodwater reached the Delta. "We were losing trees, and we were losing good habitat," says Zamora. "So we decided that we had to do something."

Beginning in 2002, he and Osvel Hinojosa, a 38-year-old, pony-tailed, improbably upbeat ecologist from the Mexican environmental group Pronatura, cobbled together money for a series of small-scale restoration projects. The pair became increasingly well-versed in the nuances of grant-writing and heavy equipment contracting. They rented backhoes to rip out tamarisk; leveled land with laser-guided mechanized graders; installed irrigation systems; and tended cottonwood, willow and mesquite seedlings by hand in a nursery in Carranza, before driving them out to the river, where local crews carefully planted them.

Zamora and Hinojosa envisioned a network of demonstration projects that would not only provide a functioning ecological corridor within the Delta, but also help people understand what an even more ambitious restoration effort might look like. The sites were small, says Zamora, "but once people saw them, they wanted to see more."

Replanting native forest was just part of the challenge. The bigger one was reintroducing the force that energized the Delta's natural dynamics: Water.

And so, even as the water agencies labored to ensure that no "fugitive" water escaped across the border, Hinojosa's boss, Pronatura director Martín Gutiérrez, successfully lobbied to change Mexican law so that nonprofit environmental organizations could purchase water for environmental restoration. In 2008, Pronatura, the Sonoran Institute and the Environmental Defense Fund formed the Colorado River Delta Water Trust, which has subsequently purchased some 3,200 acre-feet of water from local farmers to irrigate the trees in a web of restoration sites that now exceeds 800 acres. The thick cottonwood and willow forests began to spring to life with birds, including long-vanished species like the yellow-billed cuckoo and yellow-breasted chat.

But truly restarting the ecological heart of the Delta was going to take another tremendous pulse of spring floodwater, like the ones that had come charging downriver during the '80s. And to make that happen, the environmental groups had to find a way into the inner sanctum of the river's mighty bosses.

The three most powerful urban water agencies in the Western U.S. are the Metropolitan Water District of Southern California, which supplies Los Angeles and San Diego, the Southern Nevada Water Authority, which supplies Las Vegas, and the Central Arizona Project, which supplies Phoenix and Tucson. As the drought wore on, the Big Three faced a deepening crisis.

Lake Powell and Lake Mead are essentially gigantic emergency water reserves: Together, they hold four years' worth of the river's average flow. But by 2007, both were nearly half empty. That spurred the Big Three's bosses to develop a set of shortage-prevention measures – voluntary conservation strategies to slow the reservoirs' fall and keep levels from dropping to the point where the system couldn't supply enough water to meet each state's yearly needs.

The crisis was part of a bigger problem that has long plagued the river: Bad math. The foundation of the law of the river, the 1922 Colorado River Compact, assumes that an average of 16.8 million acre-feet of water is available each year. But the river's actual long-term average flow is about 13 percent less. And computer models suggest that climate change will deepen that shortfall by another 10 to 30 percent by mid-century.

Shackled together by the very river whose water they had long reveled in fighting over, the water bosses began realizing that they might have more to gain through cooperation than through warfare. They also recognized that, to spread the impacts of the drought and soften the blow to their own agencies, they had to convince Mexico to agree to share in future shortages.

"That," says Patricia Mulroy, the blunt-spoken head of the Southern Nevada Water Authority, "was all-important to us."

U.S. water agencies have traditionally refused to concede any responsibility for the damage that Colorado River dams and efficiency projects have done to Mexico's environment and agriculture. "Any impacts that were felt south of the border were just not considered," says Kara Gillon, who represented Defenders of Wildlife in its 2000 case against the U.S. government. "It was like a no-man's land."