Updated 1/17/14

Just outside the dusty Mexican town of Carranza, Francisco Zamora wheels his Toyota pickup off the highway and down a gravel road along an irrigation canal. To one side, irregular farm fields flash by, fringed with reeds, sunflowers and an occasional shaggy palm. On the other side lies the bone-dry bed of the Colorado River. Straitjacketed between two levees roughly a mile apart and choked with mean, gray-green tamarisk, or salt cedar, it nonetheless has an emphatic presence – like a prehistoric creature waiting to rumble back to life.

It's hard to imagine that, a century ago, wetlands and impenetrable cottonwood, willow and mesquite forests covered nearly 2 million acres here. After Aldo Leopold visited in 1922, he rhapsodized about the Delta's "hundred green lagoons," where the river twisted and split and braided together again before finally disappearing into the Gulf of California. Huge freshwater flows, powerful saltwater tides barreling up from the Gulf, and millions of tons of rich sediment converged to make the Delta an unlikely Eden on the edge of the Sonoran Desert. Home to more than 350 species of birds, it was a key part of the Pacific Flyway, a wild, immense and occasionally magical dreamscape prowled by jaguars and exploding with waterfowl.

But a series of big dams upstream gradually strangled the Delta. First came Hoover, in 1937. Just seven years later, Godfrey Sykes, an explorer and engineer who followed in Leopold's footsteps, likened the now-harnessed river to "a bird in a cage, or an animal in a zoo." In 1950, Mexico's Morelos Dam went into operation. And finally, in 1963, Glen Canyon Dam began impounding any leftover water to create Lake Powell.

Those formidable waterworks, along with the many smaller dams and diversions built throughout the Colorado's 224,000-square-mile drainage area, have transformed the river into a gigantic water-supply system for Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada and California, and the Mexican states of Sonora and Baja California. The Delta, meanwhile, has shrunk to less than one-tenth of its original size; over the past 50 years, the river has only occasionally reached the sea. And the stranglehold has tightened over the past 14 years, as the river suffers through its worst drought in more than a century.

But even as the drought has dragged on, Zamora, who works for the Tucson, Ariz.-based Sonoran Institute, and compatriots from several other environmental organizations on both sides of the U.S.-Mexico border, have patiently tended to the last 90 miles of the river. Zamora's field outfit – sunhat, nylon hiking pants, fly-fishing shirt and binoculars – is classic birdwatcher, and he speaks with thoughtful reserve. He and his colleagues have hired workers from nearby towns to plant a small network of native forests, nurturing them with small amounts of water purchased from local farmers.

"It's not easy, because it's not a lot of water," he says. "We've had to do the best we can."

But that's about to change.

Last November, after five years of remarkable negotiations that unfolded far from the Delta, representatives from the U.S. and Mexico agreed to a complex, multi-part water deal that will give them desperately needed flexibility for weathering the drought. More surprisingly, the two nations will join the team of environmental organizations to release a flood of more than 105,000 acre-feet of water – 3.8 million big-rig tankers' worth – into the Delta's ancient floodplain, and chase it with a smaller, permanent annual flow to sustain the ecosystem.

It is the unlikeliest of times to pull off a deal like this. Rather than hoarding all the water for themselves in this drought –– the river supplies some 35 million people –– the West's largest water agencies have pledged to send some all the way to the sea. That move is, to some extent, a long-overdue acknowledgment that the U.S. bears responsibility for the impacts its dams have caused beyond its borders. And after years of fruitless court fights in the U.S. by environmental groups, the Mexican government finally insisted that water for the Delta be a cornerstone of the broader deal.

"This wasn't just an afterthought," says a U.S. negotiator. "This was integral to the negotiations."

But Delta restoration is still far from assured. If the drought deepens, as many fear it will, the surprising alliances that led to the deal will be tested as never before. The Delta's champions are anxiously racing against time and the drought to lay the groundwork for what they hope will become a permanent commitment to restoration, before the window to return water to the Delta closes – perhaps forever.

"It has been," Zamora says, "a pretty busy summer."