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After prolonged neglect, Mexico’s Colorado River Delta is teeming with life. This summer, Ridgway’s Rails and Least Bitterns prowled in lush marshes. People jumped into the river’s water to escape record-breaking heat, or enjoyed picnics on the shore. Fish long absent shimmered in sunlit water.

This life aquatic was unthinkable until recently. Beginning in 1922, water-sharing agreements among the seven U.S. states in the Colorado River basin and Mexico claimed nearly every drop of the once-mighty waterway, which begins high in the Rocky Mountains and wends 1,450 miles to northwest Mexico, where it empties into the Sea of Cortez. Over ensuing decades, dams and diversions built to sustain farms and growing cities from Colorado to California left little water for downstream stretches south of the border. By the 1960s, when Lake Powell began to fill behind Glen Canyon Dam, the river no longer reached the sea.

Deprived of nourishment, the Colorado River Delta shrank, becoming a sliver of the expanse of lagoons and braided channels that naturalist Aldo Leopold saw when he paddled through a century ago. Ninety percent of its wetlands shriveled. Migratory birds, arriving to find far fewer places to feed and rest, made do with less appealing alternatives, such as agricultural waste ponds.

This year, however, marks a turning point in the delta’s long-sought revival: From May through October, 35,000 acre-feet of Colorado River water—about 11 billion gallons, or enough to supply at least 70,000 Southwest households for a year—is snaking from the U.S.-Mexico border to the river’s fan-shaped terminus 100 miles away. It is the first time since a brief period in 2014 that the Colorado reached the sea. And thanks to tireless advocacy , a binational alliance of six conservation groups, and a series of delicate negotiations between the two nations, the delta’s future is awash in promise. By 2026 it will receive 210,000 acre-feet of water in total.

This moment was a long time coming. After years of talks, the United States and Mexico did a test release, a 105,392-acre-foot “pulse flow” for two months in spring 2014. The surge, along with small deliveries made possible in part by Mexican farmers, helped biologists restore 1,400 acres of riparian systems, which involved planting more than 47,000 native trees and acquiring water rights.

The delta bloomed in response: Bird abundance rose 20 percent and avian diversity increased 42 percent, showing even a modest amount of water can make a big difference. “You almost can’t believe how quickly an area can be transformed from a pretty decimated landscape and disturbed ecosystem into something that really conjures the river of the past,” says Jennifer Pitt, ̽����ѡ’s Colorado River Program director. “A lot of hard work in the hot sun made something miraculous happen.”

“We know that we are never going to have the delta like it was 100 years ago, no,” says Gabriela Caloca Michel, water and wetlands conservation program coordinator at Pronatura Noroeste, a coalition member group. “But we know that we now have more birds, we have restoration, we have people working there—because we have some water.”

Buoyed by the results, in 2017 conservation groups convinced the two nations to ink a longer-term deal to send regular surges downstream and, along with nonprofits, contribute to a total of $18 million for restoration, science, and monitoring. Minute 323, an addendum to a 1944 Colorado River water-sharing accord, was born. 

Crucially, the agreement not only allocated water to the delta, but also called for both nations to make voluntary cuts in dry times and required the United States to fund water-efficiency projects in Mexico to ease the way. “Getting the two countries aligned was harder than I ever imagined it would be,” says Pitt. “And keeping them aligned remains complicated. But knowing what that takes makes it all the more special to see.”

The design of this year’s release, the first under the new accord, builds on lessons from 2014. This time, to ensure as much water as possible reaches the delta rather than sinks into the thirsty riverbed on its journey south, the water’s path is diverted near the border through irrigation channels to bypass a 35-mile dry stretch. Some irrigation water, purchased from Mexican farmers, is also sent to the river’s floodplain instead of fields. Overall, the volume dedicated to the delta is less than one percent of the river’s total average flow. But again managers expect a little will go a long way, boosting restoration efforts and transforming desiccated areas into verdant habitat for Vermilion Flycatchers, Yellow-billed Cuckoos, and more of the 380 avian species using the delta.

What makes the win even more extraordinary is that it comes amid extreme challenges. The Southwest, and two-thirds of Mexico, is in the grips of one of the most intractable droughts the region has ever experienced, fueled by climate change. This year the Colorado River Basin’s two largest reservoirs, Lake Mead and Lake Powell, dropped to record low levels. In August the U.S. Bureau of Reclamation announced that in 2022, for the first time, it would reduce water deliveries to Arizona, Nevada, and Mexico and may have to impose more cuts in coming years.

With hotter, drier times ahead, some advocates worry that water dedicated for the delta and other basin ecosystems could begin to look tempting, as farms and cities try to harness every drop. Yet despite painful shortages, those working to resurrect the delta remain cautious optimists. “The water that we’re putting in the river isn’t only for wildlife,” says Francisco Zamora Arroyo, the Sonoran Institute’s senior director of programs. “It’s for the people, too.” With locals again connected to their river, he says, the case to keep it flowing will only strengthen.

After all, Pitt adds, the same agreements that are re-watering the delta enable Mexico’s commitment to share in shortages. In an exception to the norm, an ecological revival—made possible by sharing rather than fighting over water—has inspired a new way to value the region’s most precious resource. “I see the two countries saying, ‘We need to figure out how to sustain this,’ ” says Pitt. “And that really is so important and amazing.” 

This story originally ran in the Fall 2021 issue as “Vital Signs.”​ To receive our print magazine, become a member by .