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 by Raise the River, 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, Audubon’s Colorado River Program director. “A lot of hard work in the hot sun made something miraculous happen.”