CUCAPÁ EL MAYOR, Mexico — Germán Muñoz looked out at the river before him and talked about the days when dolphins swam here, 60 miles from the sea.

“The wave made noise like a train,” he said, describing the tides that would roll up the Colorado River from the Gulf of California and then a mile or so up this tributary, past his family’s land. “There would be all kinds of fish jumping, very happy. And then the dolphins would come, chasing the fish.”

That was in the 1950s, when the Colorado still flowed regularly to the gulf — as it had for tens of thousands of years, washing sand and silt down from the Rocky Mountains to form a vast and fertile delta. In the last half-century, thanks to dams that throttled the Colorado and diverted its water to fuel the rise of the American West, the river has effectively ended at the Mexican border. The Colorado delta, once a lush network of freshwater and marine wetlands and meandering river channels and a haven for fish, migrating birds and other wildlife, is largely a parched wasteland.