Our pilot, David Kunkel, asked me to retrieve his oxygen bottle from under my seat, and when I handed it to him he gripped the plastic breathing tube with his teeth and opened the valve. We had taken off from Boulder that morning, and were flying over Rocky Mountain National Park, about thirty miles to the northwest. We were in a Maule M-7, a single-engine “backcountry” plane, and Kunkel was navigating with the help of an iPad Mini, which was resting on his legs. “People don’t usually think altitude is affecting them,” he said. “But if you ask them to count backward from a hundred by sevens they have trouble.” What struck me at that moment was not how high we were but how low: a little earlier, we had flown within what seemed like hailing distance of the sheer east face of Longs Peak, and now, as Kunkel banked steeply to the right to give a better view of a stream at the bottom of a narrow valley, his wingtip appeared to pass just feet from the jagged declivity beneath. Snow had fallen in the mountains during the night, and I half expected it to swirl up in the plane’s wake.

The other passenger, sitting in the co-pilot’s seat and leaning out the window with a camera, was Jennifer Pitt, a senior researcher for the Environmental Defense Fund. Pitt, who is in her forties, is the director of the E.D.F.’s Colorado River Program. She has long brown hair, which she had pulled back into a ponytail, and she was wearing a purple fleece. Most of her work in recent years has involved the river’s other end, in Mexico, but she had agreed to show me its source. We were bound for the Colorado’s headwaters, just over the Continental Divide, roughly fifty miles south of the Wyoming state line. “The best way to see a river system is from the air,” she had told me.

She pointed toward the Never Summer Mountains, on our right, and said, “There’s the Grand Ditch.” I saw what looked like a road or a hiking trail cut across the face of a steeply sloping forest of snow-dusted conifers; she explained that it was an aqueduct, dating to 1890. Until 1921, the section of the Colorado that’s upstream from its confluence with the Green, in eastern Utah, was called the Grand. Hence: Grand Lake, Grand Valley, Grand Junction (but not Grand Canyon, which was named for its grandness). The Grand Ditch carries water toward the state’s eastern plains. It doesn’t take water directly from the river but captures as much as forty per cent of the flow from slopes that would otherwise feed it, like a sap-gathering gash in the trunk of a rubber tree. We had already flown over later additions to the same network, including Long Draw Reservoir, completed in 1930, and five connected lakes that lie on the western side of the divide. The northernmost of those lakes spills as much as a third of a billion gallons a day into a tunnel, which carries the water under the park, through five hydroelectric generating plants, and into a distribution system that serves a large area east of the mountains, including the city of Boulder.

Kunkel dipped a wing. “We just flew over the headwaters,” he said. Our position was easier to see on his iPad than on the ground, because since we’d entered the mountains he’d had to pick his way under and around what sometimes looked like an upside-down ocean of clouds. The ceiling made flying difficult but helped to explain the existence of the water-storing-and-shifting network we’d been looking at. As moisture-laden weather systems move eastward across the western United States, they pile up over the Rockies, dumping snow and rain. Eighty per cent of Colorado’s precipitation falls on the western half of the state, yet eighty-five per cent of the population lives to the east, in what’s known as the mountains’ rain shadow. If transporting water from one side to the other were impossible, most of the people who live in Denver and other Eastern Slope cities would have to move.

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When the first Europeans to see the Grand Canyon looked down from its southern rim, in 1540, they guessed that the stream they could see at the bottom must be about eight feet wide. They’d been fooled by the scale of the canyon, but, even so, the Colorado River isn’t huge. It’s nearly a thousand miles shorter than the Mississippi and only a fraction as wide, but it’s a crucial resource for a surprisingly large part of the United States. A congressman in 1928 called it “intrinsically the most valuable stream in the world.” It and its tributaries flow through or alongside seven Western states—Colorado, Wyoming, Utah, New Mexico, Nevada, Arizona, and California—before crossing into Mexico near Yuma, Arizona. It supplies water to approximately thirty-six million people, including residents not just of Boulder and Denver but also of Salt Lake City, Las Vegas, Phoenix, Tucson, San Diego, and Los Angeles, several of which are hundreds of miles from its banks. It irrigates close to six million acres of farmland, much of which it also created, through eons of silt deposition. It powers the hydroelectric plants at the Hoover and Glen Canyon dams, is the principal source for the country’s two biggest man-made reservoirs, and supports recreational activities that are said to be worth twenty-six billion dollars a year. Some of its southern sections attract so many transient residents during the winter that you could almost believe it had overflowed its banks and left dense alluvial deposits of motorboats, Jet Skis, dirt bikes, all-terrain vehicles, trailers, mobile homes, fifth wheels, and R.V.s.

All that human utility has costs; the river suffers, in varying degrees, from many of the same kinds of overuse and environmental degradation that threaten freshwater sources around the world. The Colorado’s flow is so altered and controlled that in some ways the river functions more like a fourteen-hundred-mile-long canal. The legal right to use every gallon is owned or claimed by someone—in fact, more than every gallon, since theoretical rights to the Colorado’s flow (known as “paper water”) vastly exceed its actual flow (known as “wet water”). That imbalance has been exacerbated by the drought in the Western United States, now in its sixteenth year, but even if the drought ended tomorrow problems would remain. The river has been “over-allocated” since the states in its drainage basin first began to divide the water among themselves, nearly a century ago, and scientists expect climate change to strain it further, in part by reducing precipitation in the mountains that feed it.

Not long ago, I travelled as much of the Colorado’s length as can be followed in a car. I began near the headwaters, put three thousand miles on three rental cars, and ended, eventually, in northern Mexico, where the Colorado simply runs out. So much water is diverted from the river as it winds through the Southwest that, since the early nineteen-sixties, it has seldom flowed all the way to its natural outlet, at the upper end of the Gulf of California, and since the late nineteen-nineties it has made it there only once. People who drive into or out of the town of San Luis Río Colorado, in the Mexican state of Sonora, sometimes complain about having to pay a six-peso toll to cross a bridge that spans only sand.