ALLAH WAS INVOKED THAT DAY, after a wood-fired breakfast in an 18-degree dawn. The earth was cloaked anew; hoarfrost made the grass crunch under my boots. Wind ripped away the plume of Daniel González's breath. Winter in southern Chile, one of the southernmost places on earth. The pickup truck, already humming in the near-dark, gave me a static shock big enough to light a cigarette.

The white four-door rig was plastered with decals reading PATAGONIA SIN REPRESAS—Patagonia Without Dams—the coalition that González, the campaign's appealingly bedraggled coordinator, was here to fight for. A set of new dams is now slated to choke off Chile's biggest river, the Baker, here in the deep south. In the frozen daybreak, we were headed into a remote valley in search of a one-eyed cowboy with a little of the future in his hands.

For the next four hours we drove, crashed, slid, hiked, ice-walked, and bog-stomped, eventually arriving on foot in the pristine pastures of the isolated Nadis Valley. This region, called Aysén, is a Patagonia of the imagination, a rippling landscape that stuns even repeat visitors like González into silence. There is a reason ancient people worshipped mountains. There is a reason modern people carry trout rods. You enter, helpless as a dreamer, into a humbling realm of sheer granite and luring vistas, all of it wrapped in Andean peaks holding 13,000 square miles of glaciers. The Northern and Southern Patagonian Ice Fields are the world's largest outside the poles and Greenland, making up one of the planet's most accessible big reserves of freshwater.

Around lunchtime we stumbled upon the lonely old huaso, or cowboy. Arturo Quinto acted unsurprised that people had walked in, during the first hard freeze of the winter, to find him. He had a bushy monobrow, two sweaters, baggy blue pants tucked into black rubber boots, and a furry brown horse. Quinto tossed things onto this wide-hoofed Criollo pony—a pad of foam, a blanket, two sheepskins, more foam and blankets, and finally a thin piece of leather—until he had a Chilean saddle. Only when he mounted up did I see his age; this hairy, virile creature, with one eye gouged out by a branch and a huge knife tucked in the small of his back, was in his seventies. With a gruff command to follow, he started at a canter for his puesto, or hut, his dogs yelping around him.

Cowboys are taciturn. It's the sheep.

Quinto was one of half a dozen holdouts along the Baker who have refused to sell their pastureland to the multinational conglomerate, HidroAysén, which is on the brink of constructing five dams in the region. Three will be built on the short, turbulent Río Pascua, a Class VI favorite for expert kayakers, just to the south, but the first two will be constructed here on the Baker, the second one plugging the exit from Quinto's valley.

The Baker is a cold, intimidating snake, born in a massive lake, Lago General Carrera, on the Argentina border. It carves just 110 miles down through Aysén in a hard S, collecting glacial tributaries and raging at 30,000 cubic feet per second even in its headwaters, like a blue-green upper Colorado at flood. The torrents that now carry rafts and kayaks would be a perpetual gold mine for an energy company in a country with some of the highest electricity rates in Latin America. HidroAysén already has the key water rights to the Baker and the Pascua. Land rights are a minor issue—the five dams will drown less than 15,000 acres total, and if Quinto refuses to sell his meadow, a committee of "hombres buenos," or prominent citizens, could step in to negotiate.

When we finally got out of the wind, Quinto's 12-by-12-foot shack felt like a mansion. The room had pictures of girls in bikinis and was full of hanging meat. He fired up his woodstove, and González handed over a bag of green mate tea and a box of Santa Rita 120, a dreadful Chilean wine that tastes excellent when consumed cold, in the cold, beneath gobsmacking mountains, by a fire, with dinner rolls, ham, and cheese. Any day that includes a place like this is a good day.