The colors are not decoration; they are chemistry. The deep reds come from iron oxide, the same rust that stains an old nail left in the rain. The yellows are the signature of sulfur compounds, and the strange greens betray copper in the rock. Wind and water have done the carving, scouring softer layers away and leaving the harder stone in fins, hoodoos and slumping cliffs that locals have named for the shapes they resemble. The Atuel River drops roughly five hundred metres as it threads the canyon, and over uncounted millennia that falling water has acted like a slow blade, opening the mountainside to reveal the banded geology beneath.

The canyon does not run wild end to end. It begins, traveling upstream, at the Embalse El Nihuil and ends at the Embalse Valle Grande, two reservoirs that bracket the gorge roughly 37 kilometres from San Rafael. Between them, the Atuel's torrential energy has been put to work: a chain of power stations harnesses the river as it falls, beginning at the El Nihuil dam. This is a landscape where raw geology and human engineering share the same narrow valley, the turquoise pool of Valle Grande sitting still and bright at the canyon's foot while the river above it crashes through stone.

For all its geological drama, the Atuel is best known to many as a playground. The rapids that punctuate the river run from Class II, gentle enough for novices, up to Class IV, where the water demands real skill and a guide who knows the line. Rafts and kayaks ride the current through the gorge while horseback riders and mountain bikers trace the rim. Hotels, campsites and country clubs have grown up along the surrounding roads to serve the steady flow of visitors, making the canyon one of the anchors of tourism in southern Mendoza, an easy day trip that rewards a traveler with a full menu of ways to get wet, get high, or simply get lost in the colors.

Like all great desert canyons, the Atuel saves its finest moments for the edges of the day. In the flat noon glare the walls can read as merely dusty, but as the sun drops low the reds deepen to ember, the yellows turn to gold, and shadows pool in the side gullies to throw every ridge into relief. Photographers learn to wait. The same wall that looked ordinary at midday becomes, an hour before sunset, a vertical canvas of warm and cool tones stacked in ragged bands, the still water of Valle Grande catching the whole show upside down.