Guatín is where the Río Puritama and the Río Purifica meet, about 25 kilometres north-east of San Pedro on the road to the Termas de Puritama. Water in this desert means vegetation, and the gorge below the confluence is full of cardón cacti — columnar, ribbed, some of them seven metres tall and standing in ranks up both walls. They grow roughly a centimetre a year, which makes the big ones older than the Spanish conquest.
The walk is a straightforward hour or two down the riverbed, crossing the water repeatedly on stones, with the canyon narrowing and the cactus forest thickening as you go. There is a small colonial bridge at the top and a scatter of ruined stone corrals. Wear shoes you do not mind soaking, and start early — the gorge is at 3,000 metres and the sun comes straight down it from mid-morning.
Most agencies bundle Guatín with Puritama as a half-day, which is the sensible pairing: the walk first, the hot pools after. On your own, the turning is signposted off the B-245 and there is space to leave a car.