The Maipo River carved its canyon over three million years, slicing through sedimentary rock and exposing strata that predate the final Andean uplift. By the early twentieth century, engineers had identified the upper basin as ideal for hydroelectric expansion, but it was not until 1964 that construction began on the Embalse El Yeso, a reservoir conceived to regulate water flow for Santiago's growing metropolitan demand. The dam wall rises 65 meters above the riverbed, impounding a body of water that stretches nearly eight kilometers at full capacity, its surface mirroring the mineral-blue hue of glacial meltwater suspended with fine rock flour. Today the reservoir anchors a network of guided excursions departing the capital each morning. Visitors traverse a succession of microclimates, beginning in the semi-arid basin floor where cacti punctuate the scrubland, ascending through riparian corridors lined with native quillay and litre, and emerging above 2,500 meters into alpine steppe where only cushion plants and low grasses withstand the thin air. The route follows the Camino al Volcán, a gravel track engineered for hydroelectric maintenance that doubles as the principal access corridor for tourism. Most cajón del maipo day trip from santiago itineraries pause at the village of San Gabriel, where a chapel built in 1928 marks the transition from settled valley to high-altitude wilderness. The reservoir itself sits at 2,500 meters elevation, cradled between the Cordillera Principal to the east and lower ridges that obscure views from the valley floor. On clear days the surface reflects the summit of San José Volcano, a 5,856-meter stratovolcano whose permanent snowfields feed the watershed. Conservation protocols implemented in 2021 by the Asociación Parque Cordillera formalized the 3,450-peso entrance fee, revenue earmarked for trail maintenance and waste management infrastructure. The fee structure, paired with advance reservation requirements, has moderated visitor pressure during peak austral summer months, when midday temperatures in the canyon can exceed thirty degrees Celsius even as the reservoir remains cold enough to numb exposed skin within seconds. Geologists continue to study the basin's fault lines, which represent the subduction boundary where the Nazca Plate descends beneath South America at roughly seven centimeters per year.