Embalse del Yeso sits at 2,500 meters above sea level, impounding the Yeso River behind a 1964 concrete-arch dam that supplies a quarter of Santiago's drinking water. The reservoir's milky turquoise color comes from glacial silt suspended in runoff from the surrounding Andes peaks, a phenomenon that intensifies during the Southern Hemisphere summer melt between December and March. Access to the reservoir was restricted until 2018, when Chile's national water authority formalized a conservation protocol allowing guided tourism under permit from the Asociación Parque Cordillera, the consortium managing the 45,000-hectare buffer zone that extends east from San José de Maipo to the Argentine border. The canyon itself predates modern infrastructure by millions of years. Cajón del Maipo formed through tectonic uplift and fluvial erosion, carving a 100-kilometer corridor through the Andean foothills that indigenous Picunche communities used as a seasonal migration route between the coastal valley and high-altitude pastures. Spanish colonial records from the 1540s describe copper and silver prospecting expeditions into the gorge, though sustained mining operations did not begin until the 1800s with the establishment of El Volcán mine near the headwaters. By the mid-20th century, hydroelectric development had replaced mineral extraction as the region's dominant industry, culminating in the construction of three reservoirs—El Yeso, Laguna Negra, and Embalse Lo Encañado—that together generate over 1,000 gigawatt-hours annually. Today, the Cajón del Maipo Embalse del Yeso tour with picnic operates under strict environmental guidelines. Daily visitor quotas cap entry at 300 persons to prevent shoreline erosion and protect nesting sites for Andean condors, which patrol the basin's thermals between May and September. Tour operators are required to carry portable waste systems and observe a 50-meter buffer from the waterline, measures enforced by park rangers stationed at the G-455 checkpoint 18 kilometers south of the dam. The 3,450-peso conservation fee collected at entry funds habitat restoration and trail maintenance across the Parque Cordillera network, which expanded in 2022 to include the adjacent Morado Valley and its namesake glacier. The picnic component emerged as a logistical necessity rather than a marketing feature. With no food vendors permitted within the park boundary and the nearest restaurant 40 kilometers west in San Gabriel, tour operators began including packed lunches in the early 2000s. Contemporary Cajón del Maipo Embalse del Yeso tours now integrate the meal as a scheduled stop, typically timed to coincide with midday light conditions that render the reservoir's color at its most saturated. Some operators have added cultural elements—alpaca fiber demonstrations, folk music, or empanada preparation—transforming the functional meal break into a curated Andean experience that distinguishes one tour package from another in a crowded market.