The Río Oro o Amarillo is one of Argentina's most visually striking and geologically fascinating waterways, flowing through the Chilecito region of La Rioja Province and originating deep within the Cerro Famatina mountain range. The river earns its vivid name from its unmistakable golden-yellow color, a natural phenomenon produced by the presence of ferrous dioxide and the oxidation of iron as waters flow down from the historic La Mejicana gold mine. Heavy metals including lead, copper, and zinc are distributed throughout the riverbed for approximately 40 kilometers downstream, intensifying the river's characteristic ochre hue as the water travels further from its source. One of the most spectacular natural features along this waterway is its confluence with the Quebrada del Agua Negra river — a stream renowned for its crystal-clear transparency. At this meeting point, known locally as "la unión de los ríos," the intensely yellow-orange waters and the clear waters flow side by side for several meters before gradually merging, creating a visual phenomenon rarely witnessed anywhere else in the world.