Atrato (äträ`tō), river, c.415 mi (670 km) long, rising in the Cordillera Occidental, W Colombia. It meanders north, across the base of the Isthmus of Panama, to the Gulf of Urabá. Quibdo is the head of navigation. The Atrato drains a region of rain forests. Its headwaters are in Colombia's chief platinum-producing area. Cartagena is the chief outlet for the products of the valley.