The Atrato River () is a river of northwestern Colombia. It rises in the slopes of the Western Cordillera and flows almost due north to the Gulf of Urabá (or Gulf of Darién), where it forms a large, swampy delta. Its course crosses the Chocó Department, forming that department's border with neighboring Antioquia in two places. Its total length is about , and it is navigable as far as Quibdó (400 km / 250 mi), the capital of the department. Watershed of the Atrato River The river’s total length is about , and it is navigable as far as Quibdó (400 km / 250 mi), the capital of the department. The basin occupies an area of and has an average annual precipitation of >5,000 mm/year that reaches up to 12,000 mm/year in the upper basin. Flowing through a narrow valley between the Cordillera and coastal range, it has only short tributaries, the principal ones being the Truandó, the Sucio, and the Murrí rivers. The gold and platinum mines of Chocó line some of its confluence, and the river sands are auriferous. Mining and its toxic leavings have adversely affected river and environmental quality, damaging habitat for many species and affecting the ethnic groups: the predominantly Afro-Colombian and Native American indigenous peoples who live along the river. The river is one of the few ways to move around in the Chocó region. Northwestern Colombia encompasses an area of great diversity in wildlife. During the Pleistocene era at the height of the Atrato river, where it intersected the Cauca-Magdalena, the area was covered by a sea. It is proposed that this created a geographic barrier that may have caused many species to diverge through the process of allopatric speciation. For example, Philip Hershkovitz proposed that the cotton-top tamarin (Saguinus oedipus) and the white-footed tamarin (Saguinus leocopus) diverged because of the rise of the Atrato, and today they are principally separated by the river. History In the 19th and early 20th centuries, the San Juan and the Atrato rivers attracted considerable attention as part of a feasible route for a trans-isthmian canal in Colombia. William Kennish, an engineer and inventor from the Isle of Man and Royal British Navy veteran, proposed an aqueduct making use of the Atrato River and its tributary, the Truando River, to cross the Colombian isthmus. After publishing a report in 1855 on this proposal for a New York