Río Pilcomayo is a river in the central parts of South America, longest western tributary of the Paraguay River, and its watershed extends across 270,000 km². The Picomayo rises in the foothills of the Andes cordillera, between the Bolivian departments of Potosí and Oruro, east of Poopó Lake. From there it flows in a southeasterly direction 2,000 km through Chuquisaca and Tarija departments, passes through the Argentine province of Formosa and the Gran Chaco plains of Paraguay, forming the border between these two countries before it joins the Paraguay River near Asunción. The river basin is home to approximately 1.5 million people: one million in Bolivia, 300,000 in Argentina, and 200,000 in Paraguay.