Pilcomayo (in Hispanicized spelling) (Quechua: Pillkumayu or Pillku Mayu, pillku red, mayu river, "red river"; Guarani: Ysyry Araguay ) is a river in central South America. At long, it is the longest western tributary of the Paraguay River. Its drainage basin is in area, and its mean discharge is . Upstream the Jach'a Juqhu River successively receives the names Aguas Calientes and Kachi Mayu. From the confluence with the Chillawa (Chillahua), the river is called Pilcomayo. From there, it flows in a southeasterly direction through Chuquisaca and Tarija departments, passes through the Argentine province of Formosa and the Gran Chaco plains of Paraguay, forming most of the border between these two countries before it joins the Paraguay River near Asunción. The Río Pilcomayo National Park is located on the Argentine side of the border. It also forms a short part of the boundary between Argentina and Bolivia, just prior to the Argentina–Bolivia–Paraguay tripoint.
Human settlement
The river basin is home to approximately 1.5 million people: one million in Bolivia, 300,000 in Argentina, and 200,000 in Paraguay.
The Nakotoi Indian Tribe resides in the southern region of the Pilcomayo, a region located along the river in central Paraguay. The aborigines of this tribe are descendants of the highly renowned Redskin Tribe. The Natokoi dress in a rug, fastened around the waist-line.