The largest of the rivers through which Argentina drains into the Plata system are the Pilcomayo, which rises in Bolivia and flows south-east along the Argentine frontier for about 400 m.; the Bermejo, which rises on the northern frontier and flows south-east into the Paraguay; and the Salado del Norte (called Rio del Juramento in its upper course), which rises on the high mountain slopes of western Salta and flows south-east into the Parana.
None of the tributaries of the La Plata system thus far mentioned is navigable except the lower Pilcomayo and Bermejo for a few miles.
In the extreme north a little over a degree and a half of territory lies within the torrid zone, extending from the Pilcomayo about 500 m.
Finding their comrades did not return, Irala and his companions determined to descend the river, and on their downward journey opposite the mouth of the river Pilcomayo, finding a suitable site for colonizing, they founded (1536) what proved to be the first permanent Spanish settlement in the interior of South America, the future city of Asuncion (15th August 1536).
The Buenos Aires treaty of the 3rd of February 1876 fixed the frontier between Argentina and Paraguay, and assigned to Paraguay the portion of the Gran Chaco between Rio Verde and Bahia Negra; the appropriation of the portion between Rio Verde and the Pilcomayo was submitted to the arbitration of the president of the United States, who in 1878 assigned it to Paraguay.
The Pilcomayo, the largest western tributary of the Paraguay, and an important frontier river, is only navigable in its upper and lower reaches.
No violent constitutional change took place after 1870, though there have been spasmodic outbreaks of revolution, as in 1881, in 1894, in 1898, in December 1904 - when a somewhat serious civil war was ended by the peace of Pilcomayo - in July 1908 and in September 1909.
North of Fort Olimpo and run south-west in a straight line to an intersection with the Pilcomayo in long.
It possesses only one large river in Bolivia, the Pilcomayo, which rises on the east slopes of the Cordillera Oriental opposite the south end of Lake Pampa Aullaguas and flows east and south-east through the sierra region to the Bolivian Chaco.
Whenever the trade of southern Bolivia becomes important enough to warrant the expense of opening a navigable channel in the Pilcomayo, direct river communication with Buenos Aires and Montevideo will be possible.
The Grande is a river of enormous length, rising in a great valley of the Andes between the important cities of Sucre and Cochabamba, and having its upper waters in close touch with those of the Pilcomayo branch of the river Paraguay.