Argentina–Paraguay border

Three rivers

From the tripoint with Brazil, the border follows the Paraná downstream for about 732 km to where the Paraguay River joins it. It then goes up the Paraguay for about 383 km to the mouth of the Pilcomayo.^[1]

From there it follows the Pilcomayo for about 765 km to the tripoint with Bolivia at La Esmeralda.^[1] In one poorly drained stretch of the Gran Chaco, between Punto Horqueta and Salto Palmar, the river has no fixed, continuous course. There the border is a series of straight lines with pillars at the corners.^[1]

Before and after the war

Paraguay became independent from Spain in 1811. That year a treaty with Buenos Aires kept Paraguay's existing boundaries and put off the question of the Pedro González district on the Paraná.^[1] A treaty of 15 July 1852 made the Paraná the border, but it gave Paraguay the whole Paraguay River from bank to bank and left a neutral strip on one bank.^[1]

In 1865 Argentina, Brazil and Uruguay signed an alliance against Paraguay. It said that after the war Argentina and Paraguay would be divided by the Paraguay and Paraná rivers. That would have given Argentina the Gran Chaco as far north as Bahía Negra, and the Misiones territory.^[1] The War of the Triple Alliance lasted from 1865 to 1870.^[1]

The present line in the Paraná and the Paraguay comes from a treaty signed in Buenos Aires on 3 February 1876. Paraguay gave up its claims to Misiones and to the Chaco Central (today's Formosa) between the Pilcomayo and the Bermejo. Argentina accepted that the Chaco north of the Río Verde belonged to Paraguay.^[1] The treaty kept the island of Apipé for Argentina and Yacyretá for Paraguay.^[1]

“Paraguay should be divided from Argentina on the east and on the south by the mid-channel of the main stream of the Parana from its confluence with the Paraguay to the boundary of Brazil…”^[1]

President Hayes and the Pilcomayo

The 1876 treaty left one area, between the Río Verde and the Pilcomayo, to the President of the United States.^[1] On 12 November 1878 President Rutherford B. Hayes awarded it to Paraguay, down to the main branch of the Pilcomayo, including the town of Villa Occidental, now Villa Hayes.^[1]

That made the Pilcomayo the border, but in the flat Chaco the river splits and spreads into marshes. A 1905 convention set up a commission to work out which channel was the main one, but nothing more was done until 1939.^[1] A treaty of 5 July 1939 fixed the line along the river except between Salto Palmar and Punto Horqueta. A supplementary treaty of 1 June 1945 then set that section through the swamps, Laguna la Bella and the Estero Patiño.^[1]

Sharing the Paraná

In 1973 Argentina and Paraguay signed the Treaty of Yacyretá to build a hydroelectric plant on the Paraná. Building it took from 1978 to 2011, and the reservoir was planned to cover 1,500 square kilometres.^[2]