Argentina–Brazil border

A border made of rivers

Of the roughly 1,224 km the US Geographer measured in 1979, about 1,200 km follow the thalwegs of four rivers. The thalweg is the line of deepest water in a river's channel.^[1]

Starting in the north, where the Iguazu meets the Paraná, the border runs up the Iguazu for 124 km and then up the San Antonio for 130 km to its source. It crosses 24 km of high ground, marked by pillars, to the source of the Pepiri Guazu. It then follows the Pepiri Guazu for 225 km and the Uruguay River downstream for 721 km.^[1]

The islands were shared out by name in 1910. At the Iguazu Falls, the islands and reefs near the left bank went to Argentina and those near the right bank to Brazil.^[1]

Which river is the Pepiri Guazu?

The first boundary treaty, signed on 14 December 1857, already named the Uruguay, the Pepiri-guassu, the San Antonio and the Iguassu. It said these were the rivers identified in 1759 by surveyors working under the 1750 treaty between Portugal and Spain. Argentina never ratified it.^[1]

The trouble was that the same names were attached to different rivers. In 1885 the two countries set up a joint commission to survey four of them, known by various names: the Pepiri Guazu, the San Antonio, the Pequiri Guazu (Chapecó) and the San Antonio Guazu (Chopim). Two bounded the disputed territory in Misiones on the west and two on the east.^[1]

The commission's orders were only to survey, not to argue over rights. In 1889 the two countries agreed that if they could not settle on a line after the survey, the President of the United States would decide.^[1] A compromise treaty signed at Montevideo in January 1890 would have split the area, but Brazil rejected it the following year.^[1]

On 5 February 1895 President Grover Cleveland chose the western pair of rivers, the ones Brazil had argued for.^[1] The treaty of 6 October 1898 wrote his award into the boundary, and a mixed commission marked it on the ground between 1900 and 1904.^[1]

“…the boundary line between the Argentine Republic and the United States of Brazil in that part submitted to me for arbitration and decision is constituted and shall be established by and upon the rivers Pepiri (also called Pepiri-guazu) and San Antonio…”^[1]

The southern end and Brasilera Island

A convention of 27 December 1927 moved the line near the southern end. Instead of following the middle of the Uruguay River there, it passes between the Argentine bank and a small island called Brasilera, or Quaraí, which the convention treated as Brazilian.^[1]

Uruguay was not a party to that convention and objects to it. When it signed its own river boundary treaty with Argentina in 1961, Uruguay's foreign minister repeated a reservation first made in 1940: that the island had been given to Brazil through a mistaken reading of the 1851 Brazil–Uruguay treaty.^[1] The US Geographer noted in 1975 that this makes the three-country point disputed between Brazil and Uruguay, and that Argentina is not involved.^[3]

Crossing the Iguazu

The Tancredo Neves Bridge crosses the Iguazu between Foz do Iguaçu in Brazil and Puerto Iguazú in Argentina. It opened on 29 November 1985, about 2.9 km upstream from where the Iguazu joins the Paraná.^[2]