Argentina–Bolivia border

Peaks, rivers and a parallel

The border begins in the west at the tripoint with Chile on Cerro Zapaleri, a 5,649-metre peak.^[1] From there it runs north-north-east from summit to summit, over mountains such as Cerro Tinte (5,860 metres) and Cerro Vilama (5,210 metres).^[1]

Further east it switches between short straight lines and river courses. It follows the Cuartel and La Quiaca gorges, then the Bermejo River and the Río Grande de Tarija, and the Itau River up to the 22nd parallel of south latitude.^[1]

In the Gran Chaco the line simply follows that parallel east to the Río Pilcomayo, and then the river itself to the tripoint with Paraguay.^[1] In 1977 the US State Department's Geographer reported that the whole border was marked, either by pillars or by rivers.^[1]

Almost thirty years of treaties

Argentina and Bolivia first agreed to settle their border in a treaty of 2 May 1865. Article 20 promised a special convention once commissioners had reported on the frontier. When Argentina approved it, it reserved its claim to the territory of Tarija.^[1] A treaty of 9 July 1868 expanded that article. A protocol of 27 February 1869 then cancelled it and put off the boundary until after the Paraguayan War, with a friendly country to arbitrate any difficulties.^[1]

In 1888 the two countries fixed the 22nd parallel as a provisional boundary in the Gran Chaco, running east to the Pilcomayo.^[1] A full boundary treaty was signed in Buenos Aires on 10 May 1889. Bolivia approved it the same year, but Argentina's Congress approved it only in 1891, and with changes. Bolivia accepted them, and ratifications were exchanged on 10 March 1893.^[1]

Argentina's changes rewrote the western end. Instead of starting in the Cordillera of Atacama, the line now began by joining the highest peaks of the Andes from the northern end of the Argentina–Chile boundary, then followed the 23rd parallel to the Zapalegui range.^[1]

“…and thence by the waters of the said river as far as the 22nd parallel, which parallel it shall follow as far as the waters of the Pilcomayo River.”^[1]

Fixing the details: Yacuiba and the Pilcomayo

Applying the 1889 treaty on the ground raised questions about what it meant. A new treaty, signed in La Paz on 9 July 1925, settled them and still sets the line everywhere except in the Pilcomayo.^[1] Bolivia approved it within days, but Argentina did not approve it until 7 September 1938.^[1]

The 1925 treaty lists the peaks and streams one by one. One change stands out on a map. Instead of running straight along the 22nd parallel, the line leaves it to follow the San Roque, Yacuiba and Pocitos streams, so that the town of Yacuiba stays in Bolivia inside a small triangle.^[1]

The last piece was the Río Pilcomayo, the eastern end. In 1938 an award on the Bolivia–Paraguay boundary ended that line in the Pilcomayo at 62°37′19″ W, and a pillar named La Esmeralda was built on the riverbank.^[2] A protocol of 10 February 1941 made the river the Argentina–Bolivia border from the 22nd parallel down to La Esmeralda, following a 1940 survey by the joint demarcation commission.^[1]