Argentina–Uruguay border in the context of Colonia Department

The Río de la Plata (Spanish pronunciation: [ˈri.o ðe la ˈplata] ; lit. 'River of Silver'), also called the River Plate or La Plata River in English, is the estuary formed by the confluence of the Uruguay River and the Paraná River at Punta Gorda. It empties into the Atlantic Ocean and forms a funnel-shaped indentation on the southeastern coastline of South America. Depending on the geographer, the Río de la Plata may be considered a river, an estuary, a gulf, or a marginal sea. If considered a river, it is the widest in the world, with a maximum width of 220 kilometres (140 mi).

The river is about 290 kilometres (180 mi) long and widens from about 2 kilometres (1.2 mi) at its source to about 220 kilometres (140 mi) at its mouth. It forms part of the border between Argentina and Uruguay. The name Río de la Plata is also used to refer to the populations along the estuary, especially the main port cities of Buenos Aires and Montevideo, where Rioplatense Spanish is spoken and tango culture developed. The coasts of the river are the most densely populated areas of Uruguay and Argentina.

The Treaty between Uruguay and Argentina concerning the Río de la Plata and the Corresponding Maritime Boundary was signed in Montevideo on 19 November 1973 by Dr. Juan Carlos Blanco Estradé, Minister for Foreign Affairs of Uruguay, and Mr. Alberto J. Vignes, Minister for Foreign Affairs and Worship of Argentina.

The treaty provides the geographical coordinates for the boundary points in the Rio de la Plata, affords the status of Martín García Island and other islands in the Río de la Plata, designates Martín García Island as the seat of an Administrative Commission for the Río de la Plata (Spanish: Comisión Administradora del Río de la Plata, CARP), and regulates the adjacent maritime front.