Río de la Plata in the context of "Buenos Aires"

The Río de la Plata basin (Spanish: Cuenca del Plata, Portuguese: Bacia do Prata), more often called the River Plate basin in scholarly writings, sometimes called the Platine basin or Platine region, is the 3,170,000-square-kilometre (1,220,000 sq mi) hydrographical area in South America that drains to the Río de la Plata. It includes areas of southeastern Bolivia, southern and central Brazil, the entire country of Paraguay, most of Uruguay, and northern Argentina. Making up about one fourth of the continent's surface, it is the second largest drainage basin in South America (after the Amazon basin) and one of the largest in the world.

The main rivers of the La Plata basin are the Paraná River, the Paraguay River (the Paraná's main tributary), and the Uruguay River.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.

Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.

The Paraná River (Portuguese: Rio Paraná [ˈʁi.u paɾaˈna] ; Spanish: Río Paraná [ˈri.o paɾaˈna] ; Guarani: Ysyry Parana) is a river in south-central South America, running through Brazil, Paraguay, and Argentina for some 4,880 kilometres (3,030 mi). Among South American rivers, it is second in length only to the Amazon River. It merges with the Paraguay River and then farther downstream with the Uruguay River to form the Río de la Plata and empties into the Atlantic Ocean.

The first European to go up the Paraná River was the Venetian explorer Sebastian Cabot, in 1526, while working for Spain.

The Gran Chaco (also called Chaco or Chaco Plain), is a vast semiarid lowland region in central South America, spanning over one million square kilometers across eastern Bolivia, western Paraguay, northern Argentina, and parts of Brazil. It forms part of the Río de la Plata basin.

Gran Chaco features a mix of tropical and subtropical dry broadleaf forests, thorn scrub, savannas, wetlands, and palm groves, making it the continent’s second-largest forested ecoregion and a region of high ecological diversity.

The Viceroyalty of the Río de la Plata or Viceroyalty of Buenos Aires (Spanish: Virreinato del Río de la Plata or Virreinato de Buenos Aires or Spanish: Virreinato de las Provincias del Río de la Plata) meaning "River of the Silver", also called the "Viceroyalty of River Plate" in some scholarly writings, in southern South America, was the last to be organized and also the shortest-lived viceroyalty of the Spanish Empire in the Americas. The name "Provincias del Río de la Plata" was formally adopted in 1810 during the Cortes of Cádiz to designate the Viceroyalty of the Río de la Plata.

The Viceroyalty was established in 1776 from several former Viceroyalty of Perú dependencies that mainly extended over the Río de la Plata Basin, roughly the present-day territories of Argentina, northern Chile, Bolivia, Paraguay and Uruguay, extending inland from the Atlantic Coast. The colony of Spanish Guinea (present-day Equatorial Guinea) also depended administratively on the Viceroyalty of Rio de la Plata. Buenos Aires, located on the western shore of the Río de la Plata estuary flowing into the Atlantic Ocean, opposite the Portuguese outpost of Colonia del Sacramento, was chosen as the capital. Usually considered one of the late Bourbon Reforms, the organization of this viceroyalty was motivated on both commercial grounds (Buenos Aires was by then a major spot for illegal trade), as well as on security concerns brought about by the growing interest of competing foreign powers in the area. The Spanish Crown wanted to protect its territory against Great Britain and the Kingdom of Portugal.

Conquistadors (/kɒnˈk(w)ɪstədɔːrz/, US also /-ˈkiːs-, kɒŋˈ-/) or conquistadores (Spanish: [koŋkistaˈðoɾes]; Portuguese: [kõkiʃtɐˈðoɾɨʃ, kõkistɐˈdoɾis]; lit. 'conquerors') were Spanish and Portuguese colonizers who explored, traded with and conquered parts of the Americas, Africa, Asia, and Oceania during the Age of Discovery. Sailing beyond the Iberian Peninsula, they established numerous colonies and trade routes, and brought much of the New World under the dominion of Spain and Portugal.

After Christopher Columbus's arrival in the West Indies in 1492, the Spanish, usually led by hidalgos from the west and south of Spain, began building a colonial empire in the Caribbean using colonies such as Santo Domingo, Cuba, and Puerto Rico as their main bases. From 1519 to 1521, Hernán Cortés led the Spanish conquest of the Aztec Empire, ruled by Moctezuma II. From the territories of the Aztec Empire, conquistadors expanded Spanish rule to northern Central America and parts of what is now the southern and western United States, and from Mexico sailing the Pacific Ocean to the Spanish East Indies. Other Spanish conquistadors took over the Inca Empire after crossing the Isthmus of Panama and sailing the Pacific to northern Peru. From 1532 to 1572, Francisco Pizarro succeeded in subduing this empire in a manner similar to Cortés. Subsequently, Spanish conquistadores used Peru as a base for conquering much of Ecuador and Chile. Central Colombia, home of the Muisca, was conquered by licentiate Gonzalo Jiménez de Quesada, and its northern regions were explored by Rodrigo de Bastidas, Alonso de Ojeda, Juan de la Cosa, Pedro de Heredia and others. For southwestern Colombia, Bolivia, and Argentina, Spanish conquistadores from Peru combined parties with other conquistadors arriving more directly from the Caribbean and Río de la Plata-Paraguay respectively. These conquests founded the basis for modern Hispanic America and the Hispanosphere.

Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the Atlantic Ocean to the southeast. It is part of the Southern Cone region of South America. Uruguay covers an area of approximately 176,215 square kilometers (68,037 sq mi). It has a population of almost 3.5 million people, of whom nearly 2 million live in the metropolitan area of its capital and largest city, Montevideo.

The area that became Uruguay was first inhabited by groups of hunter gatherers 13,000 years ago. The first European explorer to reach the region was Juan Díaz de Solís in 1516, but the area was colonized later than its neighbors. At the time of European arrival, the Charrúa were the predominant tribe, alongside other groups such as the Guaraní and the Chaná. However, none of these groups were socially or politically organized, which contributed to their decline. Amid territorial disputes, the Portuguese established Colônia do Sacramento in 1680, and the Spanish founded Montevideo as a military stronghold. Uruguay secured its independence between 1811 and 1828, following a four-way struggle involving Portugal, Spain, and later the United Provinces of the Río de la Plata and the Empire of Brazil. In 1830, the country enacted its constitution and was formally established as an independent state.