Plata, Río de la (rēˈō ħā lä pläˈtä) [key], estuary, c.170 mi (270 km) long, SE South America, formed by the Paraná and Uruguay rivers. Between Argentina and Uruguay, the estuary is c.120 mi (190 km) wide at its mouth on the Atlantic Ocean and decreases to c.20 mi (30 km) near its head. Focal point of the second largest river system of the continent, the estuary receives a tremendous volume of water. Its northwestern end contains freshwater. Extensive sandbanks and shoals reduce the navigability, but constantly dredged channels permit navigation by large vessels; Buenos Aires and Montevideo are the chief ports. Discovered (1516) by Juan Díaz de Solís, it was explored by Ferdinand Magellan in 1520 and by Sebastian Cabot from 1526 to 1529. The first settlement on its banks was made (1536) at Buenos Aires by Pedro de Mendoza, the Spanish conquistador. A principal channel into the interior of SE South America, it is very important commercially. In English it is sometimes called River Plate. The viceroyalty of Río de la Plata, more or less corresponding to the present Argentina, Bolivia, Uruguay, and Paraguay, was established in 1776.