Though it is generally spoken of as a river, the Río de la Plata is considered by some geographers to be a large bay or marginal sea of the Atlantic Ocean or an the estuary of the Paraná and Uruguay rivers.

The delta of the Paraná and the mouth of the Uruguay meet at the head of the Río de la Plata. The breadth of the estuary increases from the head seaward, a distance of about 180 miles (290 kilometres): it is 31 miles from the city of Punta Lara on the southern (Argentine) shore to the port of Colonia del Sacramento on the northern (Uruguayan) shore, and 136 miles from shore to shore at the Atlantic extremity of the estuary. To those who regard the Río de la Plata as a river, it is the widest in the world, with a total area of about 13,500 square miles.

The Río de la Plata receives waters draining from the basin of these rivers, which covers much of south-central South America; the total area drained is about 1.2 million square miles (3.2 million square kilometres), or about one-fifth of the surface of the continent. Montevideo, the capital of Uruguay, is located on the northern shore of the estuary, and Buenos Aires, the capital of Argentina, is on the southwestern shore.

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