Covering about 13,500 square miles
It was discovered in 1516 by Juan Diaz de Solis, though Sebastian Cabot was the one to give it its current name.  Magellan stopped there on his world tour, and though he didn't traverse its 300 kilometers, he probably stopped at the site of the best natural harbor on it, Montevideo.  Covering about 13,500 square miles it is formed by the Parana, Paraguay, and Uruguay Rivers.  FTP, name this estuary that translates to "River of Silver" and on which Buenos Aires sits
It was discovered in 1516 by Juan Diaz de Solis, though Sebastian Cabot was the one to give it its current name.  
Rio de la Plata or River Plate [pla-TAY]