Estuary of the Paraná and Uruguay rivers, between Uruguay and Argentina. It is about 170 mi (275 km) long, with a maximum width at its mouth of about 140 mi (220 km); at Montevideo it is about 60 mi (97 km) wide but narrows gradually to the northwest as it approaches Buenos Aires, where it is about 25-28 mi (40-45 km) wide. Discovered by the Spanish in 1516, it was explored by Ferdinand Magellan in 1520 and by Englishman Sebastian Cabot (1527-29). The first permanent settlement in the area was at Asunción in 1537