PLATA, RIO DE LA or RIVER PLATE, a funnel shaped estuary, on the east side of South America, extending west-north-west from the sea about 170 miles. The discovery of the South sea by Balboa, then governor of Castilla del Oro, of which Darien formed a part, created a lively desire to learn some thing of its coast-line, and the year following (in 1514), the Spanish monarch concluded a navigation contract with Juan Diaz de Solis, then Piloto Mayor, to search for a strait connecting the Atlantic with the newly found ocean. De Solis set sail from the port of Lepe on Oct. 8, 1515, reached the Bay of Rio de Janeiro on Jan. I, 1516, and continuing southward to lat. 35° en tered the great estuary now known as the Plata, which, for a short period of time, was called the de Solis and the Mar Dulce. Ascend ing it to the vicinity of the island of Martin Garcia, near the mouth of the Parana river, de Solis was ambushed and killed early in 1516 by Guarani Indians. In 152o Magellan explored the Rio de la Plata, and discovered and navigated the straits which bear his name. In 1526 Cabot ascended the Mar Dulce, discovered the Parana river and reached a point on the Paraguay near the site of the present city of Asuncion. Here he obtained by barter from Guarani Indians silver ornaments. The receipt of these silver baubles caused the name of Rio de la Plata to be applied to the third greatest river of the western continent.
The extreme breadth of the river at its mouth is 138 miles. It narrows quickly to 57m. at Montevideo, and to 25m. at its head, where it receives the Parana, and Uruguay rivers. Its northern or Uruguayan shore is somewhat elevated and rocky, while the southern or Buenos Airean one is very low. The whole estuary is very shallow, and in no place above Montevideo exceeds 36ft. in depth at low water. The bottom generally consists of sand covered with from f o to 2oft. of water. A dredged channel of about 22ft. in depth leads from the port of Buenos Aires 20M. east to deep water. Winds and tides considerably affect the depth of the channel. The Plata is simply the estuarine receptacle of two mighty streams, the Uruguay and Parana which drain the Plata basin. This has an area of 1,198,000sq.m., or over two and one-half times that of the Pacific slope of the Andes, and com prises the most fertile, healthiest and best part of Brazil, a large portion of the Argentine republic, the whole of Paraguay and south-eastern Bolivia, and most of Uruguay.
58' and 34' S. three important tributaries join it from the east-the Ipui-guazu, the Ibicui and the Negro.
The Pepiri-guazu in its lower course is about 25oft. wide, but higher up it narrows to about 3oft., and runs with great violence between high wooded banks. It is navigable for canoes for about 70m. above its mouth, as far as its first fall. The Rio Negro has a delta of several large islands at its confluence with the Uruguay. Its head-waters are in the southern part of Rio Grande do Sul, but the main river belongs entirely to the State of Uruguay. Its lower reaches are navigable for craft of moderate draught.
From its source in the coast range of Brazil the Uruguay runs through open, hilly country, and then enters a forest belt of high lands. At the river Pepiri-guath it turns suddenly to the south-west, and continues this course to its junction with the Parana and Plata. Near Fray Bentos, 61m. before reaching the Plata, it forms a great lake, about 56m. long and from 4 to 6m. wide. At Punta Gorda, where it debouches into the Plata, it is about im. wide, but is 9oft. deep. From the Pepiri-guazU junc tion its banks are high and covered with forest as far down as 27° 30' S., where the river is 2,300f t. wide and from i o to 4oft. deep. The Uruguay is much obstructed by rocky barriers. Four miles below its confluence with the Pepiri-guath it has a cataract, about 8m. long, with a total fall of 26ft. at low water. The river near the Pepiri-guazu is 1,55oft. wide, but about fim. before reaching the cataract its width is reduced to 600ft. Along the cataract it is closed in between high precipitous walls of black rock only 7oft. apart. Above Punta Gorda, 212m., is the Salto Grande, which has a length of 15m. of rapids, the greatest single fall being i 2ft., and the difference of level for the entire length of the reefs 25ft. Nine miles below the Salto Grande is the Salto Chico, which bars navigation during six months of the year, but in flood time may be passed in craft drawing 5ft. of water. The Uruguay can be navigated at all seasons by vessels of 41ft. draught as far up as the Salto Chico, and of i4ft. up to Paysandu for a greater part of the year. Fray Bentos may be reached all the year round by any vessel that can ascend the Parana. Above the navigable lower river there is launch and canoe navigation for many hun dreds of miles upon the main artery and its branches, between the rapids which are met with from time to time. The Uruguay has its annual floods. due to t'ne rains in its upper basin. They begin at the end of July and continue to November, attaining their maximum during September and October. Except in floods, it is a clear-water stream, and even at its highest level carries comparatively little silt.