Rio De La Silver River (Plata), a river (or more properly an estuary) of South America, between Uruguay and the Argentine Eepublic, having Montevideo, the capital of the former, on its N. bank near the entrance, and Buenos Ayres, the capital of the latter, on its S. bank near the head. It is formed by the junction of the Parana with the Uruguay, whose waters empty through it into the Atlantic. From the confluence of these rivers to about 120 m. below it has all the distinctive features of a freshwater estuary; it then widens abruptly into a bay, 130 m. wide and 50 m. long, to a line drawn from Este point to Rasa point, the N. E. and S. W. limits of the entrance, where the water discharged by the rivers begins to mingle with the ocean. The estuary has a general direction of N. W. by W. and S. E. by E., and its width varies from 55 to 21 m. The northern coast is rocky, elevated, and undulating; the southern, low, marshy, and of uniform height. Bordering the former are the Sierra de las Animas and the Pan de Azucar, 15 m. within the entrance; the Monte, an isolated pyramidal hill, 50 m. within, from which Montevideo derives its name; and the triple cerro of San Juan, near the head of the estuary.

The southern coast is the termination of a vast marshy plain, which constitutes a considerable part of the state of Buenos Ayres. The bed of the Plata is of rock, covered with alternate layers of sand and mud of varying thickness. In a few places the rock rises to the surface, and extensive sand banks and shoals contract the navigable space. Of these the shoalest and most dangerous is English bank, which is chiefly rock. Among the others are the Ortiz, Archimedes, French, Chico, and Astrolabe banks, all of sand; and the Cuirassier and Piedras, composed of a kind of hard mud or rotten stone called tosca. A comparison of the most recent with the earliest surveys demonstrates that the general direction of the channels is the same, but the form and dimensions of the banks have changed. "Within the past 15 years the sands have been washed away from the Cuirassier, and the Ortiz has been prolonged 12 m. S. E. The average depth of the Plata has decreased, but that of the channels appears to remain the same. This diminishes from 15 fathoms in the entrance to 5 fathoms opposite Montevideo; thence to Buenos Ayres there is a minimum depth of 20 ft. in the channels; and across the flats in the head of the estuary the deepest channels have only 13 ft. in ordinary stages of the tide and rivers.

Three islands only are found within the limits of the Plata: Gorriti, just within the entrance, a mile long and a quarter of a mile wide; Flores, 45 m. within, three quarters of a mile long and a quarter of a mile wide; and Martin Garcia, a mass of granite in the head of the estuary, about two miles in circumference, rising in the form of a flattened cone to an elevation of 190 ft. The position of this island, between and commanding the only channels leading from the Plata to the rivers accessible to large vessels, renders it a strategic point of great importance. By a treaty signed in 1853 between the Argentine Republic and the United States, the two nations agree to use their power to prevent the island from being retained by any state which shall not adhere to the principles of the free navigation of the rivers. Bordering the northern coast are several islets and rocks, of which a cluster near Colonia is the largest. Floating islands, called camelotes, are frequently met with in the estuary, and occasionally some distance at sea.

They are detached from the low lands bordering the rivers, held together by roots, and on them are sometimes found specimens of the beasts and reptiles which inhabit the Gran Chaco. From one which stranded a short distance above Martin Garcia island, a jaguar of extraordinary size was captured in 1870. The tides and currents of the waters of the Plata follow no regular law, being greatly influenced,' if not entirely created, by the prevailing or approaching wind. This is the result of its peculiar shape and shallowness. A rise or fall of the waters, accompanied by violent currents, frequently occurs without any apparent cause. The mean difference of level is about 9 ft., the maximum 20 ft. within the limits of the estuary. The navigation of the Plata is attended with much risk. Some lights have been established, but not enough. The estuary contains no port which is secure in all weathers; and numerous wrecks and great loss of life have resulted from this want. The basin of the Plata drains an area estimated at 1,200,000 sq. m., and affords about 10,000 m. of inland navigation. The movement of shipping in the river is now (1875) about 4,000,-000 tons per annum, and the value of imports and exports about $140,000,000. The fish caught in the Plata are not suitable for food.

The hair seal abounds in the bay, and is caught for the hide and oil. - The discoverer and date of discovery of the Plata remain in doubt. The brothers Pinzon appear to have arrived within its limits in 1508; but Juan Diaz de Solis was the first to enter it, in 1512. After a short stay his vessels were driven to sea by a gale, and he returned to Spain to report the discovery. Solis revisited the estuary in 1515, and was murdered by the natives, near a small stream which now bears his name. He called the estuary Mar Dulce; after his death it was called Rio de Solis until the exploration of Sebastian Cabot in 1527, who named it Rio de la Plata, believing it to be the long sought route to El Dorado. - See "Argentine Republic, Handbook of the River Plate," by M. G. and E. T. Mulhall (New York, 1869), and "The Rio de la Plata," by Lieut. Commander Henry H. Gorringe, U. S. N. (published by government, "Washington, 1875).