At Paso de Patria, on the right (Paraguayan) bank, the Paraná receives its greatest tributary, the Paraguay River. The fifth largest river in South America, the Paraguay (Spanish: Río Paraguay; Portuguese: Rio Paraguai) is 1,584 miles (2,550 kilometres) long. The name Paraguay, also taken from the Guaraní language, could be translated “river of paraguas (coloured, plumed birds)” or “river of cockades,” an allusion, perhaps, to the plumed headdresses once worn by the riverine peoples.

The Paraguay also rises in southern Brazil, in the central plateaus of Mato Grosso state, at an altitude of 980 feet above sea level. Where it becomes navigable for small craft—about 150 miles downstream, near Cáceres, Brazil, after its confluence with the Sepotuba River—it is 275 feet wide and 20 feet deep. Another 20 miles downstream, where the Jauru River joins it at an elevation of 400 feet, the Paraguay enters the Pantanal, a vast seasonal swamp that covers much of southern Mato Grosso and northwestern Mato Grosso do Sul state. During the dry season (May to October) the swamps in the Pantanal shrink to small patches of marshy land. With the onset of the rains in November, the slow-flowing rivers are quickly filled to capacity, and a large, shallow lake is formed. Spanish missionaries mistook this for a permanent lake, and it appeared as “Lago Xarays” on early maps of the region.

The Paraguay’s main channel skirts the Pantanal’s western edge over a sandy bed, flowing around the many islands in its course. During its passage through the Pantanal, the river receives such important tributaries as the Cuiabá, Taquari, and Miranda rivers. About 470 miles downstream, it flows north-south to form the boundary between Brazil and Paraguay before being joined by a tributary, the Apa River, that flows in from the east and demarcates part of the Brazilian-Paraguayan frontier. The river then enters Paraguay, having traveled about 640 miles from its source. After flowing for more than 200 miles across Paraguay, it is joined by the Pilcomayo River at the Argentinian border, near Asunción. It then flows south-southwest along the Argentine-Paraguayan frontier for about 140 miles, until it is joined on its west bank by the Bermejo River. Continuing along the border for another 40 miles, it then empties into the Paraná River at a short distance from the Argentine city of Corrientes.

From its confluence with the Apa for the 630 miles to its mouth, the Paraguay runs on a shallow, broad bed, with an average width of about 2,000 feet. South of Asunción, the river’s right (Argentine) bank gradually lowers, whereas its left (Paraguayan) bank becomes elevated, forming cliffs. Along this stretch, floods develop principally on the western bank, spreading over the Argentine plain for distances of from three to six miles. These lands form part of the Gran Chaco.

Physiography of the lower Paraná basin

After its juncture with the Paraguay, the combined stream of the Paraná turns southward as it passes Corrientes. It now becomes a typical “plains” river, banked by its own alluvial deposits and having an extensive floodplain on its right bank, with tracts up to 24 miles wide subject to inundation. Its permanent bed, about 2.5 miles wide at Corrientes, narrows to about 8,000 feet at Bella Vista, to about 7,000 feet at Santa Fe, and to about 6,000 feet at Rosario, and it is strewn throughout with chains of islands. Santa Fe, on the right bank opposite the port of Paraná, stands where the Paraná receives its last major tributary, the Salado River. Between Santa Fe and Rosario, however, the right bank begins to rise as the river skirts the edge of the undulating plain, which flanks it down to the delta, and reaches altitudes ranging from about 30 to 65 feet. The left bank, meanwhile, is always higher than the right but has to sustain the erosive action of the water, which becomes increasingly turbid as great masses of soil are constantly falling into it; in the delta the main branch of the river runs along a break in the terrain, with its left bank consisting of a cliff about 75 feet high.

The delta of the Paraná has its apex as far north as Diamante, upstream from Rosario, where branches of the river begin to turn southeastward. About 11 miles wide at its upper end, the width of the delta grows to roughly 40 miles at the river mouth, where the separated branches of the Paraná flow into the Río de la Plata, about 200 miles from Diamante. With an area of 5,500 square miles, the delta is advancing steadily, as an estimated 165 million tons of alluvial deposits are added annually. Within the delta the river divides again and again into distributary branches, the most important being the two last great channels, the Paraná Guazú and the Paraná de las Palmas. The islands of the delta, alluvial in origin, are low-lying and of varying size. Their shores and the outer fringes of the river have protective embankments covered with trees but nevertheless may be submerged in times of flooding, when they present the appearance of flooded forests.

Physiography of the Uruguay basin

The Uruguay River (Spanish: Río Uruguay; Portuguese: Rio Uruguai) is the other major system, 990 miles (1,593 kilometres) in length, that flows into the Río de la Plata. Like the Alto Paraná and the Paraguay, the Uruguay originates in southern Brazil, formed by several small streams that rise on the western slopes of the Serra do Mar. From the south it is joined by the Pelotas River, which divides the states of Rio Grande do Sul and Santa Catarina. After flowing west, the Uruguay turns southwest at its juncture with the Peperi Guaçu River, the first sizable tributary to join it from the north. For most of its course, the fast-flowing Peperi Guaçu marks the boundary between the Argentine province of Misiones and Brazil; and after its confluence with the Uruguay, the latter river divides Brazil and Argentina. A few miles beyond the juncture with the Peperi Guaçu, the river is constricted between rocky walls in the Grande Falls, a two-mile stretch of rapids with a total descent of 26 feet in 8 miles. At the cataracts, the river narrows suddenly from 1,500 feet to a minimum of 100 feet.

Several small rivers join the Uruguay from the west and are navigable in their lower reaches by canoes and small boats. The principal ones, from north to south, are the Aguapey, Miriñay, Mocoretá (which divides Entre Ríos and Corrientes), and Gualeguaychú. The important tributaries of the Uruguay, however, come from the east. The Ijuí, Ibicuí, and the Cuareim are short rivers but of considerable volume; the last forms part of the boundary between Brazil and Uruguay. At the mouth of the Cuareim, the Uruguay becomes the boundary line between Argentina and Uruguay, and the river flows almost directly south. A dam above the falls at Salto, Uruguay, impounds Salto Grande Reservoir some 40 miles upstream. The Negro River, approximately 500 miles long and the Uruguay’s largest tributary, joins the latter only 60 miles from the Río de la Plata. The Negro rises on the Brazilian border in Rio Grande do Sul state and flows westward through central Uruguay. Like the Alto Paraná, the Uruguay generally is clear and carries little silt, except in the seasonal floods. After its juncture with the Negro, the Uruguay broadens sharply to a width of 4 to 6 miles and becomes a virtual extension of the Río de la Plata estuary.