Drainage is notably affected by the physical dissymmetry of the continent. The major basins lie east of the Andes, and the main rivers flow to the Atlantic Ocean. The four largest drainage systems—the Amazon, Río de la Plata (Paraguay, Paraná, and Uruguay rivers), Orinoco, and São Francisco—cover about two-thirds of the continent.

By far the largest system is formed by the Amazon River, which stretches some 4,000 miles (6,400 km) across equatorial South America. The volume of water it carries surpasses that of all other rivers, constituting one-fifth of the total flowing fresh water of the world. About 6,350,000 cubic feet (179,800 cubic metres) of water per second is emptied into the Atlantic by the Amazon, which is more than 10 times the outflow of the Mississippi River. The Amazon drains some 2,722,000 square miles (7,050,000 square km)—about two-fifths of South America—and has more than 1,000 tributaries, several of which are more than 1,000 miles (1,600 km) long. Rising in the central Peruvian Andes, it is named the Marañón in its upper course; after being joined by several rivers—including the Ucayali River, from which the Amazon’s length traditionally is measured—it escapes from the Andes through narrow canyons (pongos). If measured from the Marañón-Ucayali confluence, the Amazon is second in length only to the Nile. However, more recent measurements have claimed that the Amazon’s source is farther into the Andes, suggesting that the Amazon is the world’s longest river. Near Manaus, it is joined by the Negro River, which drains much of northern Brazil. The Amazon, then at full strength, winds through the low plains to pass between the Guiana Highlands and Brazilian Highlands before emptying into the Atlantic.

The second most important drainage system, estimated to cover some 1.2 million square miles (3.2 million square km), is formed by the Paraguay, Paraná, and Uruguay rivers. Those rivers empty into the Río de la Plata, which actually is an estuary and not a river. About 2,800,000 cubic feet (79,300 cubic metres) of water per second discharge from the common mouth of those rivers, an outflow second only to that of the Amazon. The Paraguay River, with a length of 1,584 miles (2,550 km), rises in the Bolivian hills and empties into the Paraná River. The Paraguay is a river of the plains, flowing across a wide stretch of marshes (the Pantanal) in its middle course; its lower course, however, is drier. The Paraná has a total length of 3,032 miles (4,880 km); its upper course (generally called the Alto Paraná) flows mainly across the eastern high plateaus before its confluence with the Paraguay, after which the river flows through a broad floodplain before emptying into the Río de la Plata. The Uruguay River, at 990 miles (1,593-km), is the shortest of the three; it flows east of the Paraná before discharging into the Paraná delta near Buenos Aires.

The Orinoco River basin is the continent’s third largest drainage system, covering about 366,000 square miles (948,000 square km). With a length of some 1,700 miles (2,740 km), the river first flows west and then north, plunging down a series of steep slopes. It then flows northeast and east along the edge of the Llanos, a flat plain that stretches westward to the Andes. Near the ocean, the Orinoco divides into a series of distributaries to form its delta. A unique feature of the upper Orinoco River is Casiquiare, a navigable waterway that allows water to flow from the Orinoco to the Amazon system, as well as in the reverse direction during high tide in the Amazon.

The basin of the São Francisco River, encompassing some 244,000 square miles (632,000 square km), is South America’s fourth largest drainage system. The river, which flows entirely within Brazil, has a total length of 1,811 miles (2,914 km). It rises in the state of Minas Gerais and flows northward for 1,000 miles (1,600 km) before curving eastward to the Atlantic. The São Francisco has been an important artery of communication since colonial times and is the location of several large hydroelectric projects; many irrigated fields are found around the São Francisco reservoir.

The remainder of the continent’s Atlantic-flowing rivers are of much less importance. Among the largest are the Magdalena in Colombia (navigable in its lower section) and the Essequibo, Maroni, and Oyapock in the Guianas.

Drainage to the Pacific is different because of the close proximity of the Andes to the Pacific coast and the scarcity of rainfall from southern Ecuador to central Chile. Consequently, the rivers are short, and few carry any large quantity of water. The major rivers are the Guayas in Ecuador, on which the port of Guayaquil is located, and the Santa in Peru. Some torrents have had great importance since early times, when good water management facilitated the development of ancient civilizations. In central Chile the valleys of the Aconcagua, Maule, and Biobío rivers have remained fertile agricultural regions.