Amazon River, the world's largest river. The Amazon is 3,900 miles long (exceeded only by the Nile), discharges an average 7 million cubic feet per second, and drains more than 2.5 million square miles. It arises in Peru, drops down the eastern slopes of the Andes, and then flows east through Brazil to the Atlantic Ocean. Its depth varies from 66 to 660 feet within Brazil, and its width ranges up to 7 miles (near where it is joined by the Xingu River). Throughout much of its lowlands plain, the Amazon is actually a labyrinth of waters, fed and interconnected by some 11,000 large and small tributaries. A northwestern tributary, the Rio Negro, is joined to the Orinoco basin by the swampy Casiquiaré Canal.
The lowlands basin is bounded to the north by the Guiana Highlands and to the south by the Brazilian Highlands. Thus the river and its tributaries are forced into a narrow course for their last several hundred miles before reaching the sea. This creates a daily tidal bore (the pororoca) up to 16 feet high, which makes navigation hazardous.
Due to the enormous size of the Amazon, the first Europeans in the region called it the River Sea. Today, Brazilians call it the Amazonas only to its junction with the Rio Negro; above there it is known as the Solimões. Spanish Americans refer to the section west of Iquitos as the Marañón. The lowlands constitute the world's largest rain forest, renowned for its incredibly rich biological diversity.
Because the drainage lies in the Northern and Southern hemispheres, the Amazon experiences two rainy seasons, in February and July. The floodwaters rise as much as 50 feet over dry-season levels. The main course of the river, fed by Andean tributaries, carries vast quantities of silt and organic matter. For this reason, the natives called it white water. The Negro, however, drains lowlands, is relatively free of silt, and picks up a dark, acidic character from mangrove roots along its course. Its waters were called black by the people who originally lived along its banks. When they meet near Manaus, the white and black waters swirl and intermingle for nearly 30 miles before blending.
In 1500 and 1501, explorers Vicente Yáñez Pinzón and Amerigo Vespucci sailed across the Amazon's mouth, which straddles both the equator and the Tordesillas Line. At that time an estimated 2.4 million natives lived in the basin.
The Spanish government claimed the river, based on the Treaty of Tordesillas. This claim was reinforced by the first exploration, carried out by a Spaniard. In 1541–1542, Francisco de Orellana took command of an expedition organized by Gonzalo Pizarro and sailed down the Amazon, claiming it for Spain. Because the South Atlantic lay within Portugal's sphere, however, the Portuguese gradually gained jurisdiction. Spain annexed Portugal in 1580, so the matter seemed moot.
In 1637–1639 the Portuguese captain Pedro Teixeira, with secret orders to secure the area for his government, led an expedition up the Amazon. When Portugal declared its independence from Spain in 1640, the Amazon remained under its control.
In 1750 Spain and Portugal signed the Treaty of Madrid, which recognized Portugal's effective occupation of the southern Amazon basin. It designated the Paraguay, Guaporé, Mamoré, and Madeira rivers as an international boundary. Over the next century and a half, Portugal (and later Brazil) managed to annex more territory in the northern and western Amazon basin.
See alsoBrazil, Geographyxml .
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Michael L. Conniff