Peru. Two Amazonian tributary rivers, the Negro and the
Madeira, rank as the fifth- and sixth- largest rivers in the
world with regard to annual discharge. By comparison,
the Mississippi River ranks about 10th and has only about
1/12 the annual discharge of the Amazon.
To the surprise of many, the Amazon River originally
flowed in the opposite direction, draining into the Pacific
Ocean near what is today the port city of Guayaquil,
Ecuador. The river changed to its present west- to- east
course as recently as 10– 15 million years ago, when the
Andean uplift profoundly altered the river’s course, as
well as patterns of biogeography, creating the Amazon
Basin. Initially the uplift of the Andes created a gigantic
lake, bordered on the west by the newly arisen mountain
chain and to the east by the extensive Guiana and
Brazilian Shields. The Amazon finally made its way to
the Atlantic during the Pleistocene, cutting through its
eastern barrier in the vicinity of Obidos, Brazil. Many
widespread trees were probably dispersed eastward by
the altered course of the river water.
The Amazon Basin, drained by the Amazon River
and its gigantic tributaries, covers an area of about 6.92
million km^2 (about 2.67 million mi^2 ) essentially 40% of
the total area of South America. Approximately 1,100
tributaries service the main river, and some of them,
like the Negro, Napo, Madeira, Tapajós, Tocantins, and
Xingu, rank as major rivers.
Amazon tributaries vary in color from cloudy mocha
to clear black depending upon where they originate and
their geological and chemical properties (chapter 6).COLOMBIANegro
BrancoTapajosJapuraJuruaPutumayoMarañónUcayaliLima
PERUPacific
OceanBOLIVIASanta CruzCusco La PazRio BrancoMadre
de DiosPucallpaIquitosCochabambaGuaporeMamoreBeniPôrto
Velho
Juruena XinguMachadoTeles PiresAraguaiaBRAZILSão PauloRio de
JaneiroBrasiliaBelemMacapáTocantinsPurus MadeiraVENEZUELABogotaQuitoCaracasGeorgetown
GUYANAAtlantic
Orinoco OceanAmazon
Santarém
ManausECUADORFigure 12– 2. Map of the Amazon River and its major tributaries. Reprinted with permission from Kricher, John. Tropical Ecology.
Princeton, NJ: Princeton University Press, 2011.
208 chapter 12 cruising the rivers to the sea