Page 491 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 15 River Systems 455
PACIFIC OCEAN
SOUTH AMERICA
Amazon Basin
Amazon River Mouth
Amazon River Mouth
BRAZIL
(a) Radar images showing elevation are combined with digitally mapped stream channels in this view of the Amazon River basin. Elevations
range from sea level in green to above 4500 m
in white. In 2007, Brazilian researchers reported finding a new source for the Amazon (white box), near Mount Mismi in southern Peru, and a new length measurement of 6800 km, making it longer than the Nile.
drainage basin, which is as large as the Australian continent.
Drainage Divides
In any drainage basin, water ini- tially moves downslope as over- land flow, which takes two forms: It can move as sheetflow, a thin film spread over the ground surface; and it can concentrate in rills, small-scale grooves in the landscape made by the downslope movement of water. Rills may develop into deeper gullies and then into stream channels leading to the valley floor.
The high ground that separates one valley from another and directs sheetflow is called an interfluve (Figure 15.2). Ridges act as drainage
(b) The mouth of the Amazon is 160 km wide and discharges a fifth of all the freshwater that enters the world's oceans. Large islands of sediment are deposited where the flow enters the Atlantic.
▲Figure 15.1 Amazon River drainage basin and mouth. [(a) nASA SRTM image by Jesse Allen, University of Maryland, global Land Cover Facility; stream data World Wildlife Fund, HydroSHEDS project (see hydrosheds .cr.usgs.gov/). (b) Terra image, nASA/gSFC/JPL.]
divides that define the catchment, or water-receiving, area of every drainage basin; such ridges are the divid- ing lines that control into which basin the surface runoff drains.
A special class of drainage divides, continental divides, separates drainage basins that empty into
Mount Mismi
Interfluves
Drainage divide Valley
Drainage divide
Rill
Gully
Valley
Drainage divide
Drainage basin Drainage divide
Drainage basin
▲Figure 15.2 Drainage basins. A drainage divide separates drainage basins.
ATLANTIC OCEAN
Sheetflow