Geoscience Reference
In-Depth Information
sustenance. Flowing water both delivers and removes many vital nutrients and other con-
stituents to and from ecosystems, but rivers also have effects that may be less immediately
obvious. The distribution of many terrestrial plant and animal species concords with the
geography of major river systems because rivers can act both as corridors for species dis-
persal but also as barriers to the dispersal of organisms. One of the first to recognize the
importance of rivers as obstacles to the movement of certain creatures was the naturalist
Alfred Russel Wallace, who in the mid-19th century defined distinct areas in South Amer-
ica bounded by major rivers in the Amazon Basin, each with its own distinct communities
of species. This idea of the river acting as a barrier is one of a number of hypotheses put
forward to explain the evolutionary origin of the astonishing richness of species found in
Amazonian forests.
The Amazon: mightiest of them all
By almost every measure, the Amazon is the greatest of all the large rivers. Encompassing
more than 7 million square kilometres, its drainage basin is the largest in the world and
makes up 5% of the global land surface. The river accounts for nearly one-fifth of all the
river water discharged into the oceans. The flow is so great that water from the Amazon can
still be identified 125 miles out in the Atlantic: early sailors could drink fresh water from
the ocean long before their first sighting of the South American continent. Nonetheless, the
lower reaches of the Amazon flow down such a gentle gradient that the physical influence
of sea tides can still be identified more than 1,000 kilometres upstream from the Atlantic.
The Amazon has some 1,100 tributaries, and 7 of these are more than 1,600 kilometres
long. The main tributaries are often classified according to the colour of their waters, which
also reflects their source. Black-water tributaries attain their tea colour from high levels of
dissolved plant matter leached from low-lying areas of sandy soils. White-water rivers are
coloured by the high loads of sediments transported from the Andes. The clear-water rivers
carry low levels of sediments and organic matter from the crystalline rocks of the Guyana
and Brazilian shields.
In the lowlands, most Amazonian rivers have extensive floodplains studded with thousands
of shallow lakes. Up to one-quarter of the entire Amazon Basin is periodically flooded, and
these lakes become progressively connected with each other as the water level rises. Re-
searchers using GPS have measured a sizeable part of South America sinking by nearly 8
centimetres because of the extra weight due to flooding in the Amazon, an area that rises
