Geoscience Reference
In-Depth Information
Some rivers are thought to be very old. Evidence from sediments deposited near the mouth
of the Amazon suggests that the river has been flowing across South America for 11 mil-
lion years. Over such great periods of time, all sorts of factors change, of course, and some
rivers come and go. An example of a river that has disappeared is the Channel River which
flowed westward in northwestern Europe some 20,000 years ago in the area now sub-
merged beneath the English Channel separating Britain and France. This was the height of
the last Ice Age, when sea levels all over the world were much lower than they are today
because more water in the hydrological cycle was present as ice. Most of the British Isles
and all of Scandinavia were covered in a thick ice sheet at this time, and the Channel River
was fed by meltwater that flowed in the rivers of southern England, including the Thames
and the Solent, that lay just beyond the permanent ice. Among the Channel River's other
tributaries to the south were the Seine, Somme, Maas, Rhine, and Elbe.
Such ancient river channels are not solely of academic interest. The world's richest gold de-
posits, in the Witwatersrand district of South Africa, were laid down in river systems more
than two billion years ago. Gold carried by these rivers was deposited in gravels where the
velocity of the flowing water slowed. These gravels, known to geologists as Witwatersrand
conglomerate, have produced nearly 50,000 tonnes, or 40%, of the gold ever mined, and
probably still contain over one-third of the world's unmined gold reserves. Rivers have also
played a key role in creating the valuable diamond deposits that stretch along the western
coast of southern Africa. Diamonds have been eroded from deposits inland and carried to
the coastline by the Vaal and Orange Rivers for 100 million years and more. This fluvial
transport is also thought to be beneficial to the quality of the diamonds found in coastal
sediments because the stones tend to break down during transport, increasing the concen-
tration of higher-quality diamonds.
Erosion, transport, and deposition
An important measure of the way a river system moulds its landscape is the 'drainage dens-
ity'. This is the sum of the channel length divided by the total area drained, which reflects
the spacing of channels. Hence, drainage density expresses the degree to which a river
dissects the landscape, effectively controlling the texture of relief. Numerous studies have
shown that drainage density has a great range in different regions, depending on conditions
of climate, vegetation, and geology particularly. The value tends to be high in arid regions
of sparse vegetation, in temperate to tropical regions subjected to frequent heavy rains, and
in areas underlain by rocks that are difficult for water to penetrate.
