Geoscience Reference
In-Depth Information
again as the waters recede. This annual rise and fall of the Earth's crust is the largest we
have detected.
Many of the Amazon's plants and animals have adapted to living in an environment that is
seasonally waterlogged, some areas for up to 11 months a year and to depths of 13 metres.
Many tree species of the Amazon rainforest depend on the floods for seed dispersal, for
example, either floating downriver or through fish species that are dependent on fruits and
seeds. The great diversity of the Amazon's aquatic habitats has played a key role in produ-
cing the most diverse fish fauna on the planet. In total, with about 2,500 fish species that
have been described by scientists (and probably more than 1,000 still awaiting description),
the Amazon's species richness comfortably exceeds that of all other large river basins. Its
two mightiest fish, the arapaima and the piraíba, each has a maximum weight of about 200
kilograms, more than twice that of an average man.
The one measure by which the Amazon is not generally regarded as the greatest of all rivers
is length. It is comfortably the longest in the Americas, but most authorities place the Nile
at the top of the world list. However, the difficulties in measuring the length of a river mean
that debate on the matter will undoubtedly continue (see above).
The Onyx: an unusual river
The longest river in Antarctica, the River Onyx is just 32 kilometres in length and is in
many respects quite different from rivers in most parts of the world. The Onyx is situated
in the McMurdo Dry Valleys region, one of a small number of ice-free desert areas that
occur along the coastline of an otherwise ice-covered continent. The climate is very dry
and bitterly cold, with an average annual temperature of −20°C. The small amount of pre-
cipitation (less than 100 millimetres a year) that does fall comes only as dry snow and has
virtually no direct effect on the river because fierce winds mean it never settles. Hence, the
Onyx, and other rivers of the McMurdo Dry Valleys, flow for only four to ten weeks a year.
This occurs during the summer when the temperature is warm enough to melt glacier ice,
the only source of river water.
The Onyx flows from the Lower Wright Glacier into Lake Vanda, which has a salinity more
than ten times that of seawater and a permanent cover of ice. There are no plants in the re-
gion and no fish or insects in the river, but worms, microscopic animals, and communities
of algae growing as mats inhabit the river bed. These algal mats can survive long periods
of desiccation, making the Onyx a relative hotspot of life in an otherwise barren landscape.
