Examples of major blackwater rivers include the Río
Negro and Río Urubu. Clearwater rivers include the Río
Tapajós, Río Trombetas, Río Xingu, and Río Curuá Una.
Whitewater rivers, abundant with sediment, include the
Río Jutaí, Río Juruá, Río Madeira, Río Purús, Río Napo,
and the upper Amazon itself.
Before the ever- increasing numbers of roads and
airstrips, these tributaries served as the only access to
the interior. Cities such as Iquitos, Peru, are, even now,
accessible only by boat or airplane.The Amazonian Flood CycleStanding in the downpour watching the trails turn into
mud, I was surprised at first to discover that the level
of the Amazon itself, near were I stood, was dropping
nearly 1 m (3.3 ft) a day, even as it was raining heavily
throughout the region. But the river’s depth was not
closely tied to the rain falling on this rain forest near
Iquitos, Peru. The river was dropping because it had
stopped snowing in the Andes, the meltwater had
already drained, absorbed by the Amazon. Now the
huge river was receding from its peak flood.
The timing of floods and the distribution of floodwaters
result from a complex pattern of seasonal precipitation,
much of it in distant mountains. Because of the vast area
of the Amazon Basin, at any given time some regions
will be experiencing flood while others are at low water.
This is because the equator divides Amazonia (though
it is north of the river itself ) and many of the major
Amazonian tributary rivers are either partly north or
entirely south of the equator. Rainy season generally
occurs in southern Amazonia from October to April.
Rainy season in Manaus, in the middle of the basin, is
from November to May, and rainfall is highest in the
northern part of the basin from April through June. The
wettest months in Iquitos, Peru, which receives between
300 and 400 cm (118– 157 in) of annual rainfall, are
February through May, though there is much variability.
In fact, according to meteorological records kept at
Iquitos Airport, every month of the year except May
has, at one year or another, been the low water month.
Seasonality in Iquitos is rather variable.
In general, flooding in the northern waters occurs
as southern waters are low and vice versa. In areas fed
by one major tributary, there is a single annual flood.
But in those regions fed by both southern and northern
tributaries, there are two annual flood periods, which
may differ from one another in intensity. As rainyseason proceeds, floodwaters build such that the peak of
the flood cycle usually occurs at the onset of dry season.
Because some parts of the Amazon are at flood while
other parts are at low water, there is little difference
between the minimum and maximum annual discharge
rates, which vary by only a factor of two or three. This is
in marked contrast to the Orinoco River, all of which lies
north of the equator (discussed above).
The low, flat geomorphology of the Amazon Basin is
conducive to flooding (plate 12- 5). Though sediment has
a strong tendency to build up along riverbanks, forming
levees, Amazonian rivers will routinely overflow their
banks at full flood. The general floodplain is characterized
by land that is not uniformly flat, creating habitats such
as temporary lakes and swamps. Floodplain forest is
estimated to occupy approximately 100,000 km^2 (38,610
mi^2 ) within the total Amazon Basin.
The ecological importance of the flood cycle should
not be underestimated. Amazonian rivers experience
an annual fluctuation that averages between 7 and 13
m (23– 43 ft), which can result in a floodplain forest
inundated to a depth of 10 m (33 ft) annually, a water
fluctuation that can bring river water as far as 20 km
(12.5 mi) into the neighboring forest. Flooding is
essential in dispersing sediment, fertilizing várzea
floodplain forests, and enabling fish and other organisms
to make use of gallery forest during high water (and
act as seed dispersers, as described in chapter 10).
Zooplankton reproduction peaks during high water,
and this resource, which washes into neighboring rivers
as the flood recedes, provides an invaluable resource for
fish, especially, but not exclusively, during their juvenilePlate 12- 5. These houses on stilts are part of the city of Iquitos,
Peru, which is subject to strong flooding by the Amazon River
during the peak of the annual flood cycle. Photo by John
Kricher.chapter 12 cruising the rivers to the sea 209