life- cycle stages. Until recently, the extensive damming
activities routinely seen in countries such as the United
States were unknown in South America. Damming
changes the flood cycle, isolating previously flooded
areas from the annual flooding. Doing so throughout
much of Amazonia would cause a substantial disruption
of many ecological relationships and interdependencies.
This topic is discussed more in chapter 18.
Diverse Riverine Habitats
In the ecological sense, rivers are really part of the
terrestrial ecosystems that border them. A river is
dynamic: it varies seasonally and, with time, turns
and twists within its floodplain, creating a diversity of
habitats.
Open River
From a ship sailing upriver on the Amazon, the
width of the vast river can take on the appearance of
a small inland sea (plate 12- 6). Strong currents and
continuously changing underwater sediment bars make
navigation challenging, and large ships sometimes run
aground. There is not much wildlife to be seen on the
open river (though there is much in it). In the central
and lower Amazon, skies are typically clear above the
widest stretches of the river, while neighboring forests
have clouds above, the result of forest transpiration. But
along the upper Amazon the humidity is often such that
the river itself is cloud-covered and subject to intense
cloudbursts.
Beaches and Sandbars
The vast tonnage of sediment washed from the Andes
and carried by the tributaries and the Amazon itself is
often deposited along the river edge or as bars in the
river itself (plates 12- 7– 8). Along the várzea forests of
the upper Amazon the sediment is deep blackish gray,
the rich volcanic soil transported from the high Andes.
Altered annually by the flood cycle, sediment deposits
form extensive beaches and sandbars that provide
habitat for birds such as plovers, Black Skimmer, and
various herons and egrets, as well as good resting
places for caiman. Swallows of various species, mostly
seen in flight over the river, can often be seen perched
on small snags and bushes along beaches and sandbars.
Sandbar ScrubAndean sediment is rich in nutrients and does not go
uninhabited for very long. Once sediment is deposited
from the action of riverine dynamics, the area is subject
to colonization by pioneer plant species. Various
plants, including those of such familiar temperate
genera as Salix, the willows, quickly invade and often
become sufficiently dense to stabilize the soil. Sandbar
scrub is typically dense, composed of a low diversity
of fast- growing, colonizing plant species (plate 12- 9).River Islands
From the riverside walk at Iquitos, Peru, you cannot see
across to the far side of the Amazon River, but that’s not
because the river is too wide. It is because the far bank
is usually blocked from view by huge sediment islands.
Padre Island is the main island visible from Iquitos,
though just to the west of the city one finds Timarca and
Tarapoto islands. River islands can be of all sizes, but
the big ones are stable, composed of years of sediment
deposit stabilized by vegetation invasion (plate 12- 10).
Forests grow on the river islands and can be managed for
sustained yield of various products. Many humans inhabit
and farm the river islands (as well as várzea floodplain
bordering rivers), planting rice, corn, peppers, beans,
and bananas. Whole towns can be found on the larger
islands. The riverine people, called ribereños in western
Amazonia and caboclos in Brazil, have long inhabited
riverine areas, and actively alter the species composition
of the forest in order to achieve economic gain. A study
from the southeastern portion of the Amazon estuary
in the vicinity of Belém, Brazil, and the Río Tocantins
demonstrated that the local people employ several
management techniques, including the active removal
of unwanted plant species (such as certain spiny palms),
removal of firewood species, cultivation of such species
as cacao, avocado, and mango, and maintenance of
potentially useful species such as rubber trees. The result
of such management activities is that the forest is altered
in species composition but is nonetheless maintained as
forest in a sustainable manner. Ribereños also are active
agriculturalists, planting maize, rice, manioc, bananas,
and other crops. Fish are the major protein source for
most river inhabitants.
River islands, beaches, sandbars, and sandbar scrub
are all related in a process that ecologists term point bar
ecological succession. In this process, when a sandbar
forms it provides habitat for plants, and through seed210 chapter 12 cruising the rivers to the sea