In contrast, soils in various places— Puerto Rico, many
parts of Central America, including much of Costa Rica,
and throughout much of the Andes Mountains— are not
mineral poor but mineral rich. Mostly volcanic in origin,
these eutrophic soils are younger, some up to 60 million
years old, some much more recent. Though exposed
to high rainfall and temperature, they can be farmed
efficiently and will maintain their fertility if basic soil-
conservation practices are applied. Because so much
sediment leaches by runoff from the land into the river,
waters that drain rich soils are typically cloudy, and are
called whitewaters. This terminology can be confusing:
whitewater rivers do not drain white, sandy soils;
blackwater rivers do. Whitewaters drain nutrient- and
sediment- rich Andean soils, and the term white refers to
the cloudy appearance of the water, loaded as it is with
sediment (plates 6- 12– 13). Mocha would perhaps be a
better term to describe the water color.
The Wedding of the Amazonian Waters
A dramatic example of the difference between blackwater
and whitewater rivers occurs at the confluence of the
Amazon River and the Río Negro near Manaus, Brazil.
The clear, dark Río Negro, a major tributary draining
some of the white, sandy soils of the ancient Guiana
Shield, meets the muddy, whitewater Amazon, rich in
nutrient load, draining mostly from the youthful though
distant Andes. The result, locally called the “wedding of
the waters,” is a swirling maelstrom of soupy mocha-
colored Amazonian water irregularly mixing with clear
blackwater from the Negro, a process that continues
downriver for anywhere from 15 to 25 km (9– 16 mi),
until the mixing is complete (plate 6- 14). The most
remarkable feature is that both soil types support
impressive rain forest, igapo in the blackwater areas,
várzea in the whitewater areas.
Why Parrots Eat Dirt
Many species of animals throughout the world have
been observed to intentionally ingest soil, a behavior
known as geophagy. Geophagy is common among
many bird, mammal, and insect species throughout
the tropics. It is also widely practiced by humans in
many tropical areas. Several reasons for geophagy have
been suggested, and they are not mutually exclusive.
Geophagy typically involves ingestion of soils high
in clay content. Clay, because of its negative charge,
binds potential toxins such as alkaloids and phenols.
Ingested, it may aid in preventing diarrhea, help treat
intestinal parasites, and supply vital minerals. In
humans, geophagy is associated with pregnancy and
may be an adaptation to eliminate plant toxins that
could cause morning sickness.
Parrots throughout the tropical world are known to
ingest clay. In western Amazonia many parrot species
(family Psittacidae) gather along certain outcroppings
of soil, usually along riverbanks, called collpas, or
clay licks (plate 6- 15). The birds, which may numberPlate 6- 15. Parrots of several species at a collpa (clay lick) along
the Río Napo in Ecuador. Photo by John Kricher.Plate 6- 14. The “wedding of the waters,” where the cloudy
mocha- like sediment- rich Amazon intersects with the clear,
dark, sediment- poor Río Negro. The confluence occurs near
Manaus, Brazil. Photo by John Kricher.90 chapter 6 essential dirt: soils and cycling