The 38-kilometer-long (about 24 miles) Lago do Erepecu (Lake
Erepecu) in Brazil runs parallel to the lower Rio Trombetas (Trombetas
River), which snakes along the lower half of this astronaut photograph.
Waterbodies in the Amazon rainforest are often so dark they can be
difficult to distinguish. In this image, however, the lake and river
stand out from the uniform green of the forest in great detail as a
result of sunglint on the water surface. Sunglint is the mirror-like
reflection of sunlight off of a surface directly back towards the
viewer, in this case an astronaut onboard the International Space
Forest soil is red, as shown by airfield clearings near Porto
Trombetas (image far lower right), a river port on the south side of
the Trombetas River. The Trombetas flows into the Amazon River from the
north about 800 kilometers (497 miles) from the Amazon mouth. Despite
being so far from the sea, seagoing ore ships export most of Brazil’s
bauxite from Porto Trombetas. Bauxite is the raw material used to
produce aluminum. (The Trombetas bauxite mine is beyond the lower edge
of the image).
Central Amazonia has many lakes like Erepecu—relatively straight,
large waterbodies located just off the main axis of the large rivers.
These lakes began as rivers that carved deeply into the landscape
during periods of low sea level accompanying ice ages in the past 1.7
million years. When sea level was low, the gradient from a river’s
headwaters to its end at the ocean was steeper, and rivers flowed
faster and carved deeper beds. During intervening warm periods, rising
sea level reduced the gradient at the river’s end so much that it faced
an impossible task—flowing uphill to the ocean.
The only way a river could have continued to flow to the sea is if
it was carrying enough sediment to fill the deep river valleys carved
during low sea level, creating a new “ground level” for the river to
flow across. Many larger rivers like the Trombetas and the Amazon
itself carried enough loose sediment to fill their deeply carved
valleys, and then to trace sinuous courses (lower part of image) across
the new beds. But smaller rivers that carried less sediment could not
fill in their deep valleys; instead, the valleys acted like troughs.
The river water poured in, but couldn’t flow out because of rising sea
level, so lakes like Erepecu formed.
Astronaut photograph ISS020-E-34693
was acquired on August 25, 2009, with a Nikon D2Xs digital camera
fitted with an 180 mm lens, and is provided by the ISS Crew Earth
Observations experiment and Image Science & Analysis Laboratory,
Johnson Space Center. The image was taken by the Expedition 20 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson.