Thirty-six years ago, an astronaut aboard the Space Shuttle Discovery focused a camera lens on South America’s Paraná River. This southward-looking panorama shows the point where the Paraná flows into northern Argentina, then bends south, and finally reaches the Atlantic Ocean at the River Plate estuary (top left). For scale, the distance from Asunción, the Paraguayan capital, to the Argentine capital of Buenos Aires is more than 1000 kilometers (600 miles).
This photo shows a very large ‘inland delta’ where the Paraná River has spread out laterally to form a huge, triangular surface (outlined in gray) that measures 425 kilometers (265 miles) from the apex to the most distant point near La Paz. The entire surface of the delta—known to geologists as a megafan—is covered by numerous channels showing where the Paraná used to flow. The channels can be seen radiating across the delta from the apex.
The photo was taken with a Hasselblad film camera—common in the days before astronauts switched to digital cameras—and before wide views were easily obtainable from the International Space Station. It remains one of the best images to demonstrate megafan landscapes. Two other megafans are partly visible: the lower 100 kilometers (60 miles) of the Pilcomayo and Bermejo megafans. The Pilcomayo megafan is the largest on the planet at 705 kilometers in length.
At the time, views like this were surprising to geologists, who had thought megafans were few and far between on Earth’s surface. (Fewer than a dozen were known at the time.) Orbital images such as this spurred research to identify the largest megafans, defined as being greater than 80 kilometers (50 miles) long. The tally now stands at nearly 300 megafans worldwide.
Megafans can spread across vast areas, especially where they coalesce in major river valleys such as the Paraná. There are many other megafans in South America, but the combined area of just the three megafans above is 376,000 square kilometers (145,200 square mile), almost equal to the land area of Japan.
Megafans are being adapted for agriculture at an ever increasing rate. Their flat landscapes are ideal for developing transport and irrigation infrastructure. Being composed of softer river sediments, they are also relatively easy to plow.
Astronaut photograph STS51D-46-22 was acquired on April 18, 1985, with a Hasselblad film camera using a focal length of 100 millimeters. It is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by a member of the STS-51D crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab 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 Justin Wilkinson, Texas State University, JETS Contract at NASA-JSC.