Alluvial Fans in Mongolia

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Text, Earth Watch. Exploring the Blue Planet by Satellite.

 

A satellite photo of Earth. White patches stand out against blue.

 

Text, Braided channels and Alluvial Fans on the Mongolia-China border.

 

May 25, 2019, Mongolia, After heavy rainfall in southern Mongolia, floodwaters rush through narrow stream channels south of the Tost Mountains.

 

They create a network of ephemeral streams that wind their way through the rocky ranges wherever the terrain allows.

 

Earth Observatory, Earth Observatory dot NASA dot gov. ASTER, Terra Satellite.

 

China on map.

 

A transition to flatter terrain allows floodwater to spread out into braided streams with interwoven channels.

 

Satellite view shows a map of the Mongolia-China border.

 

The water velocity decreases considerably as the channels widen. As the water slows down, a mixture of gravel, sand, and clay known as alluvium is deposited, forming sandbars.

 

Tan-colored streams running vertically fill the area.

 

Text, Over time, the channels and sandbars migrate back and forth to create triangular deposits known as alluvial fans.

 

In the bottom center is a patch of lighter tan.

 

Text, Geologic research indicates that the channels near the Tost Mountains are filled with a coarse sediment that was likely deposited during a cool, dry period between 23,000 and 9,000 years ago, when sudden flash floods were more common than they are today.

 

The many streams vary in width, and some branch off in other directions. There are random patches of black and white.

 

Text, To learn more, go to Earth Observatory dot NASA dot gov, A text banner runs the width of the screen.