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On the 1st day of summer, we’ve got an #EarthWord that’s a fan-just not the cooling kind...

EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science. Think of us as your terminology tour-guides, and meet us back here every week for a new word!

Image shows an alluvial fan in Iran. By NASA image created by Jesse Allen, using data from NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Rebecca Lindsey, based on interpretation provided on the ASTER Project Science Imagery Gallery Website. -, Public Domain,

The EarthWord: Alluvial Fan


  • Remember earlier when we covered fluvial? This word’s closely related to it. An alluvial fan happens when a fast-moving mountain stream empties out onto a relatively flat plain. All of the sediment it was carrying falls out as the water slows down.


  • Alluvial comes from the Latin alluvius, which meant “washed against.”

Use/Significance in the Earth Science Community:

  • Alluvial fans are part of the natural process of how mountains are eroded into plains. They also play an important role in spreading nutrients around.


Next EarthWord: See a flock of birds fall from the sky? There’s an EarthWord for that, and it isn’t “apocalypse”…

Hungry for some science, but you don’t have time for a full-course research plate? Then check out USGS Science Snippets, our snack-sized science series that focuses on the fun, weird, and fascinating stories of USGS science.