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Geography

Why Alluvial Fans Form at Mountain Front Junctions

Quick fact

Some alluvial fans in the Himalayas are so large they cover hundreds of square kilometers and can be seen from space.

Why this is interesting

Have you ever noticed the way a pile of sand spreads out when you pour it from a narrow cup? Now imagine a mountain stream doing the same thing—but on a scale that can cover a whole valley. Why does it spread so dramatically?