A gently sloping wedge-shaped mass of sediment deposited by a stream where it comes from a narrow canyon onto a plain or valley floor: Alluvial fans accumulate at and spread from the base of dry mountain ranges where a mountain stream, no longer confined to its narrow channel, deposits its coarse sediment into a large sloping apron-like structure at the point where it comes out onto the plain.

When a stream divides into branching and intertwining lower channels that are separated by islands or sandbars, they twist their way through the fan format and carry sediment from the fan's apex to its fringes and so become alluvial fans.

Alluvial fans also develop in front of melting glaciers, when the water flows out from the ice and spreads out a large amount of coarse debris with it.

Large alluvial fans have deposits that exist in different sizes within the fan, with the bigger pieces of sediment dropping closest to the mountains and the finer materials deposited farther away as the water moves along.