the.com/alluvial fan

means A fan-shaped spread of sediment deposited by a river or stream where it exits a steep slope onto flatter ground and loses the speed to keep carrying its load.

from From Latin 'alluvius,' meaning 'washed against,' itself from 'ad-' (toward) plus 'luere' (to wash) — the same root that gives us 'alluvium,' the loose sand, silt, and gravel rivers carry and drop. 'Fan' is simply the plain English word for the shape, since the deposit spreads outward from the point where the channel meets open ground, exactly like a hand fan unfolded. Geologists paired the two terms to describe the landform wherever a fast mountain stream suddenly slows on flat terrain and dumps its cargo in that telltale cone.