Dictionary.com
definitions

alluvial fan

noun Physical Geography.
  1. a fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain.

Origin of alluvial fan

First recorded in 1870–75
Also called alluvial cone.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for alluvial fan

alluvial fan

alluvial cone

noun
  1. a fan-shaped accumulation of silt, sand, gravel, and boulders deposited by fast-flowing mountain rivers when they reach flatter land
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
alluvial fan in Science

alluvial fan

[ə-lōō′vē-əl]
  1. A fan-shaped mass of sediment, especially silt, sand, gravel, and boulders, deposited by a river when its flow is suddenly slowed. Alluvial fans typically form where a river pours out from a steep valley through mountains onto a flat plain. Unlike deltas, they are not deposited into a body of standing water.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.