alluvial fan

American  

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.

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alluvial fan British  

noun

1. a fan-shaped accumulation of silt, sand, gravel, and boulders deposited by fast-flowing mountain rivers when they reach flatter land

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alluvial fan Scientific  

/ ə-lo̅o̅′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.

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Etymology

Origin of alluvial fan

First recorded in 1870–75

Example Sentences

Summer storms in Death Valley are usually more localized, closing a road or two and maybe causing an alluvial fan to flash flood, Jurado said, calling Friday’s downpour “exceptionally rare.”

From Los Angeles Times • Aug. 6, 2022

A man stands on an alluvial fan below Tumacacori mountain, a few dozen miles north of here.

From The Guardian • Mar. 1, 2017

The trip I joined was part of a 67-day, 1,058-mile “first,” paddling the Cuito tributary of the Okavango river, which rises in the Angolan Highlands before tipping through Namibia into the alluvial fan in Botswana.

From The Wall Street Journal • Jul. 30, 2015

Over the next few centuries, this creek will seek to become graded again by eroding down through its own alluvial fan.

From Textbooks • Jan. 1, 2015

We emerged from the thickets near a promontory where there was a fine view down the valley and particularly of a heavily wooded alluvial fan just below us.

From Inca Land Explorations in the Highlands of Peru by Bingham, Hiram