Encyclopedic Entry

An alluvial fan spreads out in Death Valley.

Photograph by Walter Meayers Edwards, National Geographic

Extraterrestrial Alluvium
Alluvial fans exist on other planets. The presence of alluvial fans on Mars gives evidence for the existence of liquid water on the planet billions of years ago.

An alluvial fan is a triangle-shaped deposit of gravel, sand, and even smaller pieces of sediment, such as silt. This sediment is called alluvium.
 
Alluvial fans are usually created as flowing water interacts with mountains, hills, or the steep walls of canyons. Streams carrying alluvium can be trickles of rainwater, a fast-moving creek, a powerful river, or even runoff from agriculture or industry. As a stream flows down a hill, it picks up sand and other particles—alluvium. 
 
The rushing water carries alluvium to a flat plain, where the stream leaves its channel to spread out. Alluvium is deposited as the stream fans out, creating the familiar triangle-shaped feature. 
 
The narrow point of the alluvial fan is called its apex, while the wide triangle is the fan's apron. Alluvial fans can be tiny, with an apron of just a few centimeters spreading out from the trickle of a drainpipe. They can also be enormous. Over time, water flowing down the Koshi River in Nepal, for example, has built up an alluvial fan more than 15,000 square kilometers (almost 5,800 square miles) wide. This "megafan" carries alluvium from the Himalaya Mountains. 
 
Types of Alluvial Fans
 
A bajada is the convergence, or blending, of many alluvial fans. Bajadas are common in dry climates, such as the canyons of the American Southwest. Bajadas can be narrow, from the flow of two or three streams of water, or they can be wide, where dozens of alluvial fans converge.
 
Alluvial fans and bajadas are often found in deserts, where flash floods wash alluvium down from nearby hills. They can also be found in wetter climates, where streams are more common.
 
Alluvial fans are even found underwater. A subaqueous fan is created as an underwater current deposits alluvium from a submarine hill or glacier.
 
Sometimes, fans are formed without the aid of water. These are called colluvial fans. Colluvial fans are created by mass wasting. Mass wasting is simply the downward movement of rock, soil, or other material. Alluvium is material transported by water, while colluvium is material transported by mass wasting. Landslides are an instance of mass wasting that often create colluvial fans. 
 
A debris cone is a type of alluvial fan with a steep slope, closer to the shape of a half-cone than a flat fan. Debris cones can be created by the slow accumulation of alluvium over many centuries. They can also form as boulders and other large materials gather during landslides, floods, or other instances of mass wasting.
 
Life Near the Fan
 
Alluvial fans can be very diverse habitats. Shrubs such as rabbitbrush and greasewood, or even trees such as ash or willow, are common in the area of alluvial fans. These plants have very deep roots, which can access the water that helped create the alluvial fan, but has now sunken far below it.
 
Creating a settlement on an alluvial fan can be dangerous. Alluvial fans are prone to flooding. Rushing water, mud, and debris can threaten communities many kilometers away from the apex of the alluvial fan. 

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

accumulation

Noun

a buildup of something.

agriculture

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

Encyclopedic Entry: agriculture

alluvial fan

Noun

fan-shaped deposit of eroded material, usually sediment and sand.

Encyclopedic Entry: alluvial fan

alluvium

Noun

gravel, sand, and smaller materials deposited by flowing water.

apex

adjective, noun

tip, point, top, or summit.

apron

Noun

area covered by a deposit of sediument, usually at the foot of a hill or glacier.

bajada

Noun

area where several alluvial fans meet.

boulder

Noun

large rock.

canyon

Noun

deep, narrow valley with steep sides.

Encyclopedic Entry: canyon

channel

Noun

deepest part of a shallow body of water, often a passageway for ships.

climate

Noun

all weather conditions for a given location over a period of time.

Encyclopedic Entry: climate

colluvial fan

Noun

triangle-shaped deposit of eroded material transported by mass wasting.

converge

Verb

to meet or come together.

creek

Noun

flowing body of water that is smaller than a river.

current

Noun

steady, predictable flow of fluid within a larger body of that fluid.

Encyclopedic Entry: current

debris

Noun

remains of something broken or destroyed, waste, or garbage.

debris cone

Noun

alluvial fan with a slope of more than 10 degrees.

deposit

Verb

to place or deliver an item in a different area than it originated.

desert

Noun

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

Encyclopedic Entry: desert

diverse

Adjective

varied or having many different types.

enormous

Adjective

very large.

flash flood

Noun

sudden, short, and heavy flow of water.

glacier

Noun

mass of ice that moves slowly over land.

Encyclopedic Entry: glacier

gravel

Noun

small stones or pebbles.

habitat

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Encyclopedic Entry: habitat

hill

Noun

land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

Encyclopedic Entry: hill

industry

Noun

activity that produces goods and services.

landslide

Noun

the fall of rocks, soil, and other materials from a mountain, hill, or slope.

Encyclopedic Entry: landslide

mass wasting

Noun

downward movement of rock, soil, and other material.

mountain

Noun

landmass that forms as tectonic plates interact with each other.

plain

Noun

flat, smooth area at a low elevation.

Encyclopedic Entry: plain

prone

Adjective

vulnerable or tending to act in a certain way.

rain

Noun

liquid precipitation.

Encyclopedic Entry: rain

river

Noun

large stream of flowing fresh water.

Encyclopedic Entry: river

root

Noun

part of a plant that secures it in the soil, obtains water and nutrients, and often stores food made by leaves.

runoff

Noun

overflow of fluid from a farm or industrial factory.

Encyclopedic Entry: runoff

sand

Noun

small, loose grains of disintegrated rocks.

sediment

Noun

solid material transported and deposited by water, ice, and wind.

Encyclopedic Entry: sediment

shrub

Noun

type of plant, smaller than a tree but having woody branches.

silt

Noun

small sediment particles.

Encyclopedic Entry: silt

slope

Noun

slant, either upward or downward, from a straight or flat path.

stream

Noun

body of flowing fluid.

subaqueous fan

Noun

triangle-shaped deposit of sediment transported by an underwater current or glacier.

Credits

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Editor

Jeannie Evers

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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