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. 
alluvial fan
An alluvial fan spreads out in Death Valley.

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.

accumulation
Noun

a buildup of something.
agriculture
Noun

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

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

small stones or pebbles.
habitat
Noun

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

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

activity that produces goods and services.
landslide
Noun

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

vulnerable or tending to act in a certain way.
rain
Noun

liquid precipitation.
river
Noun

large stream of flowing fresh water.
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.
sand
Noun

small, loose grains of disintegrated rocks.
sediment
Noun

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

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

small sediment particles.
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.

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    Editor

    Jeannie Evers, Emdash Editing

    Producer

    Caryl-Sue, National Geographic Society

    Last Updated

    Aug. 1, 2013

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