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.

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.
accumulation
Noun
a buildup of something.
Noun
the art and science of cultivating land for growing crops (farming) or raising livestock (ranching).
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.
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.
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.
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.
gravel
Noun
small stones or pebbles.
Noun
environment where an organism lives throughout the year or for shorter periods of time.
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.
mass wasting
Noun
downward movement of rock, soil, and other material.
mountain
Noun
landmass that forms as tectonic plates interact with each other.
prone
Adjective
vulnerable or tending to act in a certain way.
root
Noun
part of a plant that secures it in the soil, obtains water and nutrients, and often stores food made by leaves.
sand
Noun
small, loose grains of disintegrated rocks.
shrub
Noun
type of plant, smaller than a tree but having woody branches.
slope
Noun
slant, either upward or downward, from a straight or flat path.
subaqueous fan
Noun
triangle-shaped deposit of sediment transported by an underwater current or glacier.