• 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.

    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. 
  • 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.

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