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Alluvial Fan R E S O U R C E L I B R A R Y E N C Y C L O P E D I C E N T RY Alluvial fan An alluvial fan is a triangle-shaped deposit of gravel, sand, and smaller materials called alluvium. G R A D E S 4 - 12+ S U B J E C T S Earth Science, Geology, Geography, Physical Geography C O N T E N T S 5 Images For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/alluvial-fan/ 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 Part of Term Definition Speech accumulationnoun a buildup of something. the art and science of cultivating land for growing crops (farming) or agriculture noun 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. adjective, apex tip, point, top, or summit. noun area covered by a deposit of sediment, usually at the foot of a hill or apron noun 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. triangle-shaped deposit of eroded material transported by mass colluvial fan noun 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. area of land that receives no more than 25 centimeters (10 inches) of desert noun 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. environment where an organism lives throughout the year or for habitat noun shorter periods of time. land that rises above its surroundings and has a rounded summit, hill noun usually less than 300 meters (1,000 feet). Part of Term Definition Speech industry noun activity that produces goods and services. the fall of rocks, soil, and other materials from a mountain, hill, or landslide noun slope. mass wastingnoun 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. part of a plant that secures it in the soil, obtains water and nutrients, root noun 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 triangle-shaped deposit of sediment transported by an underwater noun fan current or glacier. Articles & Profiles USGS: Our Dynamic Desert—Pediments and Alluvial Fans FEMA: Alluvial Fan Flooding Maps National Geographic Magazine: Africa's Miracle Delta Map Websites University of Oregon: Dr. Marli Bryant Miller—Alluvial Fan © 1996–2021 National Geographic Society. All rights reserved..
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