| a fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain. |
| alluvial fan n. A fan-shaped accumulation of alluvium deposited at the mouth of a ravine or at the juncture of a tributary stream with the main stream. |
alluvial fan
unconsolidated sedimentary deposit that accumulates at the mouth of a mountain canyon because of a dimunition or cessation of sediment transport by the issuing stream. The deposits, which are generally fan-shaped in plan view, can develop under a wide range of climatic conditions and have been studied in the Canadian Arctic, Swedish Lappland, Japan, the Alps, the Himalayas, and other areas. They tend to be larger and more prominent in arid and semiarid regions, however, and generally are regarded as characteristic desert landforms. This is particularly true in the basin-and-range type of areas of parts of Iran, Afghanistan, Pakistan, the western United States, Chile and Peru, Sinai and western Arabia, and Central Asia, where the basic landscape configuration consists of mountains set against adjacent basins
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