Large alluvial fan in the Taklimakan Desert, Xinjiang Province, China. Image source: USGS Landsat Project.
An alluvial fan is a generally fan-shaped area of alluvium (water-borne sediment) deposited by a river when it reaches a plain on lower ground, and the speed of flow of the water is abruptly reduced.
Sand, gravel, and silt are the main constituents of the alluvium, with the coarser material deposited near the head of the fan. Organic matter is also transported, making the soil highly fertile. Valuable minerals such as cassiterite (tin ore, SnO2), diamonds, gold, and platinum are often found in alluvial fans.