An alluvial fan is a landform created by the deposition of sediment as water flows from a steep area onto a flatter surface. This process typically occurs at the base of mountains or hills, where the velocity of the water decreases, causing it to drop its sediment load. The result is a fan-shaped accumulation of materials like sand, silt, and gravel. Alluvial fans are important in geology and ecology, as they can create fertile land for agriculture and provide habitats for various species. They are often found in arid regions, where rivers may carry sediment during seasonal floods, contributing to the formation of these unique landscapes.