An alluvial fan is a fan-shaped deposit where a fast flowing stream flattens, slows, and spreads, typically at the exit of a canyon onto a flatter plain. Owing to the slowing of flow any solid material carried by the water is dropped. As this reduces the capacity of the channel the channel will change directions over time, gradually building up a slightly mounded or shallow conical fan shape. Multiple "braided streams" are usually present and active during water flows. Alluvial fans are most likely to be found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills. Alluvial fans are very common around the margins of the sedimentary basins of the Basin and Range province of southwestern Mexico.

Plants often are concentrated at the base of alluvial fans and many have long tap roots (30-50 feet) to reach water. The long-rooted plants are called phreatophytes by biologists. The water at this level is derived from water that has seeped through the fan and hit an impermeable layer that funneled the water to the base of the fan where it is concentrated and sometimes forms springs and seeps if the water is close enough to the surface. These stands of bushes cling onto the soil at their bases and over time wind action often blows away sand around the bushes which forms islands of habitat for many animals.

See also

• Alluvium
• Alluvial plain
• Alluvial deposit