Alluvial fans form where tributaries enter into broad valleys from the steep hillsides or mountain ranges. With the stream competency suddenly reduced, most of the transported sediment is rapidly deposited. The fans are usually formed by mud flow or sheetwash deposition during periods of heavy rain and runoff, although stream deposition does occur. Many alluvial fans form in arid regions.Although the coarser material is usually deposited at the top of the fan, and as a result, any stream flow quickly infiltrates, the sediments are usually very erratic because of the shifting of the stream channel from year to year. If a river flows over a pervious channel, the stream flow can infiltrate so quickly that the channel can become plugged with sediment and divert the water to a new channel. Illivation, the accumulation of material from a higher layer to a lower layer, can be an important process. The permeability is, therefore, very erratic along with the distribution of the deposits.
In cross section, fans are convex upward, sloping away from the stream channel. The slope can vary from 2-3 degrees to 30-35 degrees depending on the volume of material deposited. The depth is often several hundred feet thick.
The only difference between an alluvial fan and cone is that the cone tends to be somewhat steeper and exhibits a more conical shape.