Alluvial Fans

Alluvial fans are fan-shaped deposits of water-transported material (alluvium). They typically form at the base of topographic features where there is a marked break in slope. Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained. Alluvial fans are relatively steep (>1-2°) cones consisting of coarse- grained facies and constitute the most proximal fluvial depositional environments (usually at the break of slope on the edge of a floodplain).

Debris flows dominate on small and steep alluvial fans

Sheetfloods are common on larger and gentler alluvial fans

    Alluvial fan and center-point irrigation in Dixie Valley, California.
Two alluvial fans have grown together (coallesced). Note the numerous channels (light-colored areas)--they mark the locations of the coarsest sediment. Small fan emerging from a wineglass canyon.  Death Valley, CA. Alluvial fans at fault-bounded western edge of the Black Mountains, Death Valley, CA.

Click here to view a coarse-grained alluvial fan channel deposit.

Source: Marli Bryant Miller, Department of Earth Sciences, University Of Oregon, Eugene, Oregon, USA