Sorry, but this is one time we need to use a three-bit word.
Alluvial fans are delta shaped deposits of sediment that form at the base of a hill or mountain.
Alluvial fans form because material is constantly being eroded in narrow valleys or gullies above the fan. During occasional events in the spring and during large rain storms, debris-choked floods wash large boulders and a mixture of sand, silt, and clay down toward the base of the slope. As the base of the slope flattens out and the debris is no longer constrained by a channel it spreads out into a fan-like shape. You can see this feature on both sides of the valley throughout the Truckee Meadows. Alluvial fans are produced by debris flows, which are very nasty flows of mud, boulders, and water that travel down these canyons during very heavy rainfall.