An alluvial fan is a fan shaped deposit of sediment and organic debris that forms where a stream flows, or has flowed, out of a mountainous upland onto a level plain or valley floor. As a stream flows out of a mountainous environment, it abruptly loses capacity to transport sediment and debris resulting in an accumulation of transported materials. As materials accumulate and build on top of older deposits, stream channels can shift, and begin building in other areas.

Alluvial fan hazard areas are those areas on a fan that have the potential to damage or harm the health or welfare of the community. They generally correspond to the path of recent and potential future stream flooding, boulder flooding, and/or debris-torrents.