Alluvial fans are fan-shaped deposits of sediment (rocks, wood, gravel, and mud) that form where steep mountain streams empty onto flat valley bottoms.
An alluvial fan is a sign that catastrophic floods, often laden with sediment and debris, have occurred in the area and will occur again. These floods and sediment-laden events can be deadly because they are violent, move rapidly (15-30 feet per second), have unpredictable paths, and happen with little to no warning. Although alluvial fan floods can happen at any time, their potential to cause severe damage increases with heavy rainfall. The most damaging alluvial fan floods happen when heavy rains fall on soil that is already saturated triggering landslides. A landslide may flow directly down the stream channel, or it may form a temporary dam, which can later fail sending a surge of water and sediment onto the valley bottom below.
Whatcom County Public Works River and Flood Division works with Whatcom County Planning and Development Services and other partners to assess the risk of alluvial fan hazards to life and property in Whatcom County. Risk assessments consider both the likelihood that a hazard, such as a flood or landslide, will happen and the consequences if it does. Assessing risk for alluvial fans is challenging because it is difficult to determine the likelihood of an event, how large it will be, and how it will act on the alluvial fan.
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Whatcom County Planning and Development Services has more information on alluvial fan hazard areas and other types of geological hazards here.
There are over 100 alluvial fans in Whatcom County, not all of which are currently mapped. Maps of identified alluvial fans are available through the Whatcom County Planning and Development Services critical areas page. Even small streams exiting the mountains onto a valley bottom can form an alluvial fan. Over the next few years, Whatcom County will be working with the Washington State Department of Natural Resources (WDNR) to prepare a more comprehensive map of alluvial fans and landslides using recently acquired lidar data. Lidar is a laser-based technology that allows the user to virtually remove surface vegetation to more clearly see and accurately map landforms. View current lidar images available through the WDNR.
Whatcom County residents can take action to plan and prepare for alluvial fan hazards.