About Flooding & Floodplains: Benefits and Hazards

Definition & Causes

Floodplains are the relatively flat lands adjacent to a body of water, such as a river or stream, that become flooded (inundated with water) when channel capacity is exceeded and overtopping occurs.

Other areas in the county can be subject to flooding as well. A commonly overlooked cause of flooding is often as simple as a storm drain blocked by debris.


Floodplains are dynamic natural systems. The natural processes of periodic flooding, accompanied by erosion and deposition, bring changes to the topography, soils, vegetation, and physical features (such as meanders, braided channels and oxbow lakes) within these areas over time.


Floodplains provide a wide range of benefits to the ecosystem and community that include:

  • Flood storage and erosion control–offer a broad area for streams and rivers to spread out and accommodate temporary storage of flood water, reducing flood peaks and erosion potential
  • Water quality maintenance – reducing sediment loads, filtering nutrients and impurities, and moderating water temperature
  • Groundwater recharge
  • Biological productivity – providing fertile soils with high rate of plant growth and diversity, richer agricultural harvests, and healthier forests
  • Habitat for a variety of fish and wildlife, including rare and endangered species
  • Recreational opportunities – providing areas for active and passive activities, supporting the economic base
  • Open space


Flood waters can be deceiving in terms of their depth and how quickly they move. It is not safe to walk or drive through flood waters since as little as six inches of water can cause unstable footing, and two feet of water can cause a car to be swept away. According to the Federal Emergency Management Agency (FEMA), more people drown in their cars in floods than anywhere else.

Floods and storms also knock down power lines. Even when flood water levels appear to have subsided, electrical currents can travel through the remaining water for more than 100 yards. If you see downed power lines, stay clear of them and contact your utility company or police department to report them.

Flood waters may carry silt, raw sewage, oil, or chemical waste. If you can, wear gloves and boots to avoid coming into contact with flood waters. If you touch flood water, you are advised to wash your hands thoroughly. If your well has been flooded, you should assume the well is contaminated. If you are on a public water system, listen to the TV or radio to find out if it has become contaminated.

Flood waters can also damage structures and contents. Please contact public works solid waste at 425-388-3425 for information on safe disposal.


Some good sources for more information about living in a floodplain: