Floodplains: Natural Benefits and Hazards

What is a Floodplain?

Floodplains are areas that are prone to being inundated by floodwaters during times of heavy rain, snowmelt, or high tides. Most floodplains in Snohomish County consist of low-lying lands along rivers and streams that flood when the waterways rise high enough to spill over their banks.


Floodplains are dynamic natural systems characterized by processes such as periodic flooding, erosion and deposition. They provide a wide range of benefits to our local communities, including:

  • Natural flood and erosion control—reducing flood velocities, peak flows and erosion potential; providing flood storage and conveyance
  • Water quality benefits—filtering polluted runoff and moderating temperature fluctuations 
  • Groundwater recharge—promoting infiltration and aquifer recharge; reducing frequency and duration of low surface flows
  • Biological productivity—offering fertile, productive soils for agriculture and forestry
  • Fish and wildlife habitat—creating habitat for many species, including waterfowl and endangered species
  • Open space and scenic beauty—offering opportunities for recreation and tourism; contributing to quality of life and the economy


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 you to lose your balance, and two feet of water can sweep your car away. According to the Federal Emergency Management Agency (FEMA), during floods more people drown in their cars 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 any remaining water for more than 100 yards. If you see downed power lines, stay far away from them and contact your utility company or police department to report them.

Flood waters may carry silt, raw sewage, oil or chemical waste. If your well has been flooded, you should assume that the well is contaminated. If you are on a public water system, monitor local news to find out if your water is safe to drink.


For more information about living in a floodplain, see: