Floods cause loss of life and damage to structures, crops, land, flood control structures, roads, and utilities. Floods also cause erosion and landslides, and can transport debris and toxic products that cause secondary damage. Flood damage in Washington State exceeds damage by all other natural hazards.

There have been 36 Presidential Major Disaster Declarations for floods in Washington State since 1956 (the most recent was July 2012). Every county has received a Presidential Disaster Declaration for flooding since 1970. While not every flood creates enough damage to merit such a declaration, most are severe enough to warrant intervention by local, state or federal authorities.


The magnitude of most floods in Washington depend on the particular combinations of intensity and duration of rainfall, pre-existing soil conditions (i.e., was the ground wet or frozen before the storm), the size of the watershed, elevation of the rain or snow level, and amount of snow pack. Man-made changes to a basin also can affect the size of floods.

Although floods can happen at any time during the year, there are typical seasonal patterns for flooding in Washington State, based on the variety of natural processes that cause floods:

  • Heavy rainfall on wet or frozen ground, before a snow pack has accumulated, typically cause fall and early winter floods.
  • Rainfall combined with melting of the low-elevation snow pack typically cause winter and early spring floods.
  • Late spring floods in Eastern Washington result primarily from melting of the snow pack.
  • Thunderstorms typically cause flash flood during the summer in Eastern Washington; on rare occasions, thunderstorms embedded in winter-like rainstorms cause flash floods in Western Washington.

Tips and Information about Floods

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