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About Floods
What Causes Floods in Washington State?
Floods in different
parts of Washington State occur at different times of the year and from
different sources. For example, western Washington gets the lion's share
of precipitation in the State, mostly during the winter, so lowland flooding
in that region occurs primarily in the winter. Snowmelt generates most
of the flooding in rivers that drain the eastern side of the Cascade Range.
The sources of flooding in different regions of the State, as well as
how the floods develop and some of their characteristics (like how long
they typically last) are described in U.S. Geological Survey Fact Sheet
228-96 — What Causes Floods in Washington State?.
This Fact Sheet also gives information on how soils, vegetation, and development
play a role. The Fact Sheet is in PDF format (PDF:1.2
MB / 2 pages)
You will need to have Adobe Acrobat Reader installed on your system to read PDF files http://www.adobe.com/products/acrobat/readstep.html
Some of the information
in this Fact Sheet also is illustrated on a 48x36 inch color poster suitable
for educational or informational display. The Poster also presents information
on "The 100-Year Flood"
The 100-Year Flood
Traditionally, the
size of a flood is described relative to something called the 100-year
flood. The term "100-year flood" is misleading because it leads
people to believe that it happens only once every 100 years. The term
"100-year flood" is really a statistical designation, meaning
a flood of that magnitude has a 1-in-100 chance of occurring in a year
at a specific location. Perhaps a better term would be the "1-in-100
chance flood."
During the mid-1990's, there were numerous "100-year floods" in Washington
State causing many people to ask "Why"? This question is answered,
along with what the technical community means by the 100-year flood, and
how the size of that flood is estimated in U.S. Geological Survey Fact Sheet 229-96 — "The 100-Year Flood." In addition,
the statistics behind the term are described along with what data were
used, and why and how the numbers may change with time. A glossary of
other flood terminology is included, as well as a few tips on how to prepare
if you live on a flood plain.
The information in
this Fact Sheet also is illustrated on a 48x36 inch color poster suitable
for educational or informational display. The Poster also presents information
on What Causes Floods in Washington State?
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