In case other librarians are also looking for this information:

Colorado Road closures are online at:

U.S. Geological Survey Colorado Flood database:

U.S. Geological Survey reports, Big Thompson Flood, 1976:

U.S. Geological Survey photo archive of Big Thompson Flood, 1976:

U.S. Geological Survey, Water Education:

*100-Year Flood–It's All About Chance*

So what is a 100-year flood and how is it determined?

In the 1960's, the United States government decided to use the 1-percent
annual exceedance probability (AEP) flood as the basis for the National
Flood Insurance Program. The 1-percent AEP flood was thought to be a fair
balance between protecting the public and overly stringent regulation.
Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or
exceeded in any 1 year, and it has an average recurrence interval of 100
years, it often is referred to as the "100-year flood".

Scientists and engineers frequently use statistical probability (chance) to
put a context to floods and their occurrence. If the probability of a
particular flood magnitude being equaled or exceeded is known, then risk
can be assessed. To determine these probabilities all the annual peak
streamflow values measured at a streamgage are examined. A streamgage is a
location on a river where the height of the water and the quantity of flow
(streamflow) are recorded. The U.S. Geological Survey (USGS) operates more
than 7,500 streamgages nationwide (see map) that allow for assessment of
the probability of floods. Examining all the annual peak streamflow values
that occurred at a streamgage with time allows us to estimate the AEP for
various flood magnitudes. For example, we can say there is a 1 in 100
chance that next year's flood will equal or exceed the 1-percent AEP flood.

More recently, people talk about larger floods, such as the "500-year
flood," as tolerance for risk is reduced and increased protection from
flooding is desired. The "500-year flood" corresponds to an AEP of
0.2-percent, which means a flood of that size or greater has a 0.2-percent
chance (or 1 in 500 chance) of occurring in a given year.

Emily Wild
Librarian (Physical Scientist)
U.S. Geological Survey
Denver Library
Box 25046, MS 914
Denver Federal Center
Denver, CO 80225-0046
ph: (303) 236-1003
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