Why do the values for the 100-year flood seem to change with every flood?
The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The estimates of the flood quantiles are calculated using actual data collected at a site.
For a particular river, the USGS collects data over time, determines the largest flood in each year, and then calculate statistical data for that river. The more years of data available, the more accurate the estimates for the various flood quantiles.
As more years of data become available, the estimates become more refined, which can result in revisions to the quantiles.
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