© 1997, 1998 Streamline Publications

Since 1945 the Missouri River has had six 100-year floods. Surely, something is wrong with this way of presenting flood-risk predictions—not the least of which is the terminology. A hundred-year flood is not water that rises each century, but a convenient—and increasingly inaccurate—way of relating a one-percent chance of a flood of these proportions occurring in any single year.
Each year the chances are one in a hundred it will be a big flood year. But more than statistical chance is involved. Predictions are built on empirical evidence of past floods—the geologic record and historical accounts—but they may not accurately take into consideration changing watershed conditions that worsen runoff.
Dams, levees, and channelization speed the flow of runoff. Lands that once served as giant sponges, absorbing rainfall, have been developed, paved and shingled over. Water runoff volume increases, further burdening the river channels. Powerful flows refuse mitigation and spread across the land, testament to the watershed's altered hydrologic profile.
River-controlling structures are often built to withstand the 100-year flood, as it once was. But now, water reaching the magnitude of the once-a-century flood can be expected once a decade. And we can further expect that, as was done in the recent San Joaquin Valley floods, levees will have to be breached to relieve increased flood pressure. Decisions are forced over which lands to save and which to destroy—the levee legacy.
Only a tiny leap of logic is needed to imagine that we may have invited the remote possibility of a 500-year magnitude inundation to visit us in 50 years. The clock is running down but there is no way of knowing exactly when it started.

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