What a "100-year flood" really means

A "100-year flood" is a flood big enough that it has a 1% chance of happening in any given year. It doesn't mean it comes once every 100 years, and having one this year doesn't make next year any safer.

It's a chance each year, not a schedule

The US Geological Survey, which measures the country's rivers, defines the 100-year flood as a flood with a 1-in-100 chance of being reached or topped in any one year. Every year starts with the same 1% chance. A river can have a 100-year flood in 2026 and another in 2027; that's rare, but nothing about the first one prevents the second.

The same idea gives the other sizes: a "500-year flood" has a 0.2% chance in any year, a "10-year flood" a 10% chance. Scientists often say "1% annual chance flood" instead, because it's harder to misread.

Small each year, large over a lifetime

A 1% chance sounds small. Over many years it adds up. Over 30 years, the length of a typical mortgage, the chance of at least one 100-year flood is about 26%: roughly one in four. (That's 1 − 0.99³⁰, if each year is independent.)

The line moves

How big a "100-year flood" is comes from the years of river records so far. As more years come in, the estimate changes, and so does the river itself: new pavement, development upstream and dams all change how much water reaches it. A flood called "100-year" decades ago may be a more common flood today.

What it doesn't tell you

• It isn't how deep the water will get at your house. It's the size of the flood on the river; how much reaches you depends on your ground.
• Outside the line isn't safe. Floods don't stop at a mapped line, and flash floods from heavy rain hit places far from any river.
• It doesn't tell you what's coming this week. For that, use the river forecast and flood warnings.

Using it

Treat "100-year" as "1% every year," and think in the years you'll live there, not the century. When a flood warning is issued, follow it, whatever the river's history says: "Flooding is happening or expected. Never drive or walk into floodwater: turn around, don't drown."

Atmostorm's river gauge pages show the river's flood levels and forecast crest where the Weather Service sets them, and how today's flow compares with normal for the date; see how to read a river gauge.