Air Date: Week of April 9, 1999

Science writer Boyce Rensberger explains why this spring, we can, once again, expect so many hundred year floods.

Transcript

CURWOOD: High water is not just a Louisiana phenomenon this time of year. Spring rains and melting snows mean lots and lots of water for much of the nation. And commentator Boyce Rensberger says that with all that water comes flooding, and the annual confusion about something called the 100-year flood.

RENSBERGER: It happens every spring. Somewhere a flood-swollen river will spill out of its banks and devastate towns and farm lands. Often it will be called a 100-year flood. Then just a few weeks later, another river will go on a rampage and we'll have another 100-year flood. Then the next year, or the year after that, the same river may experience yet another 100-year flood. Clearly, you might think something has gone wrong with the weather. Lots of people point to global warming theory, which does predict more weather extremes. But government scientists say that's not the case here.

The US Geological Survey is the agency that created the concept of the 100-year flood. And monitors water levels around the country at thousands of points. Recently, its scientists checked the records for nearly 400 of the most climate-sensitive rivers. They found that the amount of water flowing in them has increased by about one third. But most of that increase has come in seasons when the water is normally low, so it doesn't cause any flooding. In fact, scientists say over the last 50 years, the trend has been toward less flooding.

They say the main reason is that instead of coming in major deluges, rain is falling more evenly around the year. So, what about all those 100-year floods? Well, the confusion stems from a common misunderstanding of the term. A 100-year flood is an amount of flooding that has a 1% chance of happening in any given year. It does not mean that after a river has a 100- year flood it's safe for the next 99 years. The next year the same river still runs a 1% chance of severe flooding. Even if it floods two years in a row, there's still a 1% chance of it flooding in the third year. Compounding the confusion is the fact that if you watch 100 rivers for one year, you can expect to see a 100-year flood. With several hundred major rivers around the country, that means we're just about certain to have a number of disastrous floods every year. And of course, with today's disaster-driven television, you can expect to hear about them over and over again.

CURWOOD: Former Washington Post science writer Boyce Rensberger heads the Knight Science Journalism Fellowship Program at MIT.