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This is Weekend Edition from NPR News. I'm Scott Simon. Levee breaks along the Mississippi River this week have spread flood waters over a wider area, prompting forecasters to revise downward some of their river crest to estimate for communities downstream. But authorities say it is too soon to tell if the danger has passed. One community that's threatened is Hull, Illinois, about a dozen miles from Hannibal, Missouri. Hull resident Susan Gill(ph) remembers the flood that hit the region 15 years ago.

Ms. SUSAN GILL (Hull, Illinois, resident): It took us about 18 months to get back last time. This time there may not be coming back. If the levee breaks this time the state probably won't let us come back. So if that does happen, our town is probably done.

SIMON: Timothy Kusky directs the Center for Environmental Sciences at St. Louis University, and joins us from the studios of member station KWMU in St. Louis. Mr. Kusky, thanks so much for being with us.

Dr. TIMOTHY KUSKY (Director, Center for Environmental Sciences, St. Louis University): Thanks for taking the time to have me on.

SIMON: We've learned this week that there have been a lot of changes along the rivers in the Midwest since that terrible flood in 1993. How do those changes affect the flooding this year?

Dr. KUSKY: To put it simply, the more changes we make to the flood plain and the levee systems along the river, the higher the flood stages or the flood heights have been coming for any given amount of water that's flowing through the river systems.

SIMON: Now, is that a simple and elemental as, when you build levees to hold back the water that almost incongruously raises the level of water.

Dr. KUSKY: Yes, it's really a very basic concept. Before we built the levees along the Mississippi River system, at St. Louis the river was 4,000 feet wide. Now it's 1,500 feet wide. Because we've taken the very wide channel and confined it to a very narrow space, and the only place that water can go in a situation like that is up.

SIMON: And have people built houses and commercial developments in places that ideally they shouldn't?

Dr. KUSKY: Yeah, the flood plain is being very hotly developed. For instance, if we go along the Missouri River system to the Chesterfield Valley just to the north and west of St. Louis, there's an area there that was under about 10 feet of water in the 1993 floods. Now it has the largest strip mall in the United States - 3 miles long. And there's about 30,000 new homes put on this area that was all completely underwater in 1993.

SIMON: Why was the largest shopping mall in the country built in a flood plain?

Dr. KUSKY: The reason that's driving it is economics. The state of Missouri has allowed developers to come in to places like flood plains through a program called tax incentive financing. And what the developers can do is, they can build the levee, and they can declare the area behind the levee to be safe and they get a very large break on the taxes they have to pay in order to stimulate the economic growth. So, they built lots of very beautiful homes and lots of economically booming businesses out on these flood plains. They declared that the areas behind them are protected by these 500-year levees.

SIMON: That term 500-year levee, could you explain that us?

Dr. KUSKY: What that means is that, it's statistically only likely to be flooded once every 500 years. So, those calculations go into the calculated risk of building or developing these areas. But it turns out there's a problem with this. And that's building the structures has raised the flood levels and changed the statistics of how likely it is to get a flood at a certain height. We've had 15 100-year floods in the past 100 years, so it's really not a 100-year flood. We've had many more 500-year floods in the past 150 years. And also, these statistics were based on the current climate regime, and most the climate models for the midwestern from part of the USA are predicting that within the next 30 years that we're going to have a slight decrease in temperature for the Midwest and a very significant increase in amount of rainfall here, perhaps even 20 percent, which equates to about a 50 percent increase in the amount water flowing through the rivers.

SIMON: Are there places you know of where the levees might be especially vulnerable?

Dr. KUSKY: Yes, we were up at Winfield, Missouri, and there's about 150 feet of levee that have just collapsed up there, and the water is spilling across the flood plain. And there's a few other places where the levee has been overtopped. But, a bigger issue is down in the metropolitan St. Louis area. Since Hurricane Katrina, the U.S. Army Corps of Engineers and FEMA have been going across the country and inspecting all the levee systems. And it turns out that the entire levee system that protects what we call the MetroEast area, that includes the cities of Granite City, East St. Louis, Cahokia and Wood River. A total of about 150...

SIMON: These are all the cities in the Illinois side of the river.

Dr. KUSKY: They are in the Illinois side, yes. And they are protected by four levee systems, and the Army Corps of Engineers has determined that all of them are structurally deficient. And they have the potential to collapse during high floodwaters. So what they've done is - they are going through a process of accreditation, and these levees will be decertified unless these communities can come up with about 100 million dollars to repair them, which is very unlikely for these rather economically depressed communities. And so if they lose the chance to be part of the federal flood insurance program and they are forced to get private flood insurance, they are going to be bankrupt, and they will be out of homes. And further, it really discourages any further economic growth in this urban area.

SIMON: Timothy Kusky, director for Center for Environmental Sciences at St. Louis University. Thank you very much.

Dr. KUSKY: Thank you.

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