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BOB: It is now about 5 and 1/2 minutes before 8 o'clock. All of Minnesota gets covered in snow, so why is the Red River Valley so much more prone to flooding than other rivers? Joining me with the answer now, Professor Don Schwert, a geologist at North Dakota State University. Good morning, Don.

DONALD SCHWERT: Good morning, Bob.

BOB: So why does the Red River flood?

DONALD SCHWERT: Well, Bob, we sit in one of the flattest places on Earth. We are on the bed of what was a large glacial lake, Glacial lake Agassiz. And so we have a river that drains that lake bed and the river really does not want to flow. It doesn't have much gradient, not much slope on it. So when we introduce a lot of waters into it, then the river is going to spill out of its channel and spill onto the lake bed, and it's going to become a very wide, shallow lake.

BOB: Where does it flow to?

DONALD SCHWERT: The Red River flows North into Lake Winnipeg, North of Winnipeg, and then ultimately into the Nelson river drainage and up into Hudson Bay.

BOB: I've always wondered this, why does the Red River flow north, almost all rivers flow South?

DONALD SCHWERT: Well, it's one of those great folklores, but we have a lot of rivers that flow north. Basically in the world, probably about one out of every four rivers flows due northward. We have a lot of rivers in the United States that flow due northward, including some in Minnesota.

So most of Canada is drained to the North. Here we have a drainage line in Canada that extends down into the northern United States and it's just one of those rivers that does it.

BOB: Is there a particular divide somewhere that makes the Red flow north?

DONALD SCHWERT: Yeah, the divide is located South of Fargo. It's a so-called Continental Divide, it's not very dramatic to look at it. It's just a little sign on the highway there. But that Continental Divide basically separates the drainage that enters into the Hudson Bay and into the Arctic Ocean from the drainage that enters into the Mississippi and down into the Gulf of Mexico.

BOB: When you look at this ancient lake bed, Lake Agassi, I believe you called it, right?

DONALD SCHWERT: That's right.

BOB: Does that have something to do with the fact that the land there is so fertile?

DONALD SCHWERT: Oh, the clays here are wonderfully fertile. We're looking at a legacy of that ancient glacial lake in terms of the fertility of these clays. But these same clays themselves also inhibit the permeability of water. So right now we have a lot of water that's being introduced by rapid snow melt onto the land surface.

The soils are still frozen, but even if they started to thaw, the soils are pretty well-saturated and they're just not going to accept very much of this water. So the water is making its way towards the Red channel and then northward from here.

BOB: Why can't they build up adequate dikes and other structures to prevent the flooding onto land?

DONALD SCHWERT: The land area is so vast. Now Fargo is fairly well-diked. The low depressions in Fargo are diked by permanent dikes. But nonetheless, we're going to have a lot of affected property owners, both in Fargo and Moorhead.

BOB: So it's just physically not possible to dike it all the way up.

DONALD SCHWERT: It would be exorbitantly expensive and it's not practical. Remember that water has got to go somewhere. And so the more dikes that we build, the more restricted becomes the flow, and probably it would increase the flooding intensities.

BOB: Professor Don Schwert, geologist at North Dakota State University, thanks so much.

DONALD SCHWERT: Thank you, Bob.