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ROBERT SIEGEL, HOST:

From NPR News, this ALL THINGS CONSIDERED. I'm Robert Siegel.

MELISSA BLOCK, HOST:

And I'm Melissa Block.

Parts of Colorado were finally drying out after heavy flooding last month, the worst the state has seen in decades. Eight people died and damage is in the hundreds of millions of dollars. Now, scientists are venturing into the hardest-hit areas to do a sort of flood forensics, trying to understand why the flooding was so bad.

NPR's Christopher Joyce went with them. He has this story.

CHRISTOPHER JOYCE, BYLINE: Geologist Jonathan Godt drives the Peak to Peak Highway in northern Colorado, up into the Rockies. The road winds past ravines where water still rushes down a stream. Front-end loaders are scooping up boulders that were tossed onto the road by that stream. Workers with acetylene torches are cutting up metal guardrails that have been twisted like pretzels.

Godt pulls his SUV to a stop. Up above, a huge tongue of muddy debris has cut a new path through the trees and down the mountainside. It looks like a ski run, except it's mud.

JONATHAN GODT: This is sort of on the scale like you'd expect off of one of the volcanoes in the Pacific Northwest.

JOYCE: Godt studies landslides at the U.S. Geological Survey. He says on these steep slopes, water keeps the soil and rock stable up to a certain tipping point.

GODT: Kind of like a little sand castle. You know, there's this sweet spot. If it's too wet, it's like a slurry, it's unstable.

JOYCE: Too wet and mountainsides collapse and flow downhill. That's what happened across Colorado's northern Rockies. The flood started with a wall of moist air from the Gulf of Mexico. It got trapped up against the mountains, and almost a year's worth of rain fell in four days. It fell into mountain ravines and canyons. It was like running a fire hydrant through a garden hose.

It wasn't just water. Streams became rivers of trees, boulders, an avalanche of concrete. Farther along the highway, we find thousands of uprooted trees, dumped in a jumble by a flooded stream. Above the jumble, the stream bed has been scoured out.

GODT: Right now, the stream channel looks like it's been eroded down to bedrock in places.

JOYCE: It may take years for streams like this one to support fish again. We get to Jamestown, a mountain village that's still digging out from under tons of rock and mud. A man died here in a mudslide. We stop beside James Creek, which runs through town, and scramble down a stream bank. Godt reconstructs what happened from what he sees. A debris-flow came racing down the stream.

GODT: A debris flow has a coarse, bouldery front at the snout of it. And then behind that is going to be a fluidized mixture of material, sand and stuff behind that, with a watery tail behind that.

JOYCE: And more flows came down from the hillsides as well. The town was hit from all sides.

GODT: It's not like a water flood. It carries a lot more momentum, it can do a lot more destruction.

JOYCE: And in this case, it came down this ravine, across the road, shot across the stream, and up the slope on the other side.

GODT: Up the slope on the other side, you can see that mud plaster which is indicative of debris flow passage. It's up 15 feet in those trees.

JOYCE: Over four days, water and mud and rock plunged 2,000 feet down out of the mountains and into towns like Boulder and Longmont. In fact, the pulse of floodwater traveled 400 miles along rivers, all the way to Nebraska. Everywhere it went, it left a muddy trail - one that scientists are now worried about.

GEOFF PLUMLEE: There were bridges that were completely overtopped and left with mud, and so now trucks are driving over it creating these massive dust clouds.

JOYCE: Dust clouds that could be toxic. Geoff Plumlee is a geologist at the U.S. Geological Survey and a dust expert. He helped identify toxic materials in the debris from the fallen Trade Center towers in New York City. Plumlee says floodwaters picked up a lot of junk on their downhill journey. In the mountains, it swept through mining operations.

PLUMLEE: Lead and uranium are probably the two elements that might be of potential concern.

JOYCE: Then farther down slope...

PLUMLEE: Oil tanks, animal feeding operations, human sewage.

JOYCE: Plumlee takes me along on a mud survey. We stop at Left Hand Creek, where it runs under a highway west of Denver. Mud along the stream bank sparkles with bits of mica from the mountains.

PLUMLEE: Left Hand Canyon is a canyon probably 20 miles upstream that has quite a bit of mining activity, so we'll see if we see elevated levels of things like lead and zinc and copper.

JOYCE: With a plastic scoop, Plumlee digs three inches into the goop.

PLUMLEE: That's what will most readily turn into dust and people can be exposed to.

JOYCE: The mud will go back to the Survey's labs for analysis. The floods have had environmental effects but a change in the environment here may have made the flooding worse. Wildfires. Wildfires burn away vegetation that holds soil together. Then when it rains hard, you get big, fast mudflows. Ten years ago, the hills above Jamestown burned. Klaus Wolter saw it happen. He lives five miles away.

KLAUS WOLTER: So this is kind of scary to contemplate that 10 years after a fire, you can still have these effects.

JOYCE: Scientists are now looking to see if there's a link between Colorado's recent wildfires and the heaviest flooding. Wolter, who's a climate scientist at the University of Colorado, says Coloradans, including himself, need to acknowledge this fire-flood dynamic.

WOLTER: Part of the equation is that the exposure to risk. We have had this increase in the number of people that live somewhere in this burn zone.

JOYCE: And that exposes them not just to fire but to the floods that follow as well. Christopher Joyce, NPR News.

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