In 2013, a rare September storm swept across the plains of Colorado. When it hit the Rockies, it dropped more than a foot of rain in places like Boulder—as much as the city sees in an entire year. The rain unleashed deadly floods and landslides that swept away roads and buildings. In fact, a new study found that a century’s worth of erosion and sedimentation took place in a matter of a few days.

“Once the flooding started, it happened quickly, and took a lot of people unawares.”

Sara Rathburn, a geoscientist at Colorado State University in Fort Collins who experienced the storm herself. On top of the damage to manmade structures, Rathburn knew that the floods moved huge amounts of sediment, wood, and the organic carbon they contain. She saw a unique opportunity to put hard numbers on what went where: At the base of one of the watersheds that flooded, a reservoir captured everything that flowed downhill.

“I was thinking about being able to track the sediment from the source to what I’m calling this anthropogenic sink—the reservoir—and really quantify it. We don’t have a lot of control on absolutely capturing everything that these large storms produce…and so the fact that the reservoir was capturing everything really seemed like a unique opportunity.”

So Rathburn and her colleagues got a Rapid Response Research grant from the National Science Foundation to study what happened. The team compared detailed maps of the landscape and the lakebed before and after the storm, then they quantified the difference. They found that half a million cubic yards of sediment washed downstream during the storm, a volume that would normally take up to 115 years to erode. About 60 percent of it accumulated in the reservoir, taking up 2 percent of its storage space. 

The rest of the material was deposited partway down the river, where it will continue to be released into the reservoir for years to come, Rathburn says, causing ongoing headaches for dam managers…who are also worried about large logs clogging the openings that they use to release water. The findings are in the journal Geology. [S. L. Rathburn et al., The fate of sediment, wood, and organic carbon eroded during an extreme flood, Colorado Front Range, USA]

The storm was an extreme event. But Rathburn says such episodes are becoming more and more common. 

“I really do think it’s climate-change driven. And that it’s something that’s just absolutely worthy and necessary of our study and our investigation. It’s too risky to ignore, given what it means for people living in places where hazards occur, which is almost everywhere.”

—Julia Rosen 

[The above text is a transcript of this podcast.]