Heavy rains in 2013 lead to devastating floods of rock, soil, and water through many cities and towns that line the Colorado Rockies. Scientists are now considering the importance of large, rare, independent storm events in determining an area’s landscape. Scott Anderson, a geomorphologist with the U.S. Geological Survey in Tacoma, Washington and lead author of a new study stated that "while it strikes us as very random, our research suggests this is one of the formative processes in this landscape."
Historic floods in Colorado in 2013 resulted in more than 1,100 landslides and debris flows in the Front Range after several days of unusually heavy rain in September. The debris flows carried huge volumes of rock and sediment down mountain valleys, scouring out river channels like sandpaper. According to Anderson, “The erosion that occurred during this one event represents the removal of hundreds to a thousand years of accumulated debris.”
To determine exactly what impact the floods had, geomorphologists first had to find how much erosion occurs in an average year due to springtime thaw, tree root movement, animal disruption, and other expected movements through radioactive dating. The upper layer of soil is constantly bombarded by cosmic rays, creating radioactive atoms that begin decaying at a regular rate. It was found that the Front Range of the Colorado Rockies is weathering away near a rate of 20-60 millimeters every 1000 years.
When the scientists compared this data to those measurements taken during and after the flooding in 2013, it was clear that this one-time event made a lasting impact. Using 3-D maps from before and after the flooding the scientist identified 120 landslides, which moved more than 114,000 cubic meters of debris. Within the debris flow channels, the floods removed up to 1 meter of rock and sediment. The total amount of erosion averaged, over the entire landscape measured in the study was 15 millimeters. That amount of erosion is comparable to the amount of lowering, that takes place over millennia along the Front Range. “The supposition is that most of this material was shot out downstream and onto the plains,” said Anderson. "The presence of so many debris flows in this semiarid landscape is pretty novel."