Just how significant are extreme events in the geologic record? Well, we now have a really exceptional measure thanks to some work due to be published in Geology by some of GG’s colleagues here at CU Boulder. Back in 2013 we had a really intense rainstorm, arguably something like a once in a thousand year rainfall in Boulder. Associated with this were numerous debris flows and landslides. It just so happened that part of the area so pounded was being studied as one of NSF’s Critical Zone Observatories, and as part of that project they had flown Lidar to get a high-resolution topographic map of the watershed. This was done in 2010. The authors then managed to use a new Lidar image obtained by FEMA to determine just how much material had moved during the flood.
The results are stunning. Over the few days in September 2013, hundreds of years of geomorphic work was accomplished in the basins with slope failures; in these basins, some 15 mm of average lowering occurred. As these basins only represent 7% of the overall drainage area of Boulder Creek, for the Boulder Creek watershed as a whole this represents removal of some 30 years of weathering as measured by geologic rates using cosmogenic isotopes–maybe less impressive but still it means that if you were out of town for a week, you missed 30 years of erosion. Interestingly, such failures were found equally on areas burned in recent fires and areas that were unburned.
These results are even more striking when considering that Boulder Creek’s drainage actually was less impacted relative to the drainages to the north. Really massive storm damage was found in the St. Vrain River drainage and parts of the Big and Little Thompson Rivers. It seems likely that the entirety of these drainages saw far more geomorphic work done in this flood. This underscores how the geomorphic evolution of some areas can occur over dramatically short times.
PS. It appears that most of the erosion was in removing soil and regolith that were the product of erosion acting on bedrock; mobilization of bedrock was rarely observed. With the removal of that material, any new large rainstorm presumably would not produce much lowering of these basins, but that also means that the water would come roaring down Boulder Creek carrying very little material, which means it would be capable of a lot of erosion.