Bedrock erosion and changes in bed sediment lithology in response to an extreme flood event: The 2013 Colorado Front Range flood

Abstract In this study, we use before and after flood observations of bed sediment lithology to determine how the lithological composition of bed sediment changes in response to an extreme flood event and use the post-flood measurements and observations to estimate values of critical shear stresses needed to erode shale bedrock and protective bank vegetation in the study streams. We present results from measurements of bed sediment lithology taken after the 2013 Colorado Front Range flood and compare these results with the same measurements taken before the flood. These two data sets provide a unique opportunity to determine how bed sediment lithology changes following an extreme flood event and allows insight into sources of channel bed sediment. After the flood, bed sediment lithology shifted distinctly towards the lithological distribution of the total catchment. Before the flood, bed sediment in most locations was primarily composed of granitic material, while after the flood, the streams were more likely to have metamorphic, volcanic, or sedimentary rocks in the bed sediment, reflecting more input from more non-local hillslopes compared to before the flood. Our post-flood measurements and observations allowed us to estimate threshold critical shear stress for the plucking of shale bedrock (212 Pa) and the removal of large cottonwood trees (364 Pa) in downstream sections of the study streams. Bank vegetation plays a critical role in protecting bedrock banks from lateral erosion, such that only the largest flood events can remove vegetation and expose bedrock on channel banks to potential erosion. We found that this large flood event plucked shale bedrock from both the channel bed and banks and that the shale bed sediment then quickly breaks down and is transported away by small flows. As such, this data set allowed us to make conclusions about the effect of large flood events on the long-term evolution of lateral and vertical erosion in bedrock rivers.