This RAPID project focuses on the impacts of flooding and erosion on Fourmile Canyon in the Colorado Front Range following the exceptional September 2013 rainfall event. The catchment is still responding to a 25 km2 wildfire in 2010 and the team will test the hypothesis that the 2013 flood reworked and deposited sediments initially mobilized by the earlier fires. The team will study the 2013 flood deposits, eroded hillslopes, side-tributary and gully deposits, and deposits freshly exposed beneath terraces along the canyon. They will revisit locations that have been part of an ongoing post-fire study, and will use short-lived fallout radionuclides (7Be, 137Cs, and 210Pb) in these deposits to help identify remobilized fire sediments. In addition, they will analyze the geochemistry and stratigraphy of valley deposits and compare measures of short-term wildfire and flooding-related erosion with ongoing studies documenting erosion rates and hillslope processes over the past 10,000-30,000 years. Sediment deposits and exposures that formed during the September 2013 flooding must be sampled and studied soon, before they degrade and disappear as road rebuilding, channel manipulation, and landscape recovery proceeds. These new exposures greatly increase the ability to analyze and date Holocene slope and valley stratigraphy using 14C.
The work will enhance understanding of consequences of increased incidence of wildfires and large-magnitude floods on the landscape and those who live there. As such, the project outcomes will inform hazard mitigation and resilience. The team will leverage existing collaborations with USGS scientists and the Boulder Creek Critical Zone Observatory, and will directly involve undergraduates and a graduate student.
Summary of Outcomes This NSF award was highly successful. All goals were met: (1) we sampled and analyzed numerous flood deposits, eroded hillslopes, side-tributary and gully deposits associated with the September 2013 flooding along Fourmile Canyon using shortlived fallout radionuclides isotopes; (2) we sampled and analyzed exposures that formed due to severe flooding to study and date slope and valley stratigraphy, including sediment associated with historic mining and Holocene flood and fire deposits; (3) we compared our results to an ongoing post-fire study of Fourmile Canyon since 2010; (4) we collaborated with colleagues at other institutions, the USGS, Boulder Creek Critical Zone Observatory and Boulder County Parks and Open Space, and used this geomorphic and geochemical data to create a more complete picture of the role that wildfire and extreme flooding play in landscape evolution for the Front Range region; and (5) we trained and educated numerous graduate and undergraduates researchers. Intellectual Merit First, this project successfully used short-lived fallout radionuclides to characterize sediment mobilization and landscape response to the combined effects of wildfire and extreme flooding along Fourmile Canyon, Front Range Colorado. In doing so, we advanced the utility of fallout radionuclides for source-to-sink tracking of sediment derived from shallow processes, particularly overland flow, compared with deeper processes such as the landslides, debris flows and channel erosion that occurs in association with intense rain and flooding. Second, this project successfully delineated the two aspects of Fourmile Canyon stratigraphy that will play a significant role in future events as well as research in the area. (1) Much of the valley is filled with placer deposits and legacy sediment associated with historic mining activity. This sediment was eroded and exposed by flooding, and will continue to erode and mobilize in future events. (2) Dated Holocene deposits help to establish a 500-1000 yr recurrence interval for the combination of fire and big flood events in the region. Broader Impacts This research project helped support a female graduate student; two undergraduate work-study students who assisted the PI in collaborating with academic institutions, the USGS and Boulder Creek CZO; and trained 7 undergraduate students in scientific techniques and first hand accounts of NSF research. It also helped to educate numerous undergraduate students at the University of Connecticut (in PI Ouimet's courses) by serving as a valuable example of the recent impacts of wildfire and flooding and their relationship to society, hazard mitigation and resilience; and how geoscientists study these events in the field.
