Patterns of debris flow initiation and periglacial sediment sourcing in the Colorado Front Range

Abstract

AbstractDuring an extreme storm in the Colorado Front Range in September 2013, 11 debris flows initiated at high elevations in Rocky Mountain National Park. We characterized these debris flows to determine controls on their initiation and found that eight of the 11 initiated in areas of convergent topography, and eight initiated at elevations > 2800 m. The high proportion of debris flows in areas of convergent topography emphasizes the importance of local topographic control on initiation. Debris flows at high elevations in the Front Range are atypical; this event broadens our understanding of debris flow regimes in the Front Range. Survey data (scarp dimensions, transport distance) suggest that transport is influenced by downslope processes rather than characteristics of the initiation site. At one site with evidence of multiple debris flows sourced from the same colluvial hollow, we obtained relative ages of debris flow deposits through geomorphic mapping and fan stratigraphy. We used radiocarbon and beryllium‐10 (10Be) analysis to age stratigraphic deposits and debris flow levees. Radiocarbon ages (n = 6) were highly variable and were interpreted to reflect secondary hillslope processes rather than debris flow ages. Large ranges in cosmogenic exposure age and anomalously old samples indicate that sampled boulders (n = 14) contained inherited 10Be concentrations. Beryllium‐10 concentrations were likely acquired as exposed bedrock surfaces or during intermediate storage in the colluvial hollow. We suggest that 10Be inheritance limits the utility of cosmogenic exposure dating of debris flows with short transport distances (10−1 to 101 km). Despite these limitations, we documented field evidence for four to seven debris flows in the last 75 ka, including two in the last 8 ka. Considered collectively, 10Be sample ages from all levees suggest that greatest sediment production occurred during Quaternary glacial stages due to periglacial processes. Subsequent evacuation of the colluvial hollow likely occurred during wetter periods of the mid‐Holocene. © 2018 John Wiley & Sons, Ltd.