Abstract

Infrequent, large magnitude geomorphic events generate quantifiable change on geologically short timescales and are crucial to understanding landscape evolution. Airborne lidar surveys and field measurements were used to investigate floodplain erosion and deposition along a 19.5km reach of Fourmile Creek, Colorado that was devastated by severe flooding in 2013 that followed a 2010 wildfire. >350mm of rain fell on the Fourmile catchment from September 9–15, 2013, generating discharge that exceeded bankfull for >120h at the Orodell gage, with local unit stream power>300Wm−2 throughout the study reach. Debris flows occurred on steep hillslopes and tributary channels in the most intensely burned areas. Lidar difference measurements and field studies highlight zones of local deposition along the study reach, but demonstrate overall net erosion of ~0.25m for the 19.5km reach of Fourmile floodplain, mainly by channel widening. Tributary junctions where debris-flow sediment entered the floodplain and local decreases in unit stream power controlled some zones of deposition. Overall, mass balance calculations show that a total sediment loss of ~91,000–161,000m3 from the Fourmile Creek floodplain and hillslopes, which is broadly consistent with channel sediment flux estimates of 71,000–111,000m3. Measurements from the Fourmile catchment demonstrate that floodplain erosion was a major source of sediment in the 2013 flood and demonstrate that infrequent events marked by long-duration flooding and high total energy expenditure can account for a large fraction of total sediment transport in mountain streams.

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