FIRE AND FLOOD IN FOURMILE CANYON, COLORADO —ANALYZING THE GEOMORPHIC EFFECTS OF CATASTROPHIC EVENTS USING LIDAR AND FIELD STUDIES

Episodic events inform our understanding of landscape evolution by providing quantifiable geologic change on short timescales. We use results from field measurements and repeat airborne LiDAR difference analysis to reconstruct sediment transport, channel changes, and sediment budgets along a 20 km reach of Fourmile Canyon, Colorado, an area devastated by fire in 2010 and by the September 2013 flood. The 2010 Fourmile Canyon fire burned 23% of the Fourmile Creek watershed, mainly in the lower reaches of the catchment. Over 350 mm of rain fell in the Front Range foothills west of Boulder, Colorado from September 9 to 15, 2013, causing extensive flooding and local mass movements. Steep-slope debris flows and road failures occurred in the most intensely burned areas and locally catastrophic reworking of the channel area. Our measurements focused on flood height and width indicators, and sedimentologic analyses of gravel-rich deposits thicker than 0.5 m. Flood channel and slackwater deposits were analyzed for concentrations of ²¹⁰Pb and ¹³⁷Cs to help identify sediment source areas. One-meter resolution DEMs from LiDAR flown before the fire (Aug. 2010) and after the flooding (Oct. 2013) allowed us to measure the geomorphic effects of the flood and fire in the same areas as our detailed field surveys. We used field-checked LiDAR-based cross-sectional profiles to estimate peak discharge using the slope-area and critical-depth methods. LiDAR difference analysis shows 34,000-55,000 m³ of local deposition despite net erosion of 44,500-96,000 m³ from the floodplain. 19 debris flows were triggered in burned areas and none in unburned zones. The mass balance of sediment on the valley floor and field observations indicate that the 2013 flood event produced net erosion, mainly by channel widening, despite significant (>100 m long, >1 m thick) depositional zones throughout the canyon. Debris flows mobilized 42,000-60,000 m³ of sediment from the hillslopes, which influenced local deposition in the floodplain. Radionuclide concentrations suggest deep sediment sources for downstream deposits, showing that sheetwash was not a significant sediment delivery mechanism in 2013. Our erosion estimates from Fourmile Canyon suggest that rare events can account for a large fraction of total sediment transport from Front Range hillslopes.