Ecological - Second Order
After wildfire, hillslope and channel erosion produce large amounts of sediment and can contribute significantly to long‐term erosion rates. However, pre‐erosion high‐resolution topographic data (e.g. lidar) is often not available and determining specific contributions from post‐fire hillslope and channel erosion is challenging. The impact of post‐fire erosion on landscape evolution is demonstrated with Structure from Motion (SfM) Multi‐View Stereo (MVS) photogrammetry in a 1 km2 Idaho Batholith catchment burned in the 2016 Pioneer Fire. We use SfM‐MVS to quantify post‐fire erosion without detailed pre‐erosion topography and hillslope transects to improve estimates of rill erosion at adequate spatial scales. Widespread rilling and channel erosion produced a runoff‐generated debris‐flow following modest precipitation in October 2016. We implemented unmanned aerial vehicle (UAV)‐based SfM‐MVS to derive a 5 cm resolution digital elevation model (DEM) of the channel scoured by debris‐flow. In the absence of cm‐resolution pre‐erosion topography, a synthetic surface was defined by the debris‐flow scour's geomorphic signature and we used a DEM of Difference (DoD) to map and quantify channel erosion. We found 3467 ± 422 m3 was eroded by debris‐flow scour. Rill dimensions along hillslope transects and Monte Carlo simulation show rilling eroded ~1100 m3 of sediment and define a volume uncertainty of 29%. The total eroded volume (4600 ± 740 m3) we measured in our study catchment is partitioned into 75% channel erosion and 25% rill erosion, reinforcing the importance of catchment size on erosion process‐dominance. The deposit volume from the 2016 event was 5700 ± 1140 m3, indicating ~60% contribution from post‐fire channel erosion. Dating of charcoal fragments preserved in stratigraphy at the catchment outlet, and reconstructions of prior deposit volumes provide a record of Holocene fire‐related debris‐flows at this site; results suggest that episodic wildfire‐driven erosion (~6 mm/year) dominate millennial‐scale erosion (~5 mm/Ka) at this site.