Fire Flipping Floodplains: Estimating Sediment Delivery from Wildfire Blowdowns and Valleybottom Tree-throw

As important vegetation-altering hillslope disturbances in forested mountain region, wildfires instigate a cascade of hydrologic and geomorphic processes that enhance sediment and wood delivery to aquatic ecosystems and shape river environments. Among those hillslope processes, tree throw can play a significant role in the downslope movement of sediment. The 2016 Pioneer Fire burned nearly 670 km² within the central Idaho Batholith, and resulted in valley bottom patches of blowdown within Clear Creek. In this initial phase of a multi-year monitoring project our study asks: What is the role of fire-related blowdowns in mobilizing sediment from floodplains to stream channels? Within six 200-m long reaches, we use repeat photogrammetry to estimate sediment volumes mobilized from floodplain tree throw from a random sample of five rootwads (sediment and organic material at the base of a fallen tree) per reach. Three-dimensional models are constructed using the Agisoft Structure from Motion (SfM) software and CloudCompare to determine volumes of sediment and visible organic material. Repeat photos, collected annually, allow us to estimate the volume of tree-thrown sediment supplied to Clear Creek, and compare fire-related tree-thrown sediment yields from valley bottoms to other hillslope processes. Quantifying the relative contribution of sediment from different sources and monitoring the temporal and spatial variation of delivery and storage processes can assist post-fire land management response and further our knowledge of post-fire geomorphic change.