A Multi-Scale Analysis of Post-Wildfire Sediment Cascades in Mountainous Southern California, USA

Wildfire is a major disturbance in vegetated mountainous regions worldwide. When intense fire burns through steep landscapes, it increases the likelihood of destructive sediment-laden floods and debris flows by increasing the amount of runoff and sediment available to erode. Despite increased work in documenting and modeling postfire erosion, there is still uncertainty regarding the amount of sediment produced and delivered to downstream communities following fire and how these processes may be impacted by greater watershed reburning rates as fire frequency increases. In this dissertation, we sought to answer the following questions organized by chapter. Chapter 2: How do sediment supplies evolve in response to repeat rainfall events? Chapter 3: Do sediment supplies in channels depend on previous fire-flood cycles? Chapter 4: What is the impact of fire-associated runoff from steep, severely burned watersheds on lake systems? Using spatiotemporally nested scales of remote sensing analysis and hydrometeorological monitoring, we show that time-dependent sediment availability is an important factor in controlling sediment yields from low-order burned catchments (Chapter 2), that inter-fire accumulations are important sediment supplies and time since previous probable fire-flood cycles could be a useful indicator of channel sediment availability (Chapter 3), and that postfire runoff delivery to downstream freshwater areas by fire is a very important component nutrient and sediment budgets over long timescales (Chapter 4). This research ultimately shows that sediment supplies in headwater channels can become limited by previous fire-flood cycles, with implications for the amount of headwater sediment yields both at the storm cycle scale (Chapter 2), over the scale of subsequent reburn and fire-flood cycles (Chapter 3), and the delivery of sediments and other sediment-associated constituents to downstream waterbodies (Chapter 4). This work advances a more detailed understanding of postfire erosional cycles and highlights additional legacy controls on postfire sediment cascades across many time (1 month-100 year) and space (1 ha to 10 sq-km) scales