No abstract for thesis

From abandoned vehicles to tiny plastic particles, anthropogenic litter ends up in the environment and damages ecosystems and economies. River ecosystems are highly impacted by anthropogenic litter and transport litter from land to the ocean. Managers, in particular, need information on watershed litter source and transport processes to make best management decisions. However, approaches to estimating anthropogenic litter fluxes are highly uncertain. In chapter two, we monitored litter accumulation on roadsides in the Inland Empire, CA. We determined that human transport was the primary process transporting litter from the sale location to the roadsides we studied. We quantified litter accumulation rates along roadsides as 1170 (917-1447) kg1km-1year-1 and established a harmonization tool in chapter three for comparing our results to other studies. Litter from roadsides is one source of macroplastics in river stormflow along with direct dumping and litter already within the channel. In chapter four, we investigated the sources of macroplastic in the Santa Ana River and tested the hypothesis that discharge controled macroplastic concentration. The particle size distribution of macroplastic particles (validated with the methodology in chapter five) did not respond to hydrologic transport mode (i.e. stormflow vs lowflow) suggesting that macroplastics in riverflow were primarily sourced from the stream channel. We found that the relationship between discharge and macroplastic concentration was nonmonotonic, and there were path-dependent effects causing variability. We estimated annual macroplastic flux in the Santa Ana as 18.2 (2.9-222.2) metric tonnes per year. Within the channel, microplastics (small plastic particles < 5 mm) can be influenced by turbulence to create concentration depth profiles. In chapter six, we tested the hypothesis that microplastics are transported via a predominant concentration depth profile, commonly assumed in other studies. We found that microplastics can be transported in any transport mode and that misapplication of transport mode assumptions to monitoring and modeling approaches increase uncertainty or systematic bias by multiple orders of magnitude. Overall, these studies support the advancement of the science and management of anthropogenic litter by bringing us closer to accurately monitoring and modeling a watershed mass balance of anthropogenic litter, and its plastics constituents.

Post-wildfire hydrological regimes can result in a dramatic increase in watershed sediment yield, particularly fine sediments (diameter < 62.5 µm) from small mountainous watersheds. The objective of this study was to evaluate changes to fine suspended sediment concentration-discharge relationships in the lower Ventura River, CA after 82% of the watershed burned in the 2017 Thomas Fire relative to persistent periods of time-varying suspended sediment dynamics over the previous four decades. We monitored fine suspended sediment concentrations during stormflow events in the Ventura River during water years 2018 and 2019, and characterized post-wildfire suspended sediment concentration-discharge relationships, which were then compared with pre-wildfire conditions monitored by the USGS from 1960 to 2015. Results show that 2018 and 2019 storm flow suspended sediment concentrations were 13.2 and 4.6 times higher, respectively, than predicted on the basis of the entire USGS monitoring record, but 32.5 and 9.5 times higher than the most recent period of persistently low suspended sediment concentrations. In comparison, previous decadal-scale periods of persistently high and low suspended sediment concentrations were 2.3 and -2.2 times that of the total USGS historical record. However, overall flux during post-wildfire years were far below the long term average because of relatively low precipitation in 2018 and 2019. These findings highlight the importance of post-wildfire storm event magnitudes for long-term sediment yield in small mountainous watersheds.

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.