Sediment loading to channels in a steep Mediterranean landscape

Sediment delivery from hillslopes is ultimately controlled by climate and climatic events. Climate influences the vegetation community in a landscape and, thus, the relevance of individual transport processes. Stochastic climatic events, such as fires and rainstorms, may cause pulses of sediment delivery so that sediment delivery, itself, is a stochastic process. Practical issues such as mitigating debris flow hazards and preserving fish habitat are intimately related to the magnitude and frequency of sediment delivery events. The synergistic effects of steep slopes, frequent fires, and intense rainfall make southern California an ideal region for investigating the stochastic nature of sediment delivery from hillslopes into the fluvial system. In addition to climatic and topographic influences, land management strategies may also have impacts on sediment delivery. In many regions throughout the West, native shrub vegetation is often cleared and replaced by exotic grasses for pasturage and, as I will demonstrate, this type conversion of vegetation can lead to wholesale changes in the rates and processes of sediment delivery.; In this thesis, I develop physically-based equations to predict the temporal distribution of sediment delivery by four types of transport processes: diffusive soil creep, overland flow, thin debris flows (TDF), and landslides. These equations are calibrated through fieldwork carried out at Sedgwick Reserve. The Sedgwick Reserve landscape is typical of the region, with gentle to steep hillslopes vegetated by coastal sage scrub or exotic annual grasses. In this environment, sediment can be delivered from hillslopes to the valley bottoms by a variety of different processes and the magnitude of each process varies by vegetation type.; The sediment transport equations developed from the fieldwork are incorporated into a numerical model that has Sedgwick Reserve as its topographic template and is driven by stochastically-generated rainfall and fire events. Results from the model suggest that the conversion of vegetation from sage scrub to grasslands increases the sediment delivered to channels by nearly 50%. Model results also suggest that changes in the spatial distribution of vegetation arising from climate change will have a greater effect on sediment delivery than changes in the magnitude and frequency of climatic events.