Graduate Thesis Or Dissertation
 

The Influence of Lithology on the Alluvial Character of Oregon Coast Range Streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pv63g5350

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  • Developing a process-based understanding of alluvial rivers is relevant to river restoration, the estimation of sediment transport, and the prediction of the effects of natural and anthropogenic disturbances on channel form and sediment flux. However, predicting downstream variations in sediment transport, and channel geometry is often challenging in mountain streams due, in part, to the strong connection between hillslopes and instream processes in many systems. These coupled streams are common in the unglaciated terrain of the Oregon Coast Range (OCR) where debris flows shape the physical characteristics of riverine landscapes. Discrete inputs of sediment from debris flows vary over space and time, and result in stochastic sediment supply regimes. Lithology dependent abrasion rates control the spatial extent of alluvial cover and the fate of sediment delivered and stored by mass wasting events. In this study, we quantified the degree to which hillslope coupled channels in the OCR display alluvial characteristics and assessed the role of lithology on the spatial extent of alluvial reaches. Using downstream measurements of bankfull geometry and grain size, we found evidence for alluvial processes shaping channels within hillslope coupled OCR streams underlain by sandstone and basalt. Contrasts in lithology between the two study basins resulted in different surface and subsurface grain sizes, channel slopes, and potential differences in sediment supply from tributaries and hillslopes. Despite these differences, both systems displayed characteristics typical of alluvial systems along most of their length, such as armored channel beds and strong relationships between shear stress and grain size. In addition, both networks adhered to the threshold theory of alluvial channels with transport stage values ––ratio of available to critical shear stress for motion ––values near one throughout the majority of reaches surveyed. Given the differences in basin and stream properties imparted by lithology, the similarity in transport stage indicates alluvial processes are an important control of channel adjustments and sediment transport at the basin scale, even within hillslope coupled systems.
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