UC Santa Cruz

Rivers are dynamic features of Earth’s surface that contain information about both past and present landscape processes. Shifts in the balances of sediment and flow in a watershed are often reflected in the character of the riverbed, channel geometry, or patterns of sediment transport, making rivers an invaluable tool for investigating changes in the overall sediment transport regime. Through three related studies, this work explores the various ways in which rivers not only respond to changing watershed conditions, but also reflect the broader climatic and lithologic character of a landscape.

This dissertation begins with a case study of the San Lorenzo River in California, in which the causes and timing of river floodplain abandonment are investigated through a combination of field measurements of channel geometry, theoretical estimates of channel depth, and radiocarbon dating of charcoal. Results suggest that significant channel incision can be attributed to historical practices associated with clear-cut logging, underscoring the long-lasting legacy of land use on landscapes and river networks.

Broader relationships between sediment transport, river characteristics, and environmental factors are explored in the following two studies. Chapter 2 uses an aridity index in tandem with suspended sediment rating curves for 71 rivers across the United States to establish a compelling relationship between climate and sediment transport efficiency, and explores the influence of vegetation density and flow variability on sediment supply and riverbed grain size sorting. Chapter 3 follows up on these results by directly examining the link between suspended sediment rating curves and the organization of the riverbed. This study strengthens the interpretations made in Chapter 2 by confirming the likely significance of riverbed grain size and armoring on sediment transport efficiency across climates.

By pairing a case study with both novel and far-reaching geomorphic analysis, this research contributes to a deeper understanding of the mechanisms governing fluvial sediment transport and river form. The specific findings and new applications of old methods provide helpful resources to researchers and policymakers seeking to mitigate the effects of climate change and environmental degradation on river ecosystems and adjacent communities.