Linking Geomorphic Process and Landscape Form: Topographic Analysis, Analog Experiments, and Numerical Modeling
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Erosive landscapes are formed from the amalgamation of individual sediment transport over space and time. Though sediment transport is readily observable at the grain and event scales, determining how these events conspire to form hillslope, rivers, and mountain ranges requires transcending considerable gaps in spatial and temporal scale. In this dissertation, I use a broad range of methods across a diverse suite of landscapes to quantify how geomorphic processes dictate landscape form. In Chapter II, I assess the magnitude of local variability in erosion in the Oregon Coast Range using the residence time of hilltop soils as a proxy for local erosion rate. I develop a new technique for measuring the soil weathering extent using visual–near-infrared spectroscopy. My results from this chapter indicate that the frequency and spatial distribution of hillslope disturbances, in this case tree throw, are primary controls on the magnitude of erosional variability. In Chapter III I take a different approach to tackling process-form linkages in eroding landscapes by systematically changing the dominant erosion process in a meter-scale laboratory landscape and quantifying the resulting topography. This approach is in contrast to most geomorphic investigations, which fit process models to static natural topography, ignoring the potential for changing process rates through time or the effect of initial conditions on landscape evolution. The steady-state topography of my experiments confirms numerical predictions that the drainage density of mountain ranges depends on the efficiency of hillslope transport relative to the efficiency of channel incision. Finally, in Chapter IV, I present my investigation of the early stages of landscape evolution on a Holocene lava flow in the Oregon High Cascades that has been incised by a fluvial channel. Here, I use lidar measurements, alluvial stratigraphy, and numerical modeling to constrain the type and magnitudes of channel-forming events. In contrast to past work in flood basalts, which point to the dominant role of megafloods in forming channels, my results demonstrate that both large outburst floods and smaller annual snowmelt flows are responsible for channel incision. This dissertation includes previously published and co-authored material.