Theses and Dissertations at Montana State University (MSU)
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Item Soil storage on steep forested and non-forested mountain hillslopes in the Bitterroot Mountains, Montana(Montana State University - Bozeman, College of Letters & Science, 2018) Quinn, Colin Aidan; Chairperson, Graduate Committee: Jean DixonMountain hillslopes are dynamic settings with discontinuous soils affected by a suite of variables including climate, lithology, hydrology, and vegetation. Our study seeks to understand how forest cover influences soil and rock distribution at decadal to century timescales. We focus on a series of post-glacial hillslopes in Lost Horse Creek of the Bitterroot Mountains, Montana. In this system, avalanche paths maintain parallel, topographically similar swaths of forested and non-forested slopes with uniform aspect, lithology, and climate. We combine field observations, fallout radionuclide analysis (210 Pbex & 137 Cs), and remote sensing data to understand both landscape- and fine-scale patterns in soil and rock distribution. Local soil and rock measurements indicate more extensive soil cover (forest = 94.4 + or = 2.6%; non-forest = 88.3 + or = 1.9%) and thicker soils (6cm greater median) in the forested system. We compare landcover-classified rock to topographic metrics from LiDAR data and find a doubling of rock cover (from 40% to 80%) as hillslope angles transition across slopes of ~24-42 ?. Topographic roughness, calculated as the standard deviation of slope, is predictive of only ~60% of total landscape rock cover, but can identify large boulders and coarse-scale outcrops with higher accuracy (79%). These calibrated remote sensing metrics indicate higher rock cover in non-forested regions (34%, compared to 20% in forested areas), though with high uncertainty. Additionally, we measure fallout-radionuclide inventories in soils to explore variations in decadal transport processes and soil residence times. We find distinct 210 Pb and 137 Cs behaviors in forested and non-forested systems, controlled both by unique partitioning of each nuclide within organic and mineral soil horizons, but also due to depth-driven differences in their physical mobility. Average 210 Pb ex inventories in non-forested soils are 33% lower, and half as variable as soils in the forested region (10.45 + or = 0.97 and 15.49 + or = 1.91 kBq/m 2 respectively), while 137 Cs inventories are indistinguishable (4.04 + or = 0.34 and 3.73 + or = 0.42 kBq/m 2). Together, our spatial, field, and isotope analyses suggest forested systems have greater soil storage and longer residence times than non-forested soils, mediated by differences in surface erosion processes within a larger fire disturbance landscape.Item Landslide morphology and its insight into the timing and causes of slope failure: case study of post-glacial landslides in Yellowstone National Park(Montana State University - Bozeman, College of Letters & Science, 2018) Nicholas, Grace Ellen; Chairperson, Graduate Committee: Jean DixonLandslides are ubiquitous to post-glacial landscapes worldwide. Withdrawal of glacier ice exposes over-steepened landscapes that may be unstable, and consequently susceptible to landsliding. Glacial debuttressing may directly destabilize slopes; however, seismicity and transitions to interglacial climates associated with greater effective moisture and subsequent degradation of permafrost may also play a role. Here, we explore disparate potential mechanisms of slope failure in a set of post-glacial landslides in northwest Yellowstone National Park. We quantify spatial relationships, topographic metrics, and relative age of eight landslides within the north entrance to the park, a system traversed by over 700,000 visitors every year. Analysis of high-resolution topography indicates increasing surface roughness of non-active landslides southward. These roughness values in ancient slides are roughly half those of the active Slide Lake Landslide, and suggest younging ages along the retreat path of the Yellowstone Ice Cap, consistent with glacial debutressing as the likely trigger for these slides. However, roughness values and their application for relative age dating are strongly confounded by topographic biases such as gullying, fluvial erosional contacts, and anthropogenic features (e.g., roads, structures). Once roughness biases are removed, we find roughness differences between landslides decrease, and do not support younging ages along the path of ice retreat. Thus, glacial debuttressing most likely only had a preparatory influence on slope failure, and was not the direct trigger. Analysis of subsurface soils at landslide toes indicate a >17 plasticity index, pointing to highly expansive clays that are sensitive to moisture addition. Stratigraphic relationships between post-glacial terraces and soil analyses suggest a late Pleistocene (~13 - 11.5 ka) timing for slide initiation, a period coincident with high available moisture. Stream power analysis indicates that Holocene incision of the Gardiner River is focused at a knickpoint locally coincident with the toe of the active Slide Lake Landslide, providing a mechanism for modern, local reactivation of the ancient slides. Together, our findings broadly show how quantifying the temporal and spatial patterns of landslides can be diagnostic of the controls on slope failure, and can be used to understand risk. They also highlight the importance of careful site calibrations and bias removals in roughness analysis.